[PATCH/RFC] MM: fix writeback for NFS

[PATCH/RFC] MM: fix writeback for NFS

[NeilBrown]

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Page account happens a little differently for NFS, and writeback is
aware of this, but handles it wrongly.
With NFS, pages go throught three states which are accounted separately:
 NR_FILE_DIRTY - page is dirty and unwritten
 NR_WRITEBACK  - page is being written back
 NR_UNSTABLE_NFS - page has been written, but might need to be written
		again if server restarts.  Once an NFS COMMIT
		completes, page becomes freeable.

writeback combines both the NF_FILE_DIRTY counters and NF_UNSTABLE_NFS
together, which doesn't make a lot of sense.  NR_UNSTABLE_NFS is more
like NR_WRITEBACK in that there is nothing that can be done for these
pages except wait.

Current behaviour is that when there are a lot of NR_UNSTABLE_NFS pages,
balance_dirty_pages() repeatedly schedules the writeback thread, even
when the number of dirty pages is below the threshold.  This result is
each ->write_pages() call into NFS only finding relatively few DIRTY
pages, so it creates requests that are well below the maximum size.
Depending on performance of characteristics of the server, this can
substantially reduce throughput.

There are two places where balance_dirty_pages() schedules the writeback
thread, and each of them schedule writeback too often.

1/ When balance_dirty_pages() determines that it must wait for the
   number of dirty pages to shrink, it unconditionally schedules
   writeback.
   This is probably not ideal for any filesystem but I measured it to be
   harmful for NFS.  This writeback should only be scheduled if the
   number of dirty pages is above the threshold.  While it is below,
   waiting for the pending writes to complete (and be committed) is a more
   appropriate behaviour.

2/ When balance_dirty_pages() finishes, it again shedules writeback
   if nr_reclaimable is above the background threshold.  This is
   conceptually correct, but wrong because nr_reclaimable is wrong.
   The impliciation of this usage is that nr_reclaimable is the number
   of pages that can be reclaimed if writeback is triggered.  In fact it
   is NR_FILE_DIRTY plus NR_UNSTABLE_NFS, which is not a meaningful
   number.

So this patch removes NR_UNSTABLE_NFS from nr_reclaimable, and only
schedules writeback when the new nr_reclaimable is greater than
->thresh or ->bg_thresh, depending on context.

The effect of this can be measured by looking at the count of
nfs_writepages calls while performing a large streaming write (larger
than dirty_threshold)

e.g.
  mount -t nfs server:/path /mnt
  dd if=/dev/zero of=/mnt/zeros bs=1M count=1000
  awk '/events:/ {print $13}' /proc/self/mountstats

Each writepages call should normally submit writes for 4M (1024 pages)
or more (MIN_WRITEBACK_PAGES) (unless memory size is tiny).
With the patch I measure typically 200-300 writepages calls with the
above, which suggests an average of about 850 pages being made available to
each writepages call.  This is less than MIN_WRITEBACK_PAGES, but enough
to assemble large 1MB WRITE requests fairly often.

Without the patch, numbers range from about 2000 to over 10000, meaning
an average of maybe 100 pages per call, which means we rarely get large
1M requests, and often get small requests

Note that there are other places where NR_FILE_DIRTY and NR_UNSTABLE_NFS
are combined (wb_over_bg_thresh() and get_nr_dirty_pages()).  I suspect
they are wrong too.  I also suspect that unstable-NFS pages should not be
included in the WB_RECLAIMABLE wb_stat, but I haven't explored that in
detail yet.

Note that the current behaviour of NFS w.r.t sending COMMIT requests is
that as soon as a batch of writes all complete, a COMMIT is scheduled.
Prior to commit 919e3bd9a875 ("NFS: Ensure we commit after writeback is
complete"), other mechanisms triggered the COMMIT.  It is possible that
the current writeback code made more sense before that commit.

Signed-off-by: NeilBrown <neilb@suse.de>
---
 mm/page-writeback.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 2caf780a42e7..99419cba1e7a 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -1593,10 +1593,11 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
 		 * written to the server's write cache, but has not yet
 		 * been flushed to permanent storage.
 		 */
-		nr_reclaimable = global_node_page_state(NR_FILE_DIRTY) +
-					global_node_page_state(NR_UNSTABLE_NFS);
+		nr_reclaimable = global_node_page_state(NR_FILE_DIRTY);
 		gdtc->avail = global_dirtyable_memory();
-		gdtc->dirty = nr_reclaimable + global_node_page_state(NR_WRITEBACK);
+		gdtc->dirty = nr_reclaimable +
+			global_node_page_state(NR_WRITEBACK) +
+			global_node_page_state(NR_UNSTABLE_NFS);
 
 		domain_dirty_limits(gdtc);
 
@@ -1664,7 +1665,8 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
 			break;
 		}
 
-		if (unlikely(!writeback_in_progress(wb)))
+		if (nr_reclaimable > gdtc->thresh &&
+		    unlikely(!writeback_in_progress(wb)))
 			wb_start_background_writeback(wb);
 
 		mem_cgroup_flush_foreign(wb);
-- 
2.25.2


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Writeback fixes for NFS

[NeilBrown]

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Please ignore my previous patch (which this is in reply to), it was
flawed in various ways.

I now understand the code a bit better and have a somewhat simpler patch
which appears to address the same problem.

The problem is that writeback to NFS often produces lots of small writes
(10s of K) rather than fewer large writes (1M).  This pattern can often
hurt throughput, but in certain circumstances it can hurt NFS throughput
more than expected.

Each nfs_writepages() call results in an NFS commit being sent to the
server.  If writeback triggers lots of smaller nfs_writepages calls,
this means lots of COMMITs.  If the server is slow to handle the COMMIT
(I've seen the Ganesha NFS server take over 200ms per commit), these
COMMITs can overlap, queue up, and choke the NFS server and cause
order-of-magnitude drop in throughput.

So we really want to only call nfs_writepages when there are a largish
number of pages to be written - i.e. that are 'dirty'.

For historical reasons that I didn't thoroughly research but I'm
confident are no longer relevant, pages that have been written to the
NFS server but have not yet been the subject of a COMMIT - so-called
"unstable" pages - are effectively accounted that same as "dirty" pages
(sometimes called "reclaimable").
This can result in writeback thinking there are lots of "dirty" pages to
reclaim, while nfs_writepages can only find a few that it can write out.

The second patch following changes the accounting for these "unstable"
pages.  They are now always accounted exactly the same was writeback
pages.  Conceptually they can be thought of as still in writeback, but the
writeback is now happening on the server.

A COMMIT will always automatically follow the writes generated by
nfs_writepages, so from the perspective of the VM, there really is no
difference: It has scheduled the write and there is nothing else it can
do except wait.

Testing this patch showed that loop-back NFS is prone to deadlocks
again.  I cannot see exactly how the change to 'unstable' accounting
affected this, but I can see that the old +25% heuristic can no longer
be justified given the complexity of writeback calculations.
So the first patch following changes how writeback is handled for NFS
servers handling loop-back requests (and other similar services) so that
it is more obviously safe against excessive dirty pages scheduled for
other devices.

Thanks,
NeilBrown

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[PATCH 1/2] MM: replace PF_LESS_THROTTLE with PF_LOCAL_THROTTLE

[NeilBrown]

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PF_LESS_THROTTLE exists for loop-back nfsd, and a similar need in the
loop block driver, where a daemon needs to write to one bdi in
order to free up writes queued to another bdi.

The daemon sets PF_LESS_THROTTLE and gets a larger allowance of dirty
pages, so that it can still dirty pages after other processses have been
throttled.

This approach was designed when all threads were blocked equally,
independently on which device they were writing to, or how fast it was.
Since that time the writeback algorithm has changed substantially with
different threads getting different allowances based on non-trivial
heuristics.  This means the simple "add 25%" heuristic is no longer
reliable.

This patch changes the heuristic to ignore the global limits and
consider only the limit relevant to the bdi being written to.  This
approach is already available for BDI_CAP_STRICTLIMIT users (fuse) and
should not introduce surprises.  This has the desired result of
protecting the task from the consequences of large amounts of dirty data
queued for other devices.

This approach of "only consider the target bdi" is consistent with the
other use of PF_LESS_THROTTLE in current_may_throttle(), were it causes
attention to be focussed only on the target bdi.

So this patch
 - renames PF_LESS_THROTTLE to PF_LOCAL_THROTTLE,
 - remove the 25% bonus that that flag gives, and
 - imposes 'strictlimit' handling for any process with PF_LOCAL_THROTTLE
   set.

Note that previously realtime threads were treated the same as
PF_LESS_THROTTLE threads.  This patch does *not* change the behvaiour for
real-time threads, so it is now different from the behaviour of nfsd and
loop tasks.  I don't know what is wanted for realtime.

Signed-off-by: NeilBrown <neilb@suse.de>
---
 drivers/block/loop.c  |  2 +-
 fs/nfsd/vfs.c         |  9 +++++----
 include/linux/sched.h |  2 +-
 kernel/sys.c          |  2 +-
 mm/page-writeback.c   | 10 ++++++----
 mm/vmscan.c           |  4 ++--
 6 files changed, 16 insertions(+), 13 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 739b372a5112..2c59371ce936 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -897,7 +897,7 @@ static void loop_unprepare_queue(struct loop_device *lo)
 
 static int loop_kthread_worker_fn(void *worker_ptr)
 {
-	current->flags |= PF_LESS_THROTTLE | PF_MEMALLOC_NOIO;
+	current->flags |= PF_LOCAL_THROTTLE | PF_MEMALLOC_NOIO;
 	return kthread_worker_fn(worker_ptr);
 }
 
diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
index 0aa02eb18bd3..c3fbab1753ec 100644
--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c
@@ -979,12 +979,13 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp, struct nfsd_file *nf,
 
 	if (test_bit(RQ_LOCAL, &rqstp->rq_flags))
 		/*
-		 * We want less throttling in balance_dirty_pages()
-		 * and shrink_inactive_list() so that nfs to
+		 * We want throttling in balance_dirty_pages()
+		 * and shrink_inactive_list() to only consider
+		 * the backingdev we are writing to, so that nfs to
 		 * localhost doesn't cause nfsd to lock up due to all
 		 * the client's dirty pages or its congested queue.
 		 */
-		current->flags |= PF_LESS_THROTTLE;
+		current->flags |= PF_LOCAL_THROTTLE;
 
 	exp = fhp->fh_export;
 	use_wgather = (rqstp->rq_vers == 2) && EX_WGATHER(exp);
@@ -1037,7 +1038,7 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp, struct nfsd_file *nf,
 		nfserr = nfserrno(host_err);
 	}
 	if (test_bit(RQ_LOCAL, &rqstp->rq_flags))
-		current_restore_flags(pflags, PF_LESS_THROTTLE);
+		current_restore_flags(pflags, PF_LOCAL_THROTTLE);
 	return nfserr;
 }
 
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 04278493bf15..5dcd27abc8cd 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1473,7 +1473,7 @@ extern struct pid *cad_pid;
 #define PF_KSWAPD		0x00020000	/* I am kswapd */
 #define PF_MEMALLOC_NOFS	0x00040000	/* All allocation requests will inherit GFP_NOFS */
 #define PF_MEMALLOC_NOIO	0x00080000	/* All allocation requests will inherit GFP_NOIO */
-#define PF_LESS_THROTTLE	0x00100000	/* Throttle me less: I clean memory */
+#define PF_LOCAL_THROTTLE	0x00100000	/* Throttle me less: I clean memory */
 #define PF_KTHREAD		0x00200000	/* I am a kernel thread */
 #define PF_RANDOMIZE		0x00400000	/* Randomize virtual address space */
 #define PF_SWAPWRITE		0x00800000	/* Allowed to write to swap */
diff --git a/kernel/sys.c b/kernel/sys.c
index d325f3ab624a..180a2fa33f7f 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2262,7 +2262,7 @@ int __weak arch_prctl_spec_ctrl_set(struct task_struct *t, unsigned long which,
 	return -EINVAL;
 }
 
-#define PR_IO_FLUSHER (PF_MEMALLOC_NOIO | PF_LESS_THROTTLE)
+#define PR_IO_FLUSHER (PF_MEMALLOC_NOIO | PF_LOCAL_THROTTLE)
 
 SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 		unsigned long, arg4, unsigned long, arg5)
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 2caf780a42e7..2afb09fa2fe0 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -387,8 +387,7 @@ static unsigned long global_dirtyable_memory(void)
  * Calculate @dtc->thresh and ->bg_thresh considering
  * vm_dirty_{bytes|ratio} and dirty_background_{bytes|ratio}.  The caller
  * must ensure that @dtc->avail is set before calling this function.  The
- * dirty limits will be lifted by 1/4 for PF_LESS_THROTTLE (ie. nfsd) and
- * real-time tasks.
+ * dirty limits will be lifted by 1/4 for real-time tasks.
  */
 static void domain_dirty_limits(struct dirty_throttle_control *dtc)
 {
@@ -436,7 +435,7 @@ static void domain_dirty_limits(struct dirty_throttle_control *dtc)
 	if (bg_thresh >= thresh)
 		bg_thresh = thresh / 2;
 	tsk = current;
-	if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) {
+	if (rt_task(tsk)) {
 		bg_thresh += bg_thresh / 4 + global_wb_domain.dirty_limit / 32;
 		thresh += thresh / 4 + global_wb_domain.dirty_limit / 32;
 	}
@@ -486,7 +485,7 @@ static unsigned long node_dirty_limit(struct pglist_data *pgdat)
 	else
 		dirty = vm_dirty_ratio * node_memory / 100;
 
-	if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk))
+	if (rt_task(tsk))
 		dirty += dirty / 4;
 
 	return dirty;
@@ -1580,6 +1579,9 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
 	bool strictlimit = bdi->capabilities & BDI_CAP_STRICTLIMIT;
 	unsigned long start_time = jiffies;
 
+	if (current->flags & PF_LOCAL_THROTTLE)
+		/* This task must only be throttled by its own writeback */
+		strictlimit = true;
 	for (;;) {
 		unsigned long now = jiffies;
 		unsigned long dirty, thresh, bg_thresh;
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 876370565455..c5cf25938c56 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1880,13 +1880,13 @@ static unsigned noinline_for_stack move_pages_to_lru(struct lruvec *lruvec,
 
 /*
  * If a kernel thread (such as nfsd for loop-back mounts) services
- * a backing device by writing to the page cache it sets PF_LESS_THROTTLE.
+ * a backing device by writing to the page cache it sets PF_LOCAL_THROTTLE.
  * In that case we should only throttle if the backing device it is
  * writing to is congested.  In other cases it is safe to throttle.
  */
 static int current_may_throttle(void)
 {
-	return !(current->flags & PF_LESS_THROTTLE) ||
+	return !(current->flags & PF_LOCAL_THROTTLE) ||
 		current->backing_dev_info == NULL ||
 		bdi_write_congested(current->backing_dev_info);
 }
-- 
2.26.0


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[PATCH 2/2] Deprecate NR_UNSTABLE_NFS, use NR_WRITEBACK

[NeilBrown]

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After an NFS page has been written it is considered "unstable" until a
COMMIT request succeeds.  If the COMMIT fails, the page will be
re-written.

These "unstable" pages are currently accounted as "reclaimable",
either in WB_RECLAIMABLE, or in NR_UNSTABLE_NFS which is included in a
'reclaimable' count.  This might have made sense when sending the COMMIT
required a separate action by the VFS/MM (e.g. releasepage() used to
send a COMMIT).  However now that all writes generated by ->writepages()
will automatically be followed by a COMMIT, it makes more sense to
treat them as writeback pages.

So this page deprecates NR_UNSTABLE_NFS and accounts unstable pages in
NR_WRITEBACK and WB_WRITEBACK.

A particular effect of this change is that when
wb_check_background_flush() calls wb_over_bg_threshold(), the latter
will report 'true' a lot less often as the 'unstable' pages are no
longer considered 'dirty' (and there is nothing that writeback can do
about them anyway).

Currently wb_check_background_flush() will trigger writeback to NFS even
when there are relatively few dirty pages (if there are lots of unstable
pages), this can result in small writes going to the server (10s of
Kilobytes rather than a Megabyte) which hurts throughput.
With this that, there are fewer writes which are each larger on average.

Signed-off-by: NeilBrown <neilb@suse.de>
---
 fs/fs-writeback.c      | 1 -
 fs/nfs/internal.h      | 7 +++++--
 fs/nfs/write.c         | 4 ++--
 include/linux/mmzone.h | 2 +-
 mm/memcontrol.c        | 1 -
 mm/page-writeback.c    | 7 ++-----
 6 files changed, 10 insertions(+), 12 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 76ac9c7d32ec..c5bdf46e3b4b 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -1070,7 +1070,6 @@ static void bdi_split_work_to_wbs(struct backing_dev_info *bdi,
 static unsigned long get_nr_dirty_pages(void)
 {
 	return global_node_page_state(NR_FILE_DIRTY) +
-		global_node_page_state(NR_UNSTABLE_NFS) +
 		get_nr_dirty_inodes();
 }
 
diff --git a/fs/nfs/internal.h b/fs/nfs/internal.h
index f80c47d5ff27..ba1ff5adeccd 100644
--- a/fs/nfs/internal.h
+++ b/fs/nfs/internal.h
@@ -660,8 +660,11 @@ void nfs_mark_page_unstable(struct page *page, struct nfs_commit_info *cinfo)
 	if (!cinfo->dreq) {
 		struct inode *inode = page_file_mapping(page)->host;
 
-		inc_node_page_state(page, NR_UNSTABLE_NFS);
-		inc_wb_stat(&inode_to_bdi(inode)->wb, WB_RECLAIMABLE);
+		/* This page is really still in write-back - just that the
+		 * writeback is happening on the server now.
+		 */
+		inc_node_page_state(page, NR_WRITEBACK);
+		inc_wb_stat(&inode_to_bdi(inode)->wb, WB_WRITEBACK);
 		__mark_inode_dirty(inode, I_DIRTY_DATASYNC);
 	}
 }
diff --git a/fs/nfs/write.c b/fs/nfs/write.c
index c478b772cc49..2e15a56620b3 100644
--- a/fs/nfs/write.c
+++ b/fs/nfs/write.c
@@ -958,9 +958,9 @@ nfs_mark_request_commit(struct nfs_page *req, struct pnfs_layout_segment *lseg,
 static void
 nfs_clear_page_commit(struct page *page)
 {
-	dec_node_page_state(page, NR_UNSTABLE_NFS);
+	dec_node_page_state(page, NR_WRITEBACK);
 	dec_wb_stat(&inode_to_bdi(page_file_mapping(page)->host)->wb,
-		    WB_RECLAIMABLE);
+		    WB_WRITEBACK);
 }
 
 /* Called holding the request lock on @req */
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 462f6873905a..227fcb8cd0e6 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -237,7 +237,7 @@ enum node_stat_item {
 	NR_FILE_THPS,
 	NR_FILE_PMDMAPPED,
 	NR_ANON_THPS,
-	NR_UNSTABLE_NFS,	/* NFS unstable pages */
+	NR_UNSTABLE_NFS,	/* NFS unstable pages - DEPRECATED DO NOT USE */
 	NR_VMSCAN_WRITE,
 	NR_VMSCAN_IMMEDIATE,	/* Prioritise for reclaim when writeback ends */
 	NR_DIRTIED,		/* page dirtyings since bootup */
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 7ddf91c4295f..fad8e8a23235 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4317,7 +4317,6 @@ void mem_cgroup_wb_stats(struct bdi_writeback *wb, unsigned long *pfilepages,
 
 	*pdirty = memcg_exact_page_state(memcg, NR_FILE_DIRTY);
 
-	/* this should eventually include NR_UNSTABLE_NFS */
 	*pwriteback = memcg_exact_page_state(memcg, NR_WRITEBACK);
 	*pfilepages = memcg_exact_page_state(memcg, NR_INACTIVE_FILE) +
 			memcg_exact_page_state(memcg, NR_ACTIVE_FILE);
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 2afb09fa2fe0..d1f03c799d11 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -504,7 +504,6 @@ bool node_dirty_ok(struct pglist_data *pgdat)
 	unsigned long nr_pages = 0;
 
 	nr_pages += node_page_state(pgdat, NR_FILE_DIRTY);
-	nr_pages += node_page_state(pgdat, NR_UNSTABLE_NFS);
 	nr_pages += node_page_state(pgdat, NR_WRITEBACK);
 
 	return nr_pages <= limit;
@@ -1595,8 +1594,7 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
 		 * written to the server's write cache, but has not yet
 		 * been flushed to permanent storage.
 		 */
-		nr_reclaimable = global_node_page_state(NR_FILE_DIRTY) +
-					global_node_page_state(NR_UNSTABLE_NFS);
+		nr_reclaimable = global_node_page_state(NR_FILE_DIRTY);
 		gdtc->avail = global_dirtyable_memory();
 		gdtc->dirty = nr_reclaimable + global_node_page_state(NR_WRITEBACK);
 
@@ -1940,8 +1938,7 @@ bool wb_over_bg_thresh(struct bdi_writeback *wb)
 	 * as we're trying to decide whether to put more under writeback.
 	 */
 	gdtc->avail = global_dirtyable_memory();
-	gdtc->dirty = global_node_page_state(NR_FILE_DIRTY) +
-		      global_node_page_state(NR_UNSTABLE_NFS);
+	gdtc->dirty = global_node_page_state(NR_FILE_DIRTY);
 	domain_dirty_limits(gdtc);
 
 	if (gdtc->dirty > gdtc->bg_thresh)
-- 
2.26.0


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Re: [PATCH 2/2] Deprecate NR_UNSTABLE_NFS, use NR_WRITEBACK

[Christoph Hellwig]

On Thu, Apr 02, 2020 at 10:54:07AM +1100, NeilBrown wrote:
> 
> After an NFS page has been written it is considered "unstable" until a
> COMMIT request succeeds.  If the COMMIT fails, the page will be
> re-written.
> 
> These "unstable" pages are currently accounted as "reclaimable",
> either in WB_RECLAIMABLE, or in NR_UNSTABLE_NFS which is included in a
> 'reclaimable' count.  This might have made sense when sending the COMMIT
> required a separate action by the VFS/MM (e.g. releasepage() used to
> send a COMMIT).  However now that all writes generated by ->writepages()
> will automatically be followed by a COMMIT, it makes more sense to
> treat them as writeback pages.
> 
> So this page deprecates NR_UNSTABLE_NFS and accounts unstable pages in
> NR_WRITEBACK and WB_WRITEBACK.

Please remove it entirely if it isn't used any more.

[PATCH 2/2 - v2] MM: Discard NR_UNSTABLE_NFS, use NR_WRITEBACK instead.

[NeilBrown]

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After an NFS page has been written it is considered "unstable" until a
COMMIT request succeeds.  If the COMMIT fails, the page will be
re-written.

These "unstable" pages are currently accounted as "reclaimable", either
in WB_RECLAIMABLE, or in NR_UNSTABLE_NFS which is included in a
'reclaimable' count.  This might have made sense when sending the COMMIT
required a separate action by the VFS/MM (e.g.  releasepage() used to
send a COMMIT).  However now that all writes generated by ->writepages()
will automatically be followed by a COMMIT (since commit 919e3bd9a875
("NFS: Ensure we commit after writeback is complete")) it makes more
sense to treat them as writeback pages.

So this patch removes NR_UNSTABLE_NFS and accounts unstable pages in
NR_WRITEBACK and WB_WRITEBACK.

A particular effect of this change is that when
wb_check_background_flush() calls wb_over_bg_threshold(), the latter
will report 'true' a lot less often as the 'unstable' pages are no
longer considered 'dirty' (and there is nothing that writeback can do
about them anyway).

Currently wb_check_background_flush() will trigger writeback to NFS even
when there are relatively few dirty pages (if there are lots of unstable
pages), this can result in small writes going to the server (10s of
Kilobytes rather than a Megabyte) which hurts throughput.
With this patch, there are fewer writes which are each larger on average.

Signed-off-by: NeilBrown <neilb@suse.de>
---

NR_UNSTABLE_NFS completely removed as recommended by Christoph, removal
of an unnecessary comment, and improvements to commit message.
Thanks.

 Documentation/filesystems/proc.txt |  3 ---
 drivers/base/node.c                |  2 --
 fs/fs-writeback.c                  |  1 -
 fs/nfs/internal.h                  | 10 +++++++---
 fs/nfs/write.c                     |  4 ++--
 fs/proc/meminfo.c                  |  2 --
 include/linux/mmzone.h             |  1 -
 include/trace/events/writeback.h   |  5 +----
 mm/memcontrol.c                    |  1 -
 mm/page-writeback.c                | 17 ++++-------------
 mm/page_alloc.c                    |  5 +----
 mm/vmstat.c                        |  1 -
 12 files changed, 15 insertions(+), 37 deletions(-)

diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
index 99ca040e3f90..690c712f5f79 100644
--- a/Documentation/filesystems/proc.txt
+++ b/Documentation/filesystems/proc.txt
@@ -904,7 +904,6 @@ Slab:           284364 kB
 SReclaimable:   159856 kB
 SUnreclaim:     124508 kB
 PageTables:      24448 kB
-NFS_Unstable:        0 kB
 Bounce:              0 kB
 WritebackTmp:        0 kB
 CommitLimit:   7669796 kB
@@ -975,8 +974,6 @@ SReclaimable: Part of Slab, that might be reclaimed, such as caches
   SUnreclaim: Part of Slab, that cannot be reclaimed on memory pressure
   PageTables: amount of memory dedicated to the lowest level of page
               tables.
-NFS_Unstable: NFS pages sent to the server, but not yet committed to stable
-	      storage
       Bounce: Memory used for block device "bounce buffers"
 WritebackTmp: Memory used by FUSE for temporary writeback buffers
  CommitLimit: Based on the overcommit ratio ('vm.overcommit_ratio'),
diff --git a/drivers/base/node.c b/drivers/base/node.c
index 98a31bafc8a2..7059021ce2af 100644
--- a/drivers/base/node.c
+++ b/drivers/base/node.c
@@ -416,7 +416,6 @@ static ssize_t node_read_meminfo(struct device *dev,
 		       "Node %d Shmem:          %8lu kB\n"
 		       "Node %d KernelStack:    %8lu kB\n"
 		       "Node %d PageTables:     %8lu kB\n"
-		       "Node %d NFS_Unstable:   %8lu kB\n"
 		       "Node %d Bounce:         %8lu kB\n"
 		       "Node %d WritebackTmp:   %8lu kB\n"
 		       "Node %d KReclaimable:   %8lu kB\n"
@@ -439,7 +438,6 @@ static ssize_t node_read_meminfo(struct device *dev,
 		       nid, K(i.sharedram),
 		       nid, sum_zone_node_page_state(nid, NR_KERNEL_STACK_KB),
 		       nid, K(sum_zone_node_page_state(nid, NR_PAGETABLE)),
-		       nid, K(node_page_state(pgdat, NR_UNSTABLE_NFS)),
 		       nid, K(sum_zone_node_page_state(nid, NR_BOUNCE)),
 		       nid, K(node_page_state(pgdat, NR_WRITEBACK_TEMP)),
 		       nid, K(sreclaimable +
diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 76ac9c7d32ec..c5bdf46e3b4b 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -1070,7 +1070,6 @@ static void bdi_split_work_to_wbs(struct backing_dev_info *bdi,
 static unsigned long get_nr_dirty_pages(void)
 {
 	return global_node_page_state(NR_FILE_DIRTY) +
-		global_node_page_state(NR_UNSTABLE_NFS) +
 		get_nr_dirty_inodes();
 }
 
diff --git a/fs/nfs/internal.h b/fs/nfs/internal.h
index f80c47d5ff27..749da02b547a 100644
--- a/fs/nfs/internal.h
+++ b/fs/nfs/internal.h
@@ -652,7 +652,8 @@ void nfs_super_set_maxbytes(struct super_block *sb, __u64 maxfilesize)
 }
 
 /*
- * Record the page as unstable and mark its inode as dirty.
+ * Record the page as unstable (an extra writeback period) and mark its
+ * inode as dirty.
  */
 static inline
 void nfs_mark_page_unstable(struct page *page, struct nfs_commit_info *cinfo)
@@ -660,8 +661,11 @@ void nfs_mark_page_unstable(struct page *page, struct nfs_commit_info *cinfo)
 	if (!cinfo->dreq) {
 		struct inode *inode = page_file_mapping(page)->host;
 
-		inc_node_page_state(page, NR_UNSTABLE_NFS);
-		inc_wb_stat(&inode_to_bdi(inode)->wb, WB_RECLAIMABLE);
+		/* This page is really still in write-back - just that the
+		 * writeback is happening on the server now.
+		 */
+		inc_node_page_state(page, NR_WRITEBACK);
+		inc_wb_stat(&inode_to_bdi(inode)->wb, WB_WRITEBACK);
 		__mark_inode_dirty(inode, I_DIRTY_DATASYNC);
 	}
 }
diff --git a/fs/nfs/write.c b/fs/nfs/write.c
index c478b772cc49..2e15a56620b3 100644
--- a/fs/nfs/write.c
+++ b/fs/nfs/write.c
@@ -958,9 +958,9 @@ nfs_mark_request_commit(struct nfs_page *req, struct pnfs_layout_segment *lseg,
 static void
 nfs_clear_page_commit(struct page *page)
 {
-	dec_node_page_state(page, NR_UNSTABLE_NFS);
+	dec_node_page_state(page, NR_WRITEBACK);
 	dec_wb_stat(&inode_to_bdi(page_file_mapping(page)->host)->wb,
-		    WB_RECLAIMABLE);
+		    WB_WRITEBACK);
 }
 
 /* Called holding the request lock on @req */
diff --git a/fs/proc/meminfo.c b/fs/proc/meminfo.c
index 8c1f1bb1a5ce..1378a132ff7e 100644
--- a/fs/proc/meminfo.c
+++ b/fs/proc/meminfo.c
@@ -106,8 +106,6 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
 	show_val_kb(m, "PageTables:     ",
 		    global_zone_page_state(NR_PAGETABLE));
 
-	show_val_kb(m, "NFS_Unstable:   ",
-		    global_node_page_state(NR_UNSTABLE_NFS));
 	show_val_kb(m, "Bounce:         ",
 		    global_zone_page_state(NR_BOUNCE));
 	show_val_kb(m, "WritebackTmp:   ",
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 462f6873905a..a18611197bea 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -237,7 +237,6 @@ enum node_stat_item {
 	NR_FILE_THPS,
 	NR_FILE_PMDMAPPED,
 	NR_ANON_THPS,
-	NR_UNSTABLE_NFS,	/* NFS unstable pages */
 	NR_VMSCAN_WRITE,
 	NR_VMSCAN_IMMEDIATE,	/* Prioritise for reclaim when writeback ends */
 	NR_DIRTIED,		/* page dirtyings since bootup */
diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
index d94def25e4dc..45b5fbdb1f62 100644
--- a/include/trace/events/writeback.h
+++ b/include/trace/events/writeback.h
@@ -542,7 +542,6 @@ TRACE_EVENT(global_dirty_state,
 	TP_STRUCT__entry(
 		__field(unsigned long,	nr_dirty)
 		__field(unsigned long,	nr_writeback)
-		__field(unsigned long,	nr_unstable)
 		__field(unsigned long,	background_thresh)
 		__field(unsigned long,	dirty_thresh)
 		__field(unsigned long,	dirty_limit)
@@ -553,7 +552,6 @@ TRACE_EVENT(global_dirty_state,
 	TP_fast_assign(
 		__entry->nr_dirty	= global_node_page_state(NR_FILE_DIRTY);
 		__entry->nr_writeback	= global_node_page_state(NR_WRITEBACK);
-		__entry->nr_unstable	= global_node_page_state(NR_UNSTABLE_NFS);
 		__entry->nr_dirtied	= global_node_page_state(NR_DIRTIED);
 		__entry->nr_written	= global_node_page_state(NR_WRITTEN);
 		__entry->background_thresh = background_thresh;
@@ -561,12 +559,11 @@ TRACE_EVENT(global_dirty_state,
 		__entry->dirty_limit	= global_wb_domain.dirty_limit;
 	),
 
-	TP_printk("dirty=%lu writeback=%lu unstable=%lu "
+	TP_printk("dirty=%lu writeback=%lu "
 		  "bg_thresh=%lu thresh=%lu limit=%lu "
 		  "dirtied=%lu written=%lu",
 		  __entry->nr_dirty,
 		  __entry->nr_writeback,
-		  __entry->nr_unstable,
 		  __entry->background_thresh,
 		  __entry->dirty_thresh,
 		  __entry->dirty_limit,
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 7ddf91c4295f..fad8e8a23235 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4317,7 +4317,6 @@ void mem_cgroup_wb_stats(struct bdi_writeback *wb, unsigned long *pfilepages,
 
 	*pdirty = memcg_exact_page_state(memcg, NR_FILE_DIRTY);
 
-	/* this should eventually include NR_UNSTABLE_NFS */
 	*pwriteback = memcg_exact_page_state(memcg, NR_WRITEBACK);
 	*pfilepages = memcg_exact_page_state(memcg, NR_INACTIVE_FILE) +
 			memcg_exact_page_state(memcg, NR_ACTIVE_FILE);
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 2afb09fa2fe0..dbc73522609e 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -504,7 +504,6 @@ bool node_dirty_ok(struct pglist_data *pgdat)
 	unsigned long nr_pages = 0;
 
 	nr_pages += node_page_state(pgdat, NR_FILE_DIRTY);
-	nr_pages += node_page_state(pgdat, NR_UNSTABLE_NFS);
 	nr_pages += node_page_state(pgdat, NR_WRITEBACK);
 
 	return nr_pages <= limit;
@@ -758,7 +757,7 @@ static void mdtc_calc_avail(struct dirty_throttle_control *mdtc,
  * bounded by the bdi->min_ratio and/or bdi->max_ratio parameters, if set.
  *
  * Return: @wb's dirty limit in pages. The term "dirty" in the context of
- * dirty balancing includes all PG_dirty, PG_writeback and NFS unstable pages.
+ * dirty balancing includes all PG_dirty and PG_writeback pages.
  */
 static unsigned long __wb_calc_thresh(struct dirty_throttle_control *dtc)
 {
@@ -1566,7 +1565,7 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
 	struct dirty_throttle_control * const mdtc = mdtc_valid(&mdtc_stor) ?
 						     &mdtc_stor : NULL;
 	struct dirty_throttle_control *sdtc;
-	unsigned long nr_reclaimable;	/* = file_dirty + unstable_nfs */
+	unsigned long nr_reclaimable;	/* = file_dirty */
 	long period;
 	long pause;
 	long max_pause;
@@ -1589,14 +1588,7 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
 		unsigned long m_thresh = 0;
 		unsigned long m_bg_thresh = 0;
 
-		/*
-		 * Unstable writes are a feature of certain networked
-		 * filesystems (i.e. NFS) in which data may have been
-		 * written to the server's write cache, but has not yet
-		 * been flushed to permanent storage.
-		 */
-		nr_reclaimable = global_node_page_state(NR_FILE_DIRTY) +
-					global_node_page_state(NR_UNSTABLE_NFS);
+		nr_reclaimable = global_node_page_state(NR_FILE_DIRTY);
 		gdtc->avail = global_dirtyable_memory();
 		gdtc->dirty = nr_reclaimable + global_node_page_state(NR_WRITEBACK);
 
@@ -1940,8 +1932,7 @@ bool wb_over_bg_thresh(struct bdi_writeback *wb)
 	 * as we're trying to decide whether to put more under writeback.
 	 */
 	gdtc->avail = global_dirtyable_memory();
-	gdtc->dirty = global_node_page_state(NR_FILE_DIRTY) +
-		      global_node_page_state(NR_UNSTABLE_NFS);
+	gdtc->dirty = global_node_page_state(NR_FILE_DIRTY);
 	domain_dirty_limits(gdtc);
 
 	if (gdtc->dirty > gdtc->bg_thresh)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 3c4eb750a199..6bd1112d590d 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5239,7 +5239,7 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
 
 	printk("active_anon:%lu inactive_anon:%lu isolated_anon:%lu\n"
 		" active_file:%lu inactive_file:%lu isolated_file:%lu\n"
-		" unevictable:%lu dirty:%lu writeback:%lu unstable:%lu\n"
+		" unevictable:%lu dirty:%lu writeback:%lu\n"
 		" slab_reclaimable:%lu slab_unreclaimable:%lu\n"
 		" mapped:%lu shmem:%lu pagetables:%lu bounce:%lu\n"
 		" free:%lu free_pcp:%lu free_cma:%lu\n",
@@ -5252,7 +5252,6 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
 		global_node_page_state(NR_UNEVICTABLE),
 		global_node_page_state(NR_FILE_DIRTY),
 		global_node_page_state(NR_WRITEBACK),
-		global_node_page_state(NR_UNSTABLE_NFS),
 		global_node_page_state(NR_SLAB_RECLAIMABLE),
 		global_node_page_state(NR_SLAB_UNRECLAIMABLE),
 		global_node_page_state(NR_FILE_MAPPED),
@@ -5285,7 +5284,6 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
 			" anon_thp: %lukB"
 #endif
 			" writeback_tmp:%lukB"
-			" unstable:%lukB"
 			" all_unreclaimable? %s"
 			"\n",
 			pgdat->node_id,
@@ -5307,7 +5305,6 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
 			K(node_page_state(pgdat, NR_ANON_THPS) * HPAGE_PMD_NR),
 #endif
 			K(node_page_state(pgdat, NR_WRITEBACK_TEMP)),
-			K(node_page_state(pgdat, NR_UNSTABLE_NFS)),
 			pgdat->kswapd_failures >= MAX_RECLAIM_RETRIES ?
 				"yes" : "no");
 	}
diff --git a/mm/vmstat.c b/mm/vmstat.c
index 78d53378db99..d1291537bbb9 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1162,7 +1162,6 @@ const char * const vmstat_text[] = {
 	"nr_file_hugepages",
 	"nr_file_pmdmapped",
 	"nr_anon_transparent_hugepages",
-	"nr_unstable",
 	"nr_vmscan_write",
 	"nr_vmscan_immediate_reclaim",
 	"nr_dirtied",
-- 
2.26.0


[-- Attachment #2: signature.asc --]
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Re: [PATCH 2/2 - v2] MM: Discard NR_UNSTABLE_NFS, use NR_WRITEBACK instead.

[Jan Kara]

On Fri 03-04-20 09:35:21, NeilBrown wrote:
> 
> After an NFS page has been written it is considered "unstable" until a
> COMMIT request succeeds.  If the COMMIT fails, the page will be
> re-written.
> 
> These "unstable" pages are currently accounted as "reclaimable", either
> in WB_RECLAIMABLE, or in NR_UNSTABLE_NFS which is included in a
> 'reclaimable' count.  This might have made sense when sending the COMMIT
> required a separate action by the VFS/MM (e.g.  releasepage() used to
> send a COMMIT).  However now that all writes generated by ->writepages()
> will automatically be followed by a COMMIT (since commit 919e3bd9a875
> ("NFS: Ensure we commit after writeback is complete")) it makes more
> sense to treat them as writeback pages.
> 
> So this patch removes NR_UNSTABLE_NFS and accounts unstable pages in
> NR_WRITEBACK and WB_WRITEBACK.
> 
> A particular effect of this change is that when
> wb_check_background_flush() calls wb_over_bg_threshold(), the latter
> will report 'true' a lot less often as the 'unstable' pages are no
> longer considered 'dirty' (and there is nothing that writeback can do
> about them anyway).
> 
> Currently wb_check_background_flush() will trigger writeback to NFS even
> when there are relatively few dirty pages (if there are lots of unstable
> pages), this can result in small writes going to the server (10s of
> Kilobytes rather than a Megabyte) which hurts throughput.
> With this patch, there are fewer writes which are each larger on average.
> 
> Signed-off-by: NeilBrown <neilb@suse.de>
> ---
> 
> NR_UNSTABLE_NFS completely removed as recommended by Christoph, removal
> of an unnecessary comment, and improvements to commit message.
> Thanks.
> 
>  Documentation/filesystems/proc.txt |  3 ---
>  drivers/base/node.c                |  2 --
>  fs/fs-writeback.c                  |  1 -
>  fs/nfs/internal.h                  | 10 +++++++---
>  fs/nfs/write.c                     |  4 ++--
>  fs/proc/meminfo.c                  |  2 --
>  include/linux/mmzone.h             |  1 -
>  include/trace/events/writeback.h   |  5 +----
>  mm/memcontrol.c                    |  1 -
>  mm/page-writeback.c                | 17 ++++-------------
>  mm/page_alloc.c                    |  5 +----
>  mm/vmstat.c                        |  1 -
>  12 files changed, 15 insertions(+), 37 deletions(-)
> 
> diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
> index 99ca040e3f90..690c712f5f79 100644
> --- a/Documentation/filesystems/proc.txt
> +++ b/Documentation/filesystems/proc.txt
> @@ -904,7 +904,6 @@ Slab:           284364 kB
>  SReclaimable:   159856 kB
>  SUnreclaim:     124508 kB
>  PageTables:      24448 kB
> -NFS_Unstable:        0 kB
>  Bounce:              0 kB
>  WritebackTmp:        0 kB
>  CommitLimit:   7669796 kB
> @@ -975,8 +974,6 @@ SReclaimable: Part of Slab, that might be reclaimed, such as caches
>    SUnreclaim: Part of Slab, that cannot be reclaimed on memory pressure
>    PageTables: amount of memory dedicated to the lowest level of page
>                tables.
> -NFS_Unstable: NFS pages sent to the server, but not yet committed to stable
> -	      storage
>        Bounce: Memory used for block device "bounce buffers"
>  WritebackTmp: Memory used by FUSE for temporary writeback buffers
>   CommitLimit: Based on the overcommit ratio ('vm.overcommit_ratio'),
> diff --git a/drivers/base/node.c b/drivers/base/node.c
> index 98a31bafc8a2..7059021ce2af 100644
> --- a/drivers/base/node.c
> +++ b/drivers/base/node.c
> @@ -416,7 +416,6 @@ static ssize_t node_read_meminfo(struct device *dev,
>  		       "Node %d Shmem:          %8lu kB\n"
>  		       "Node %d KernelStack:    %8lu kB\n"
>  		       "Node %d PageTables:     %8lu kB\n"
> -		       "Node %d NFS_Unstable:   %8lu kB\n"
>  		       "Node %d Bounce:         %8lu kB\n"
>  		       "Node %d WritebackTmp:   %8lu kB\n"
>  		       "Node %d KReclaimable:   %8lu kB\n"
> @@ -439,7 +438,6 @@ static ssize_t node_read_meminfo(struct device *dev,
>  		       nid, K(i.sharedram),
>  		       nid, sum_zone_node_page_state(nid, NR_KERNEL_STACK_KB),
>  		       nid, K(sum_zone_node_page_state(nid, NR_PAGETABLE)),
> -		       nid, K(node_page_state(pgdat, NR_UNSTABLE_NFS)),
>  		       nid, K(sum_zone_node_page_state(nid, NR_BOUNCE)),
>  		       nid, K(node_page_state(pgdat, NR_WRITEBACK_TEMP)),
>  		       nid, K(sreclaimable +

So I don't think we can just remove lines from procfs files like this. That
has a high potential of breaking some userspace app that is not careful
enough when parsing the file. So I think that we need to leave there the
format string and just replace K(node_page_state(pgdat, NR_UNSTABLE_NFS))
with 0.

> diff --git a/fs/proc/meminfo.c b/fs/proc/meminfo.c
> index 8c1f1bb1a5ce..1378a132ff7e 100644
> --- a/fs/proc/meminfo.c
> +++ b/fs/proc/meminfo.c
> @@ -106,8 +106,6 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
>  	show_val_kb(m, "PageTables:     ",
>  		    global_zone_page_state(NR_PAGETABLE));
>  
> -	show_val_kb(m, "NFS_Unstable:   ",
> -		    global_node_page_state(NR_UNSTABLE_NFS));
>  	show_val_kb(m, "Bounce:         ",
>  		    global_zone_page_state(NR_BOUNCE));
>  	show_val_kb(m, "WritebackTmp:   ",

Similarly here.

> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index 78d53378db99..d1291537bbb9 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -1162,7 +1162,6 @@ const char * const vmstat_text[] = {
>  	"nr_file_hugepages",
>  	"nr_file_pmdmapped",
>  	"nr_anon_transparent_hugepages",
> -	"nr_unstable",
>  	"nr_vmscan_write",
>  	"nr_vmscan_immediate_reclaim",
>  	"nr_dirtied",

This is probably the most tricky to deal with given how /proc/vmstat is
formatted. OTOH for this file there's good chance we'd get away with just
deleting nr_unstable line because there are entries added to it in the
middle (e.g. in 60fbf0ab5da1 last September) and nobody complained yet.

What do mm people think? How were changes to vmstat counters handled in the
past?

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Re: [PATCH 2/2 - v2] MM: Discard NR_UNSTABLE_NFS, use NR_WRITEBACK instead.

[Michal Hocko]

On Fri 03-04-20 11:42:20, Jan Kara wrote:
[...]
> > diff --git a/mm/vmstat.c b/mm/vmstat.c
> > index 78d53378db99..d1291537bbb9 100644
> > --- a/mm/vmstat.c
> > +++ b/mm/vmstat.c
> > @@ -1162,7 +1162,6 @@ const char * const vmstat_text[] = {
> >  	"nr_file_hugepages",
> >  	"nr_file_pmdmapped",
> >  	"nr_anon_transparent_hugepages",
> > -	"nr_unstable",
> >  	"nr_vmscan_write",
> >  	"nr_vmscan_immediate_reclaim",
> >  	"nr_dirtied",
> 
> This is probably the most tricky to deal with given how /proc/vmstat is
> formatted. OTOH for this file there's good chance we'd get away with just
> deleting nr_unstable line because there are entries added to it in the
> middle (e.g. in 60fbf0ab5da1 last September) and nobody complained yet.
> 
> What do mm people think? How were changes to vmstat counters handled in the
> past?

Adding new counters in the middle seems to be generally OK. I would be
more worried about removing counters though. So if we can simply print a
phone value at the very end then this should be a reasonable workaround.
-- 
Michal Hocko
SUSE Labs

Re: [PATCH 2/2 - v2] MM: Discard NR_UNSTABLE_NFS, use NR_WRITEBACK instead.

[NeilBrown]

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On Fri, Apr 03 2020, Michal Hocko wrote:

> On Fri 03-04-20 11:42:20, Jan Kara wrote:
> [...]
>> > diff --git a/mm/vmstat.c b/mm/vmstat.c
>> > index 78d53378db99..d1291537bbb9 100644
>> > --- a/mm/vmstat.c
>> > +++ b/mm/vmstat.c
>> > @@ -1162,7 +1162,6 @@ const char * const vmstat_text[] = {
>> >  	"nr_file_hugepages",
>> >  	"nr_file_pmdmapped",
>> >  	"nr_anon_transparent_hugepages",
>> > -	"nr_unstable",
>> >  	"nr_vmscan_write",
>> >  	"nr_vmscan_immediate_reclaim",
>> >  	"nr_dirtied",
>> 
>> This is probably the most tricky to deal with given how /proc/vmstat is
>> formatted. OTOH for this file there's good chance we'd get away with just
>> deleting nr_unstable line because there are entries added to it in the
>> middle (e.g. in 60fbf0ab5da1 last September) and nobody complained yet.
>> 
>> What do mm people think? How were changes to vmstat counters handled in the
>> past?
>
> Adding new counters in the middle seems to be generally OK. I would be
> more worried about removing counters though. So if we can simply print a
> phone value at the very end then this should be a reasonable workaround.

At the very end?
Do you mean not have "nr_unstable 0" appear at all, but having "dummy 0"
appear at the end just so that the number of lines doesn't decrease?
Am I misunderstanding?

Thanks,
NeilBrown

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Re: [PATCH 2/2 - v2] MM: Discard NR_UNSTABLE_NFS, use NR_WRITEBACK instead.

[Michal Hocko]

On Mon 06-04-20 10:14:16, Neil Brown wrote:
> On Fri, Apr 03 2020, Michal Hocko wrote:
> 
> > On Fri 03-04-20 11:42:20, Jan Kara wrote:
> > [...]
> >> > diff --git a/mm/vmstat.c b/mm/vmstat.c
> >> > index 78d53378db99..d1291537bbb9 100644
> >> > --- a/mm/vmstat.c
> >> > +++ b/mm/vmstat.c
> >> > @@ -1162,7 +1162,6 @@ const char * const vmstat_text[] = {
> >> >  	"nr_file_hugepages",
> >> >  	"nr_file_pmdmapped",
> >> >  	"nr_anon_transparent_hugepages",
> >> > -	"nr_unstable",
> >> >  	"nr_vmscan_write",
> >> >  	"nr_vmscan_immediate_reclaim",
> >> >  	"nr_dirtied",
> >> 
> >> This is probably the most tricky to deal with given how /proc/vmstat is
> >> formatted. OTOH for this file there's good chance we'd get away with just
> >> deleting nr_unstable line because there are entries added to it in the
> >> middle (e.g. in 60fbf0ab5da1 last September) and nobody complained yet.
> >> 
> >> What do mm people think? How were changes to vmstat counters handled in the
> >> past?
> >
> > Adding new counters in the middle seems to be generally OK. I would be
> > more worried about removing counters though. So if we can simply print a
> > phone value at the very end then this should be a reasonable workaround.
> 
> At the very end?
> Do you mean not have "nr_unstable 0" appear at all, but having "dummy 0"
> appear at the end just so that the number of lines doesn't decrease?
> Am I misunderstanding?

Sorry for not being clear. I meant semething like
diff --git a/mm/vmstat.c b/mm/vmstat.c
index 78d53378db99..836e3f7a7aff 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1705,8 +1705,16 @@ static void *vmstat_start(struct seq_file *m, loff_t *pos)
 static void *vmstat_next(struct seq_file *m, void *arg, loff_t *pos)
 {
 	(*pos)++;
-	if (*pos >= NR_VMSTAT_ITEMS)
+	if (*pos >= NR_VMSTAT_ITEMS) {
+		/*
+		 * deprecated counters which are no longer represented
+		 * in vmstat arrays. We just lie about them to be always
+		 * 0 to not break userspace which might expect them in
+		 * int the output.
+		 */
+		seq_puts(m, "nr_unstable 0")
 		return NULL;
+	}
 	return (unsigned long *)m->private + *pos;
 }
 

-- 
Michal Hocko
SUSE Labs

Re: [PATCH 2/2 - v2] MM: Discard NR_UNSTABLE_NFS, use NR_WRITEBACK instead.

[NeilBrown]

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On Fri, Apr 03 2020, Jan Kara wrote:
>
> So I don't think we can just remove lines from procfs files like this. That
> has a high potential of breaking some userspace app that is not careful
> enough when parsing the file. So I think that we need to leave there the
> format string and just replace K(node_page_state(pgdat, NR_UNSTABLE_NFS))
> with 0.

OK.  I assume changing the static trace points isn't a problem though?

Thanks,
NeilBrown

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Re: [PATCH 2/2 - v2] MM: Discard NR_UNSTABLE_NFS, use NR_WRITEBACK instead.

[Michal Hocko]

On Tue 07-04-20 09:28:19, Neil Brown wrote:
> On Fri, Apr 03 2020, Jan Kara wrote:
> >
> > So I don't think we can just remove lines from procfs files like this. That
> > has a high potential of breaking some userspace app that is not careful
> > enough when parsing the file. So I think that we need to leave there the
> > format string and just replace K(node_page_state(pgdat, NR_UNSTABLE_NFS))
> > with 0.
> 
> OK.  I assume changing the static trace points isn't a problem though?

It shouldn't be until we learn that somebody depends on it...
-- 
Michal Hocko
SUSE Labs

Re: [PATCH 2/2] Deprecate NR_UNSTABLE_NFS, use NR_WRITEBACK

[Jan Kara]

On Thu 02-04-20 10:54:07, NeilBrown wrote:
> 
> After an NFS page has been written it is considered "unstable" until a
> COMMIT request succeeds.  If the COMMIT fails, the page will be
> re-written.
> 
> These "unstable" pages are currently accounted as "reclaimable",
> either in WB_RECLAIMABLE, or in NR_UNSTABLE_NFS which is included in a
> 'reclaimable' count.  This might have made sense when sending the COMMIT
> required a separate action by the VFS/MM (e.g. releasepage() used to
> send a COMMIT).  However now that all writes generated by ->writepages()
> will automatically be followed by a COMMIT, it makes more sense to
> treat them as writeback pages.
> 
> So this page deprecates NR_UNSTABLE_NFS and accounts unstable pages in
> NR_WRITEBACK and WB_WRITEBACK.
> 
> A particular effect of this change is that when
> wb_check_background_flush() calls wb_over_bg_threshold(), the latter
> will report 'true' a lot less often as the 'unstable' pages are no
> longer considered 'dirty' (and there is nothing that writeback can do
> about them anyway).
> 
> Currently wb_check_background_flush() will trigger writeback to NFS even
> when there are relatively few dirty pages (if there are lots of unstable
> pages), this can result in small writes going to the server (10s of
> Kilobytes rather than a Megabyte) which hurts throughput.
> With this that, there are fewer writes which are each larger on average.
> 
> Signed-off-by: NeilBrown <neilb@suse.de>

The patch looks good to me. I agree with Christoph that it would be best to
remove NR_UNSTABLE_NFS completely. We just have to be careful, not change
format of any entries in /proc/ where it currently gets reported...

								Honza

> ---
>  fs/fs-writeback.c      | 1 -
>  fs/nfs/internal.h      | 7 +++++--
>  fs/nfs/write.c         | 4 ++--
>  include/linux/mmzone.h | 2 +-
>  mm/memcontrol.c        | 1 -
>  mm/page-writeback.c    | 7 ++-----
>  6 files changed, 10 insertions(+), 12 deletions(-)
> 
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 76ac9c7d32ec..c5bdf46e3b4b 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -1070,7 +1070,6 @@ static void bdi_split_work_to_wbs(struct backing_dev_info *bdi,
>  static unsigned long get_nr_dirty_pages(void)
>  {
>  	return global_node_page_state(NR_FILE_DIRTY) +
> -		global_node_page_state(NR_UNSTABLE_NFS) +
>  		get_nr_dirty_inodes();
>  }
>  
> diff --git a/fs/nfs/internal.h b/fs/nfs/internal.h
> index f80c47d5ff27..ba1ff5adeccd 100644
> --- a/fs/nfs/internal.h
> +++ b/fs/nfs/internal.h
> @@ -660,8 +660,11 @@ void nfs_mark_page_unstable(struct page *page, struct nfs_commit_info *cinfo)
>  	if (!cinfo->dreq) {
>  		struct inode *inode = page_file_mapping(page)->host;
>  
> -		inc_node_page_state(page, NR_UNSTABLE_NFS);
> -		inc_wb_stat(&inode_to_bdi(inode)->wb, WB_RECLAIMABLE);
> +		/* This page is really still in write-back - just that the
> +		 * writeback is happening on the server now.
> +		 */
> +		inc_node_page_state(page, NR_WRITEBACK);
> +		inc_wb_stat(&inode_to_bdi(inode)->wb, WB_WRITEBACK);
>  		__mark_inode_dirty(inode, I_DIRTY_DATASYNC);
>  	}
>  }
> diff --git a/fs/nfs/write.c b/fs/nfs/write.c
> index c478b772cc49..2e15a56620b3 100644
> --- a/fs/nfs/write.c
> +++ b/fs/nfs/write.c
> @@ -958,9 +958,9 @@ nfs_mark_request_commit(struct nfs_page *req, struct pnfs_layout_segment *lseg,
>  static void
>  nfs_clear_page_commit(struct page *page)
>  {
> -	dec_node_page_state(page, NR_UNSTABLE_NFS);
> +	dec_node_page_state(page, NR_WRITEBACK);
>  	dec_wb_stat(&inode_to_bdi(page_file_mapping(page)->host)->wb,
> -		    WB_RECLAIMABLE);
> +		    WB_WRITEBACK);
>  }
>  
>  /* Called holding the request lock on @req */
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 462f6873905a..227fcb8cd0e6 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -237,7 +237,7 @@ enum node_stat_item {
>  	NR_FILE_THPS,
>  	NR_FILE_PMDMAPPED,
>  	NR_ANON_THPS,
> -	NR_UNSTABLE_NFS,	/* NFS unstable pages */
> +	NR_UNSTABLE_NFS,	/* NFS unstable pages - DEPRECATED DO NOT USE */
>  	NR_VMSCAN_WRITE,
>  	NR_VMSCAN_IMMEDIATE,	/* Prioritise for reclaim when writeback ends */
>  	NR_DIRTIED,		/* page dirtyings since bootup */
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 7ddf91c4295f..fad8e8a23235 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4317,7 +4317,6 @@ void mem_cgroup_wb_stats(struct bdi_writeback *wb, unsigned long *pfilepages,
>  
>  	*pdirty = memcg_exact_page_state(memcg, NR_FILE_DIRTY);
>  
> -	/* this should eventually include NR_UNSTABLE_NFS */
>  	*pwriteback = memcg_exact_page_state(memcg, NR_WRITEBACK);
>  	*pfilepages = memcg_exact_page_state(memcg, NR_INACTIVE_FILE) +
>  			memcg_exact_page_state(memcg, NR_ACTIVE_FILE);
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 2afb09fa2fe0..d1f03c799d11 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -504,7 +504,6 @@ bool node_dirty_ok(struct pglist_data *pgdat)
>  	unsigned long nr_pages = 0;
>  
>  	nr_pages += node_page_state(pgdat, NR_FILE_DIRTY);
> -	nr_pages += node_page_state(pgdat, NR_UNSTABLE_NFS);
>  	nr_pages += node_page_state(pgdat, NR_WRITEBACK);
>  
>  	return nr_pages <= limit;
> @@ -1595,8 +1594,7 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
>  		 * written to the server's write cache, but has not yet
>  		 * been flushed to permanent storage.
>  		 */
> -		nr_reclaimable = global_node_page_state(NR_FILE_DIRTY) +
> -					global_node_page_state(NR_UNSTABLE_NFS);
> +		nr_reclaimable = global_node_page_state(NR_FILE_DIRTY);
>  		gdtc->avail = global_dirtyable_memory();
>  		gdtc->dirty = nr_reclaimable + global_node_page_state(NR_WRITEBACK);
>  
> @@ -1940,8 +1938,7 @@ bool wb_over_bg_thresh(struct bdi_writeback *wb)
>  	 * as we're trying to decide whether to put more under writeback.
>  	 */
>  	gdtc->avail = global_dirtyable_memory();
> -	gdtc->dirty = global_node_page_state(NR_FILE_DIRTY) +
> -		      global_node_page_state(NR_UNSTABLE_NFS);
> +	gdtc->dirty = global_node_page_state(NR_FILE_DIRTY);
>  	domain_dirty_limits(gdtc);
>  
>  	if (gdtc->dirty > gdtc->bg_thresh)
> -- 
> 2.26.0
> 


-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Re: [PATCH 1/2] MM: replace PF_LESS_THROTTLE with PF_LOCAL_THROTTLE

[Jan Kara]

On Thu 02-04-20 10:53:20, NeilBrown wrote:
> 
> PF_LESS_THROTTLE exists for loop-back nfsd, and a similar need in the
> loop block driver, where a daemon needs to write to one bdi in
> order to free up writes queued to another bdi.
> 
> The daemon sets PF_LESS_THROTTLE and gets a larger allowance of dirty
> pages, so that it can still dirty pages after other processses have been
> throttled.
> 
> This approach was designed when all threads were blocked equally,
> independently on which device they were writing to, or how fast it was.
> Since that time the writeback algorithm has changed substantially with
> different threads getting different allowances based on non-trivial
> heuristics.  This means the simple "add 25%" heuristic is no longer
> reliable.
> 
> This patch changes the heuristic to ignore the global limits and
> consider only the limit relevant to the bdi being written to.  This
> approach is already available for BDI_CAP_STRICTLIMIT users (fuse) and
> should not introduce surprises.  This has the desired result of
> protecting the task from the consequences of large amounts of dirty data
> queued for other devices.
> 
> This approach of "only consider the target bdi" is consistent with the
> other use of PF_LESS_THROTTLE in current_may_throttle(), were it causes
> attention to be focussed only on the target bdi.
> 
> So this patch
>  - renames PF_LESS_THROTTLE to PF_LOCAL_THROTTLE,
>  - remove the 25% bonus that that flag gives, and
>  - imposes 'strictlimit' handling for any process with PF_LOCAL_THROTTLE
>    set.
> 
> Note that previously realtime threads were treated the same as
> PF_LESS_THROTTLE threads.  This patch does *not* change the behvaiour for
> real-time threads, so it is now different from the behaviour of nfsd and
> loop tasks.  I don't know what is wanted for realtime.
> 
> Signed-off-by: NeilBrown <neilb@suse.de>

This makes sense to me and the patch looks good. You can add:

Reviewed-by: Jan Kara <jack@suse.cz>

Thanks.

								Honza

> ---
>  drivers/block/loop.c  |  2 +-
>  fs/nfsd/vfs.c         |  9 +++++----
>  include/linux/sched.h |  2 +-
>  kernel/sys.c          |  2 +-
>  mm/page-writeback.c   | 10 ++++++----
>  mm/vmscan.c           |  4 ++--
>  6 files changed, 16 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index 739b372a5112..2c59371ce936 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -897,7 +897,7 @@ static void loop_unprepare_queue(struct loop_device *lo)
>  
>  static int loop_kthread_worker_fn(void *worker_ptr)
>  {
> -	current->flags |= PF_LESS_THROTTLE | PF_MEMALLOC_NOIO;
> +	current->flags |= PF_LOCAL_THROTTLE | PF_MEMALLOC_NOIO;
>  	return kthread_worker_fn(worker_ptr);
>  }
>  
> diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
> index 0aa02eb18bd3..c3fbab1753ec 100644
> --- a/fs/nfsd/vfs.c
> +++ b/fs/nfsd/vfs.c
> @@ -979,12 +979,13 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp, struct nfsd_file *nf,
>  
>  	if (test_bit(RQ_LOCAL, &rqstp->rq_flags))
>  		/*
> -		 * We want less throttling in balance_dirty_pages()
> -		 * and shrink_inactive_list() so that nfs to
> +		 * We want throttling in balance_dirty_pages()
> +		 * and shrink_inactive_list() to only consider
> +		 * the backingdev we are writing to, so that nfs to
>  		 * localhost doesn't cause nfsd to lock up due to all
>  		 * the client's dirty pages or its congested queue.
>  		 */
> -		current->flags |= PF_LESS_THROTTLE;
> +		current->flags |= PF_LOCAL_THROTTLE;
>  
>  	exp = fhp->fh_export;
>  	use_wgather = (rqstp->rq_vers == 2) && EX_WGATHER(exp);
> @@ -1037,7 +1038,7 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp, struct nfsd_file *nf,
>  		nfserr = nfserrno(host_err);
>  	}
>  	if (test_bit(RQ_LOCAL, &rqstp->rq_flags))
> -		current_restore_flags(pflags, PF_LESS_THROTTLE);
> +		current_restore_flags(pflags, PF_LOCAL_THROTTLE);
>  	return nfserr;
>  }
>  
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 04278493bf15..5dcd27abc8cd 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1473,7 +1473,7 @@ extern struct pid *cad_pid;
>  #define PF_KSWAPD		0x00020000	/* I am kswapd */
>  #define PF_MEMALLOC_NOFS	0x00040000	/* All allocation requests will inherit GFP_NOFS */
>  #define PF_MEMALLOC_NOIO	0x00080000	/* All allocation requests will inherit GFP_NOIO */
> -#define PF_LESS_THROTTLE	0x00100000	/* Throttle me less: I clean memory */
> +#define PF_LOCAL_THROTTLE	0x00100000	/* Throttle me less: I clean memory */
>  #define PF_KTHREAD		0x00200000	/* I am a kernel thread */
>  #define PF_RANDOMIZE		0x00400000	/* Randomize virtual address space */
>  #define PF_SWAPWRITE		0x00800000	/* Allowed to write to swap */
> diff --git a/kernel/sys.c b/kernel/sys.c
> index d325f3ab624a..180a2fa33f7f 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -2262,7 +2262,7 @@ int __weak arch_prctl_spec_ctrl_set(struct task_struct *t, unsigned long which,
>  	return -EINVAL;
>  }
>  
> -#define PR_IO_FLUSHER (PF_MEMALLOC_NOIO | PF_LESS_THROTTLE)
> +#define PR_IO_FLUSHER (PF_MEMALLOC_NOIO | PF_LOCAL_THROTTLE)
>  
>  SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
>  		unsigned long, arg4, unsigned long, arg5)
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 2caf780a42e7..2afb09fa2fe0 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -387,8 +387,7 @@ static unsigned long global_dirtyable_memory(void)
>   * Calculate @dtc->thresh and ->bg_thresh considering
>   * vm_dirty_{bytes|ratio} and dirty_background_{bytes|ratio}.  The caller
>   * must ensure that @dtc->avail is set before calling this function.  The
> - * dirty limits will be lifted by 1/4 for PF_LESS_THROTTLE (ie. nfsd) and
> - * real-time tasks.
> + * dirty limits will be lifted by 1/4 for real-time tasks.
>   */
>  static void domain_dirty_limits(struct dirty_throttle_control *dtc)
>  {
> @@ -436,7 +435,7 @@ static void domain_dirty_limits(struct dirty_throttle_control *dtc)
>  	if (bg_thresh >= thresh)
>  		bg_thresh = thresh / 2;
>  	tsk = current;
> -	if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) {
> +	if (rt_task(tsk)) {
>  		bg_thresh += bg_thresh / 4 + global_wb_domain.dirty_limit / 32;
>  		thresh += thresh / 4 + global_wb_domain.dirty_limit / 32;
>  	}
> @@ -486,7 +485,7 @@ static unsigned long node_dirty_limit(struct pglist_data *pgdat)
>  	else
>  		dirty = vm_dirty_ratio * node_memory / 100;
>  
> -	if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk))
> +	if (rt_task(tsk))
>  		dirty += dirty / 4;
>  
>  	return dirty;
> @@ -1580,6 +1579,9 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
>  	bool strictlimit = bdi->capabilities & BDI_CAP_STRICTLIMIT;
>  	unsigned long start_time = jiffies;
>  
> +	if (current->flags & PF_LOCAL_THROTTLE)
> +		/* This task must only be throttled by its own writeback */
> +		strictlimit = true;
>  	for (;;) {
>  		unsigned long now = jiffies;
>  		unsigned long dirty, thresh, bg_thresh;
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 876370565455..c5cf25938c56 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1880,13 +1880,13 @@ static unsigned noinline_for_stack move_pages_to_lru(struct lruvec *lruvec,
>  
>  /*
>   * If a kernel thread (such as nfsd for loop-back mounts) services
> - * a backing device by writing to the page cache it sets PF_LESS_THROTTLE.
> + * a backing device by writing to the page cache it sets PF_LOCAL_THROTTLE.
>   * In that case we should only throttle if the backing device it is
>   * writing to is congested.  In other cases it is safe to throttle.
>   */
>  static int current_may_throttle(void)
>  {
> -	return !(current->flags & PF_LESS_THROTTLE) ||
> +	return !(current->flags & PF_LOCAL_THROTTLE) ||
>  		current->backing_dev_info == NULL ||
>  		bdi_write_congested(current->backing_dev_info);
>  }
> -- 
> 2.26.0
> 


-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Re: [PATCH 1/2] MM: replace PF_LESS_THROTTLE with PF_LOCAL_THROTTLE

[Michal Hocko]

On Thu 02-04-20 10:53:20, Neil Brown wrote:
> 
> PF_LESS_THROTTLE exists for loop-back nfsd, and a similar need in the
> loop block driver, where a daemon needs to write to one bdi in
> order to free up writes queued to another bdi.
> 
> The daemon sets PF_LESS_THROTTLE and gets a larger allowance of dirty
> pages, so that it can still dirty pages after other processses have been
> throttled.
> 
> This approach was designed when all threads were blocked equally,
> independently on which device they were writing to, or how fast it was.
> Since that time the writeback algorithm has changed substantially with
> different threads getting different allowances based on non-trivial
> heuristics.  This means the simple "add 25%" heuristic is no longer
> reliable.
> 
> This patch changes the heuristic to ignore the global limits and
> consider only the limit relevant to the bdi being written to.  This
> approach is already available for BDI_CAP_STRICTLIMIT users (fuse) and
> should not introduce surprises.  This has the desired result of
> protecting the task from the consequences of large amounts of dirty data
> queued for other devices.

While I understand that you want to have per bdi throttling for those
"special" files I am still missing how this is going to provide the
additional room that the additnal 25% gave them previously. I might
misremember or things have changed (what you mention as non-trivial
heuristics) but PF_LESS_THROTTLE really needed that room to guarantee a
forward progress. Care to expan some more on how this is handled now?
Maybe we do not need it anymore but calling that out explicitly would be
really helpful.
-- 
Michal Hocko
SUSE Labs

Re: [PATCH 1/2] MM: replace PF_LESS_THROTTLE with PF_LOCAL_THROTTLE

[NeilBrown]

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On Fri, Apr 03 2020, Michal Hocko wrote:

> On Thu 02-04-20 10:53:20, Neil Brown wrote:
>> 
>> PF_LESS_THROTTLE exists for loop-back nfsd, and a similar need in the
>> loop block driver, where a daemon needs to write to one bdi in
>> order to free up writes queued to another bdi.
>> 
>> The daemon sets PF_LESS_THROTTLE and gets a larger allowance of dirty
>> pages, so that it can still dirty pages after other processses have been
>> throttled.
>> 
>> This approach was designed when all threads were blocked equally,
>> independently on which device they were writing to, or how fast it was.
>> Since that time the writeback algorithm has changed substantially with
>> different threads getting different allowances based on non-trivial
>> heuristics.  This means the simple "add 25%" heuristic is no longer
>> reliable.
>> 
>> This patch changes the heuristic to ignore the global limits and
>> consider only the limit relevant to the bdi being written to.  This
>> approach is already available for BDI_CAP_STRICTLIMIT users (fuse) and
>> should not introduce surprises.  This has the desired result of
>> protecting the task from the consequences of large amounts of dirty data
>> queued for other devices.
>
> While I understand that you want to have per bdi throttling for those
> "special" files I am still missing how this is going to provide the
> additional room that the additnal 25% gave them previously. I might
> misremember or things have changed (what you mention as non-trivial
> heuristics) but PF_LESS_THROTTLE really needed that room to guarantee a
> forward progress. Care to expan some more on how this is handled now?
> Maybe we do not need it anymore but calling that out explicitly would be
> really helpful.

The 25% was a means to an end, not an end in itself.

The problem is that the NFS server needs to be able to write to the
backing filesystem when the dirty memory limits have been reached by
being totally consumed by dirty pages on the NFS filesystem.

The 25% was just a way of giving an allowance of dirty pages to nfsd
that could not be consumed by processes writing to an NFS filesystem.
i.e. it doesn't need 25% MORE, it needs 25% PRIVATELY.  Actually it only
really needs 1 page privately, but a few pages give better throughput
and 25% seemed like a good idea at the time.

per-bdi throttling focuses on the "PRIVATELY" (the important bit) and
de-emphasises the 25% (the irrelevant detail).

Thanks,
NeilBrown

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Re: [PATCH 1/2] MM: replace PF_LESS_THROTTLE with PF_LOCAL_THROTTLE

[Michal Hocko]

On Sat 04-04-20 08:40:17, Neil Brown wrote:
> On Fri, Apr 03 2020, Michal Hocko wrote:
> 
> > On Thu 02-04-20 10:53:20, Neil Brown wrote:
> >> 
> >> PF_LESS_THROTTLE exists for loop-back nfsd, and a similar need in the
> >> loop block driver, where a daemon needs to write to one bdi in
> >> order to free up writes queued to another bdi.
> >> 
> >> The daemon sets PF_LESS_THROTTLE and gets a larger allowance of dirty
> >> pages, so that it can still dirty pages after other processses have been
> >> throttled.
> >> 
> >> This approach was designed when all threads were blocked equally,
> >> independently on which device they were writing to, or how fast it was.
> >> Since that time the writeback algorithm has changed substantially with
> >> different threads getting different allowances based on non-trivial
> >> heuristics.  This means the simple "add 25%" heuristic is no longer
> >> reliable.
> >> 
> >> This patch changes the heuristic to ignore the global limits and
> >> consider only the limit relevant to the bdi being written to.  This
> >> approach is already available for BDI_CAP_STRICTLIMIT users (fuse) and
> >> should not introduce surprises.  This has the desired result of
> >> protecting the task from the consequences of large amounts of dirty data
> >> queued for other devices.
> >
> > While I understand that you want to have per bdi throttling for those
> > "special" files I am still missing how this is going to provide the
> > additional room that the additnal 25% gave them previously. I might
> > misremember or things have changed (what you mention as non-trivial
> > heuristics) but PF_LESS_THROTTLE really needed that room to guarantee a
> > forward progress. Care to expan some more on how this is handled now?
> > Maybe we do not need it anymore but calling that out explicitly would be
> > really helpful.
> 
> The 25% was a means to an end, not an end in itself.
> 
> The problem is that the NFS server needs to be able to write to the
> backing filesystem when the dirty memory limits have been reached by
> being totally consumed by dirty pages on the NFS filesystem.
> 
> The 25% was just a way of giving an allowance of dirty pages to nfsd
> that could not be consumed by processes writing to an NFS filesystem.
> i.e. it doesn't need 25% MORE, it needs 25% PRIVATELY.  Actually it only
> really needs 1 page privately, but a few pages give better throughput
> and 25% seemed like a good idea at the time.

Yes this part is clear to me.
 
> per-bdi throttling focuses on the "PRIVATELY" (the important bit) and
> de-emphasises the 25% (the irrelevant detail).

It is still not clear to me how this patch is going to behave when the
global dirty throttling is essentially equal to the per-bdi - e.g. there
is only a single bdi and now the PF_LOCAL_THROTTLE process doesn't have
anything private.

-- 
Michal Hocko
SUSE Labs

Re: [PATCH 1/2] MM: replace PF_LESS_THROTTLE with PF_LOCAL_THROTTLE

[Jan Kara]

On Mon 06-04-20 09:44:53, Michal Hocko wrote:
> On Sat 04-04-20 08:40:17, Neil Brown wrote:
> > On Fri, Apr 03 2020, Michal Hocko wrote:
> > 
> > > On Thu 02-04-20 10:53:20, Neil Brown wrote:
> > >> 
> > >> PF_LESS_THROTTLE exists for loop-back nfsd, and a similar need in the
> > >> loop block driver, where a daemon needs to write to one bdi in
> > >> order to free up writes queued to another bdi.
> > >> 
> > >> The daemon sets PF_LESS_THROTTLE and gets a larger allowance of dirty
> > >> pages, so that it can still dirty pages after other processses have been
> > >> throttled.
> > >> 
> > >> This approach was designed when all threads were blocked equally,
> > >> independently on which device they were writing to, or how fast it was.
> > >> Since that time the writeback algorithm has changed substantially with
> > >> different threads getting different allowances based on non-trivial
> > >> heuristics.  This means the simple "add 25%" heuristic is no longer
> > >> reliable.
> > >> 
> > >> This patch changes the heuristic to ignore the global limits and
> > >> consider only the limit relevant to the bdi being written to.  This
> > >> approach is already available for BDI_CAP_STRICTLIMIT users (fuse) and
> > >> should not introduce surprises.  This has the desired result of
> > >> protecting the task from the consequences of large amounts of dirty data
> > >> queued for other devices.
> > >
> > > While I understand that you want to have per bdi throttling for those
> > > "special" files I am still missing how this is going to provide the
> > > additional room that the additnal 25% gave them previously. I might
> > > misremember or things have changed (what you mention as non-trivial
> > > heuristics) but PF_LESS_THROTTLE really needed that room to guarantee a
> > > forward progress. Care to expan some more on how this is handled now?
> > > Maybe we do not need it anymore but calling that out explicitly would be
> > > really helpful.
> > 
> > The 25% was a means to an end, not an end in itself.
> > 
> > The problem is that the NFS server needs to be able to write to the
> > backing filesystem when the dirty memory limits have been reached by
> > being totally consumed by dirty pages on the NFS filesystem.
> > 
> > The 25% was just a way of giving an allowance of dirty pages to nfsd
> > that could not be consumed by processes writing to an NFS filesystem.
> > i.e. it doesn't need 25% MORE, it needs 25% PRIVATELY.  Actually it only
> > really needs 1 page privately, but a few pages give better throughput
> > and 25% seemed like a good idea at the time.
> 
> Yes this part is clear to me.
>  
> > per-bdi throttling focuses on the "PRIVATELY" (the important bit) and
> > de-emphasises the 25% (the irrelevant detail).
> 
> It is still not clear to me how this patch is going to behave when the
> global dirty throttling is essentially equal to the per-bdi - e.g. there
> is only a single bdi and now the PF_LOCAL_THROTTLE process doesn't have
> anything private.

Let me think out loud so see whether I understand this properly. There are
two BDIs involved in NFS loop mount - the NFS virtual BDI (let's call it
simply NFS-bdi) and the bdi of the real filesystem that is backing NFS
(let's call this real-bdi). The case we are concerned about is when NFS-bdi
is full of dirty pages so that global dirty limit of the machine is
exceeded. Then flusher thread will take dirty pages from NFS-bdi and send
them over localhost to nfsd. Nfsd, which has PF_LOCAL_THROTTLE set, will take
these pages and write them to real-bdi. Now because PF_LOCAL_THROTTLE is
set for nfsd, the fact that we are over global limit does not take effect
and nfsd is still able to write to real-bdi until dirty limit on real-bdi
is reached. So things should work as Neil writes AFAIU.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Re: [PATCH 1/2] MM: replace PF_LESS_THROTTLE with PF_LOCAL_THROTTLE

[Michal Hocko]

On Mon 06-04-20 11:36:01, Jan Kara wrote:
> On Mon 06-04-20 09:44:53, Michal Hocko wrote:
> > On Sat 04-04-20 08:40:17, Neil Brown wrote:
> > > On Fri, Apr 03 2020, Michal Hocko wrote:
> > > 
> > > > On Thu 02-04-20 10:53:20, Neil Brown wrote:
> > > >> 
> > > >> PF_LESS_THROTTLE exists for loop-back nfsd, and a similar need in the
> > > >> loop block driver, where a daemon needs to write to one bdi in
> > > >> order to free up writes queued to another bdi.
> > > >> 
> > > >> The daemon sets PF_LESS_THROTTLE and gets a larger allowance of dirty
> > > >> pages, so that it can still dirty pages after other processses have been
> > > >> throttled.
> > > >> 
> > > >> This approach was designed when all threads were blocked equally,
> > > >> independently on which device they were writing to, or how fast it was.
> > > >> Since that time the writeback algorithm has changed substantially with
> > > >> different threads getting different allowances based on non-trivial
> > > >> heuristics.  This means the simple "add 25%" heuristic is no longer
> > > >> reliable.
> > > >> 
> > > >> This patch changes the heuristic to ignore the global limits and
> > > >> consider only the limit relevant to the bdi being written to.  This
> > > >> approach is already available for BDI_CAP_STRICTLIMIT users (fuse) and
> > > >> should not introduce surprises.  This has the desired result of
> > > >> protecting the task from the consequences of large amounts of dirty data
> > > >> queued for other devices.
> > > >
> > > > While I understand that you want to have per bdi throttling for those
> > > > "special" files I am still missing how this is going to provide the
> > > > additional room that the additnal 25% gave them previously. I might
> > > > misremember or things have changed (what you mention as non-trivial
> > > > heuristics) but PF_LESS_THROTTLE really needed that room to guarantee a
> > > > forward progress. Care to expan some more on how this is handled now?
> > > > Maybe we do not need it anymore but calling that out explicitly would be
> > > > really helpful.
> > > 
> > > The 25% was a means to an end, not an end in itself.
> > > 
> > > The problem is that the NFS server needs to be able to write to the
> > > backing filesystem when the dirty memory limits have been reached by
> > > being totally consumed by dirty pages on the NFS filesystem.
> > > 
> > > The 25% was just a way of giving an allowance of dirty pages to nfsd
> > > that could not be consumed by processes writing to an NFS filesystem.
> > > i.e. it doesn't need 25% MORE, it needs 25% PRIVATELY.  Actually it only
> > > really needs 1 page privately, but a few pages give better throughput
> > > and 25% seemed like a good idea at the time.
> > 
> > Yes this part is clear to me.
> >  
> > > per-bdi throttling focuses on the "PRIVATELY" (the important bit) and
> > > de-emphasises the 25% (the irrelevant detail).
> > 
> > It is still not clear to me how this patch is going to behave when the
> > global dirty throttling is essentially equal to the per-bdi - e.g. there
> > is only a single bdi and now the PF_LOCAL_THROTTLE process doesn't have
> > anything private.
> 
> Let me think out loud so see whether I understand this properly. There are
> two BDIs involved in NFS loop mount - the NFS virtual BDI (let's call it
> simply NFS-bdi) and the bdi of the real filesystem that is backing NFS
> (let's call this real-bdi). The case we are concerned about is when NFS-bdi
> is full of dirty pages so that global dirty limit of the machine is
> exceeded. Then flusher thread will take dirty pages from NFS-bdi and send
> them over localhost to nfsd. Nfsd, which has PF_LOCAL_THROTTLE set, will take
> these pages and write them to real-bdi. Now because PF_LOCAL_THROTTLE is
> set for nfsd, the fact that we are over global limit does not take effect
> and nfsd is still able to write to real-bdi until dirty limit on real-bdi
> is reached. So things should work as Neil writes AFAIU.

Thanks for the clarification. I was not aware of the 2 bdi situation.
This makes more sense now. Maybe this is a trivial fact for everybody
who is more familiar with nfs internals but it would be so much esier to
follow if it was explicit in the changelog.
-- 
Michal Hocko
SUSE Labs

Re: [PATCH 1/2] MM: replace PF_LESS_THROTTLE with PF_LOCAL_THROTTLE

[NeilBrown]

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On Mon, Apr 06 2020, Jan Kara wrote:

> On Mon 06-04-20 09:44:53, Michal Hocko wrote:
>> On Sat 04-04-20 08:40:17, Neil Brown wrote:
>> > On Fri, Apr 03 2020, Michal Hocko wrote:
>> > 
>> > > On Thu 02-04-20 10:53:20, Neil Brown wrote:
>> > >> 
>> > >> PF_LESS_THROTTLE exists for loop-back nfsd, and a similar need in the
>> > >> loop block driver, where a daemon needs to write to one bdi in
>> > >> order to free up writes queued to another bdi.
>> > >> 
>> > >> The daemon sets PF_LESS_THROTTLE and gets a larger allowance of dirty
>> > >> pages, so that it can still dirty pages after other processses have been
>> > >> throttled.
>> > >> 
>> > >> This approach was designed when all threads were blocked equally,
>> > >> independently on which device they were writing to, or how fast it was.
>> > >> Since that time the writeback algorithm has changed substantially with
>> > >> different threads getting different allowances based on non-trivial
>> > >> heuristics.  This means the simple "add 25%" heuristic is no longer
>> > >> reliable.
>> > >> 
>> > >> This patch changes the heuristic to ignore the global limits and
>> > >> consider only the limit relevant to the bdi being written to.  This
>> > >> approach is already available for BDI_CAP_STRICTLIMIT users (fuse) and
>> > >> should not introduce surprises.  This has the desired result of
>> > >> protecting the task from the consequences of large amounts of dirty data
>> > >> queued for other devices.
>> > >
>> > > While I understand that you want to have per bdi throttling for those
>> > > "special" files I am still missing how this is going to provide the
>> > > additional room that the additnal 25% gave them previously. I might
>> > > misremember or things have changed (what you mention as non-trivial
>> > > heuristics) but PF_LESS_THROTTLE really needed that room to guarantee a
>> > > forward progress. Care to expan some more on how this is handled now?
>> > > Maybe we do not need it anymore but calling that out explicitly would be
>> > > really helpful.
>> > 
>> > The 25% was a means to an end, not an end in itself.
>> > 
>> > The problem is that the NFS server needs to be able to write to the
>> > backing filesystem when the dirty memory limits have been reached by
>> > being totally consumed by dirty pages on the NFS filesystem.
>> > 
>> > The 25% was just a way of giving an allowance of dirty pages to nfsd
>> > that could not be consumed by processes writing to an NFS filesystem.
>> > i.e. it doesn't need 25% MORE, it needs 25% PRIVATELY.  Actually it only
>> > really needs 1 page privately, but a few pages give better throughput
>> > and 25% seemed like a good idea at the time.
>> 
>> Yes this part is clear to me.
>>  
>> > per-bdi throttling focuses on the "PRIVATELY" (the important bit) and
>> > de-emphasises the 25% (the irrelevant detail).
>> 
>> It is still not clear to me how this patch is going to behave when the
>> global dirty throttling is essentially equal to the per-bdi - e.g. there
>> is only a single bdi and now the PF_LOCAL_THROTTLE process doesn't have
>> anything private.
>
> Let me think out loud so see whether I understand this properly. There are
> two BDIs involved in NFS loop mount - the NFS virtual BDI (let's call it
> simply NFS-bdi) and the bdi of the real filesystem that is backing NFS
> (let's call this real-bdi). The case we are concerned about is when NFS-bdi
> is full of dirty pages so that global dirty limit of the machine is
> exceeded. Then flusher thread will take dirty pages from NFS-bdi and send
> them over localhost to nfsd. Nfsd, which has PF_LOCAL_THROTTLE set, will take
> these pages and write them to real-bdi. Now because PF_LOCAL_THROTTLE is
> set for nfsd, the fact that we are over global limit does not take effect
> and nfsd is still able to write to real-bdi until dirty limit on real-bdi
> is reached. So things should work as Neil writes AFAIU.

Exactly.  The 'loop' block device follows a similar pattern - there is
the 'loop' bdi that might consume all the allowed dirty pages, and the
backing bdi that we need to write to so those dirty pages can be
cleaned.

The intention for PR_SET_IO_FLUSHER as described in 'man 2 prctl'
is much the same.  The thread that sets this is expected to be working
on behalf of a "block layer or filesystem" such as "FUSE daemons,  SCSI
device  emulation daemons" - each of these would be serving a bdi
"above" by writing to a bdi "below".

I'll add some more text to the changelog to make this clearer.

Thanks,
NeilBrown

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Re: [PATCH 1/2] MM: replace PF_LESS_THROTTLE with PF_LOCAL_THROTTLE

[Hillf Danton]

On Thu, 02 Apr 2020 10:53:20 +1100 NeilBrown wrote:
> 
> PF_LESS_THROTTLE exists for loop-back nfsd, and a similar need in the
> loop block driver, where a daemon needs to write to one bdi in
> order to free up writes queued to another bdi.
> 
> The daemon sets PF_LESS_THROTTLE and gets a larger allowance of dirty
> pages, so that it can still dirty pages after other processses have been
> throttled.
> 
> This approach was designed when all threads were blocked equally,
> independently on which device they were writing to, or how fast it was.
> Since that time the writeback algorithm has changed substantially with
> different threads getting different allowances based on non-trivial
> heuristics.  This means the simple "add 25%" heuristic is no longer
> reliable.
> 
> This patch changes the heuristic to ignore the global limits and
> consider only the limit relevant to the bdi being written to.  This
> approach is already available for BDI_CAP_STRICTLIMIT users (fuse) and
> should not introduce surprises.  This has the desired result of
> protecting the task from the consequences of large amounts of dirty data
> queued for other devices.
> 
> This approach of "only consider the target bdi" is consistent with the
> other use of PF_LESS_THROTTLE in current_may_throttle(), were it causes
> attention to be focussed only on the target bdi.
> 
> So this patch
>  - renames PF_LESS_THROTTLE to PF_LOCAL_THROTTLE,
>  - remove the 25% bonus that that flag gives, and
>  - imposes 'strictlimit' handling for any process with PF_LOCAL_THROTTLE
>    set.

	/*
	 * The strictlimit feature is a tool preventing mistrusted filesystems
	 * from growing a large number of dirty pages before throttling. For

Based on the comment snippet, I suspect it is applicable to IO flushers
unless they are likely generating tons of dirty pages. If they are,
however, cutting their bonuses seem questionable.

> 
> Note that previously realtime threads were treated the same as
> PF_LESS_THROTTLE threads.  This patch does *not* change the behvaiour for
> real-time threads, so it is now different from the behaviour of nfsd and
> loop tasks.  I don't know what is wanted for realtime.
> 
> Signed-off-by: NeilBrown <neilb@suse.de>
> =2D--

Hrm corrupted delivery?

>  drivers/block/loop.c  |  2 +-
>  fs/nfsd/vfs.c         |  9 +++++----
>  include/linux/sched.h |  2 +-
>  kernel/sys.c          |  2 +-
>  mm/page-writeback.c   | 10 ++++++----
>  mm/vmscan.c           |  4 ++--
>  6 files changed, 16 insertions(+), 13 deletions(-)

Re: [PATCH 1/2] MM: replace PF_LESS_THROTTLE with PF_LOCAL_THROTTLE

[NeilBrown]

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On Thu, Apr 02 2020, Hillf Danton wrote:

> On Thu, 02 Apr 2020 10:53:20 +1100 NeilBrown wrote:
>> 
>> PF_LESS_THROTTLE exists for loop-back nfsd, and a similar need in the
>> loop block driver, where a daemon needs to write to one bdi in
>> order to free up writes queued to another bdi.
>> 
>> The daemon sets PF_LESS_THROTTLE and gets a larger allowance of dirty
>> pages, so that it can still dirty pages after other processses have been
>> throttled.
>> 
>> This approach was designed when all threads were blocked equally,
>> independently on which device they were writing to, or how fast it was.
>> Since that time the writeback algorithm has changed substantially with
>> different threads getting different allowances based on non-trivial
>> heuristics.  This means the simple "add 25%" heuristic is no longer
>> reliable.
>> 
>> This patch changes the heuristic to ignore the global limits and
>> consider only the limit relevant to the bdi being written to.  This
>> approach is already available for BDI_CAP_STRICTLIMIT users (fuse) and
>> should not introduce surprises.  This has the desired result of
>> protecting the task from the consequences of large amounts of dirty data
>> queued for other devices.
>> 
>> This approach of "only consider the target bdi" is consistent with the
>> other use of PF_LESS_THROTTLE in current_may_throttle(), were it causes
>> attention to be focussed only on the target bdi.
>> 
>> So this patch
>>  - renames PF_LESS_THROTTLE to PF_LOCAL_THROTTLE,
>>  - remove the 25% bonus that that flag gives, and
>>  - imposes 'strictlimit' handling for any process with PF_LOCAL_THROTTLE
>>    set.
>
> 	/*
> 	 * The strictlimit feature is a tool preventing mistrusted filesystems
> 	 * from growing a large number of dirty pages before throttling. For
>
> Based on the comment snippet, I suspect it is applicable to IO flushers
> unless they are likely generating tons of dirty pages. If they are,
> however, cutting their bonuses seem questionable.

The purpose of the strictlimit feature was to isolate one filesystem
(bdi) from all others, so that the one cannot create dirty pages which
unfairly disadvantage the others - this is what that comment says.
But the implementation appears to focus on the isolation, not the
specific purpose, and isolation works both ways.  It protects the others
from the one, and the one from the others.

fuse needs to be isolated so it doesn't harm others.
nfsd and loop need to be isolate so they aren't harmed by others.
I'm less familiar with IO flushers but I suspect that have exactly the
same need as nfsd and loop - they need to be isolated from dirty pages
other than on the device they are writing to.
The 25% bonus was never about giving them a bonus because they need it.
It was about protecting them from excess usage elsewhere.  I strongly
suspect that my change will provide a conceptually better service for IO
flushers. (whether it is better in a practical measurable sense I cannot
say, but I'd be surprised if it was worse).

One possible problem with strictlimit isolation is suggested by the
comment

	 *
	 * In strictlimit case make decision based on the wb counters
	 * and limits. Small writeouts when the wb limits are ramping
	 * up are the price we consciously pay for strictlimit-ing.
	 *

This suggests that starting transients may be worse in some cases.
I haven't noticed any problems in my (limited) testing so while I
suspect there is a genuine difference here, I don't expect it to be problematic.

>
>> 
>> Note that previously realtime threads were treated the same as
>> PF_LESS_THROTTLE threads.  This patch does *not* change the behvaiour for
>> real-time threads, so it is now different from the behaviour of nfsd and
>> loop tasks.  I don't know what is wanted for realtime.
>> 
>> Signed-off-by: NeilBrown <neilb@suse.de>
>> =2D--
>
> Hrm corrupted delivery?

No, that's just normal email quotation for a multi-part message (needed
for crypto-signing which I do by default)
'git am', for example, is quite capable of coping with it.

Thanks,
NeilBrown

>
>>  drivers/block/loop.c  |  2 +-
>>  fs/nfsd/vfs.c         |  9 +++++----
>>  include/linux/sched.h |  2 +-
>>  kernel/sys.c          |  2 +-
>>  mm/page-writeback.c   | 10 ++++++----
>>  mm/vmscan.c           |  4 ++--
>>  6 files changed, 16 insertions(+), 13 deletions(-)

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Re: [PATCH 1/2] MM: replace PF_LESS_THROTTLE with PF_LOCAL_THROTTLE

[Hillf Danton]

On Thu, 02 Apr 2020 15:57:56 +1100 NeilBrown wrote:
> 
> On Thu, Apr 02 2020, Hillf Danton wrote:
> 
> > On Thu, 02 Apr 2020 10:53:20 +1100 NeilBrown wrote:
> >>=20
> >> PF_LESS_THROTTLE exists for loop-back nfsd, and a similar need in the
> >> loop block driver, where a daemon needs to write to one bdi in
> >> order to free up writes queued to another bdi.
> >>=20
> >> The daemon sets PF_LESS_THROTTLE and gets a larger allowance of dirty
> >> pages, so that it can still dirty pages after other processses have been
> >> throttled.
> >>=20
> >> This approach was designed when all threads were blocked equally,
> >> independently on which device they were writing to, or how fast it was.
> >> Since that time the writeback algorithm has changed substantially with
> >> different threads getting different allowances based on non-trivial
> >> heuristics.  This means the simple "add 25%" heuristic is no longer
> >> reliable.
> >>=20
> >> This patch changes the heuristic to ignore the global limits and
> >> consider only the limit relevant to the bdi being written to.  This
> >> approach is already available for BDI_CAP_STRICTLIMIT users (fuse) and
> >> should not introduce surprises.  This has the desired result of
> >> protecting the task from the consequences of large amounts of dirty data
> >> queued for other devices.
> >>=20
> >> This approach of "only consider the target bdi" is consistent with the
> >> other use of PF_LESS_THROTTLE in current_may_throttle(), were it causes
> >> attention to be focussed only on the target bdi.
> >>=20
> >> So this patch
> >>  - renames PF_LESS_THROTTLE to PF_LOCAL_THROTTLE,
> >>  - remove the 25% bonus that that flag gives, and
> >>  - imposes 'strictlimit' handling for any process with PF_LOCAL_THROTTLE
> >>    set.
> >
> > 	/*
> > 	 * The strictlimit feature is a tool preventing mistrusted filesystems
> > 	 * from growing a large number of dirty pages before throttling. For
> >
> > Based on the comment snippet, I suspect it is applicable to IO flushers
> > unless they are likely generating tons of dirty pages. If they are,
> > however, cutting their bonuses seem questionable.
> 
> The purpose of the strictlimit feature was to isolate one filesystem
> (bdi) from all others, so that the one cannot create dirty pages which
> unfairly disadvantage the others - this is what that comment says.
> But the implementation appears to focus on the isolation, not the
> specific purpose, and isolation works both ways.  It protects the others
> from the one, and the one from the others.
> 
> fuse needs to be isolated so it doesn't harm others.
> nfsd and loop need to be isolate so they aren't harmed by others.

For those working in emergency services, extra N95 face masks and Covid-19
testing kits, say 25%, would be preserved, too, if isolation doesn't help
them.

> I'm less familiar with IO flushers but I suspect that have exactly the
> same need as nfsd and loop - they need to be isolated from dirty pages
> other than on the device they are writing to.
> The 25% bonus was never about giving them a bonus because they need it.
> It was about protecting them from excess usage elsewhere.

For example,

> I strongly
> suspect that my change will provide a conceptually better service for IO
> flushers. (whether it is better in a practical measurable sense I cannot
> say, but I'd be surprised if it was worse).

Writeback fixes for NFS - V2

[NeilBrown]

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This is a second version of my patches to improve writeback over NFS.
This was prompted by a customer report of very slow throughput to a
server running Ganesha NFS server which was taking about 200ms to handle
COMMIT requests.  While slow COMMIT requests do exacerbate the
problem, there are still problems when COMMIT is comparatively fast.
Latest test results from customer are that these changes provide
substantial improvements.

Changes over first version include improvements to the explanation in
the commit message, and preservation of "NFS Unstable" counts in various
statistics files, despite that fact that the internal accounting is
completely gone (merged with writeback accounting).

Thanks,
NeilBrown

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[PATCH 1/2] MM: replace PF_LESS_THROTTLE with PF_LOCAL_THROTTLE

[NeilBrown]

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PF_LESS_THROTTLE exists for loop-back nfsd, and a similar need in the
loop block driver, where a daemon needs to write to one bdi (the final
bdi) in order to free up writes queued to another bdi (the client bdi).

The daemon sets PF_LESS_THROTTLE and gets a larger allowance of dirty
pages, so that it can still dirty pages after other processses have been
throttled.

This approach was designed when all threads were blocked equally,
independently on which device they were writing to, or how fast it was.
Since that time the writeback algorithm has changed substantially with
different threads getting different allowances based on non-trivial
heuristics.  This means the simple "add 25%" heuristic is no longer
reliable.

The important issue is not that the daemon needs a *larger* dirty page
allowance, but that it needs a *private* dirty page allowance, so that
dirty pages for the "client" bdi that it is helping to clear (the bdi for
an NFS filesystem or loop block device etc) do not affect the throttling
of the deamon writing to the "final" bdi.

This patch changes the heuristic to ignore the global limits and
consider only the limit relevant to the bdi being written to.  This
approach is already available for BDI_CAP_STRICTLIMIT users (fuse) and
should not introduce surprises.  This has the desired result of
protecting the task from the consequences of large amounts of dirty data
queued for other devices.

This approach of "only consider the target bdi" is consistent with the
other use of PF_LESS_THROTTLE in current_may_throttle(), were it causes
attention to be focussed only on the target bdi.

So this patch
 - renames PF_LESS_THROTTLE to PF_LOCAL_THROTTLE,
 - removes the 25% bonus that that flag gives, and
 - imposes 'strictlimit' handling for any process with PF_LOCAL_THROTTLE
   set.

Note that previously realtime threads were treated the same as
PF_LESS_THROTTLE threads.  This patch does *not* change the behvaiour for
real-time threads, so it is now different from the behaviour of nfsd and
loop tasks.  I don't know what is wanted for realtime.

Note that the worst-case situation with this patch is that the threshold
might be calculated as zero.  In that case the daemon may block when
there are any dirty pages for the final bdi.  These will eventually
clear and the daemon will be able to proceed.  The writing of those
dirty pages will increase the apparent throughput of the final bdi and
thus increase its threshold for future calculations.

Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: NeilBrown <neilb@suse.de>
---
 drivers/block/loop.c  |  2 +-
 fs/nfsd/vfs.c         |  9 +++++----
 include/linux/sched.h |  3 ++-
 kernel/sys.c          |  2 +-
 mm/page-writeback.c   | 10 ++++++----
 mm/vmscan.c           |  4 ++--
 6 files changed, 17 insertions(+), 13 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index a42c49e04954..0e13b9fc8dfa 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -899,7 +899,7 @@ static void loop_unprepare_queue(struct loop_device *lo)
 
 static int loop_kthread_worker_fn(void *worker_ptr)
 {
-	current->flags |= PF_LESS_THROTTLE | PF_MEMALLOC_NOIO;
+	current->flags |= PF_LOCAL_THROTTLE | PF_MEMALLOC_NOIO;
 	return kthread_worker_fn(worker_ptr);
 }
 
diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
index 0aa02eb18bd3..c3fbab1753ec 100644
--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c
@@ -979,12 +979,13 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp, struct nfsd_file *nf,
 
 	if (test_bit(RQ_LOCAL, &rqstp->rq_flags))
 		/*
-		 * We want less throttling in balance_dirty_pages()
-		 * and shrink_inactive_list() so that nfs to
+		 * We want throttling in balance_dirty_pages()
+		 * and shrink_inactive_list() to only consider
+		 * the backingdev we are writing to, so that nfs to
 		 * localhost doesn't cause nfsd to lock up due to all
 		 * the client's dirty pages or its congested queue.
 		 */
-		current->flags |= PF_LESS_THROTTLE;
+		current->flags |= PF_LOCAL_THROTTLE;
 
 	exp = fhp->fh_export;
 	use_wgather = (rqstp->rq_vers == 2) && EX_WGATHER(exp);
@@ -1037,7 +1038,7 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp, struct nfsd_file *nf,
 		nfserr = nfserrno(host_err);
 	}
 	if (test_bit(RQ_LOCAL, &rqstp->rq_flags))
-		current_restore_flags(pflags, PF_LESS_THROTTLE);
+		current_restore_flags(pflags, PF_LOCAL_THROTTLE);
 	return nfserr;
 }
 
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 4418f5cb8324..5955a089df32 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1481,7 +1481,8 @@ extern struct pid *cad_pid;
 #define PF_KSWAPD		0x00020000	/* I am kswapd */
 #define PF_MEMALLOC_NOFS	0x00040000	/* All allocation requests will inherit GFP_NOFS */
 #define PF_MEMALLOC_NOIO	0x00080000	/* All allocation requests will inherit GFP_NOIO */
-#define PF_LESS_THROTTLE	0x00100000	/* Throttle me less: I clean memory */
+#define PF_LOCAL_THROTTLE	0x00100000	/* Throttle writes only agasint the bdi I write to,
+						 * I am cleaning dirty pages from some other bdi. */
 #define PF_KTHREAD		0x00200000	/* I am a kernel thread */
 #define PF_RANDOMIZE		0x00400000	/* Randomize virtual address space */
 #define PF_SWAPWRITE		0x00800000	/* Allowed to write to swap */
diff --git a/kernel/sys.c b/kernel/sys.c
index d325f3ab624a..180a2fa33f7f 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2262,7 +2262,7 @@ int __weak arch_prctl_spec_ctrl_set(struct task_struct *t, unsigned long which,
 	return -EINVAL;
 }
 
-#define PR_IO_FLUSHER (PF_MEMALLOC_NOIO | PF_LESS_THROTTLE)
+#define PR_IO_FLUSHER (PF_MEMALLOC_NOIO | PF_LOCAL_THROTTLE)
 
 SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 		unsigned long, arg4, unsigned long, arg5)
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 7326b54ab728..4c9875971de5 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -387,8 +387,7 @@ static unsigned long global_dirtyable_memory(void)
  * Calculate @dtc->thresh and ->bg_thresh considering
  * vm_dirty_{bytes|ratio} and dirty_background_{bytes|ratio}.  The caller
  * must ensure that @dtc->avail is set before calling this function.  The
- * dirty limits will be lifted by 1/4 for PF_LESS_THROTTLE (ie. nfsd) and
- * real-time tasks.
+ * dirty limits will be lifted by 1/4 for real-time tasks.
  */
 static void domain_dirty_limits(struct dirty_throttle_control *dtc)
 {
@@ -436,7 +435,7 @@ static void domain_dirty_limits(struct dirty_throttle_control *dtc)
 	if (bg_thresh >= thresh)
 		bg_thresh = thresh / 2;
 	tsk = current;
-	if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) {
+	if (rt_task(tsk)) {
 		bg_thresh += bg_thresh / 4 + global_wb_domain.dirty_limit / 32;
 		thresh += thresh / 4 + global_wb_domain.dirty_limit / 32;
 	}
@@ -486,7 +485,7 @@ static unsigned long node_dirty_limit(struct pglist_data *pgdat)
 	else
 		dirty = vm_dirty_ratio * node_memory / 100;
 
-	if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk))
+	if (rt_task(tsk))
 		dirty += dirty / 4;
 
 	return dirty;
@@ -1580,6 +1579,9 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
 	bool strictlimit = bdi->capabilities & BDI_CAP_STRICTLIMIT;
 	unsigned long start_time = jiffies;
 
+	if (current->flags & PF_LOCAL_THROTTLE)
+		/* This task must only be throttled by its own writeback */
+		strictlimit = true;
 	for (;;) {
 		unsigned long now = jiffies;
 		unsigned long dirty, thresh, bg_thresh;
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 2e8e690d2813..b776da4bb8c8 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1879,13 +1879,13 @@ static unsigned noinline_for_stack move_pages_to_lru(struct lruvec *lruvec,
 
 /*
  * If a kernel thread (such as nfsd for loop-back mounts) services
- * a backing device by writing to the page cache it sets PF_LESS_THROTTLE.
+ * a backing device by writing to the page cache it sets PF_LOCAL_THROTTLE.
  * In that case we should only throttle if the backing device it is
  * writing to is congested.  In other cases it is safe to throttle.
  */
 static int current_may_throttle(void)
 {
-	return !(current->flags & PF_LESS_THROTTLE) ||
+	return !(current->flags & PF_LOCAL_THROTTLE) ||
 		current->backing_dev_info == NULL ||
 		bdi_write_congested(current->backing_dev_info);
 }
-- 
2.26.0


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Re: [PATCH 1/2] MM: replace PF_LESS_THROTTLE with PF_LOCAL_THROTTLE

[Chuck Lever]

> On Apr 6, 2020, at 7:43 PM, NeilBrown <neilb@suse.de> wrote:
> 
> 
> PF_LESS_THROTTLE exists for loop-back nfsd, and a similar need in the
> loop block driver, where a daemon needs to write to one bdi (the final
> bdi) in order to free up writes queued to another bdi (the client bdi).
> 
> The daemon sets PF_LESS_THROTTLE and gets a larger allowance of dirty
> pages, so that it can still dirty pages after other processses have been
> throttled.
> 
> This approach was designed when all threads were blocked equally,
> independently on which device they were writing to, or how fast it was.
> Since that time the writeback algorithm has changed substantially with
> different threads getting different allowances based on non-trivial
> heuristics.  This means the simple "add 25%" heuristic is no longer
> reliable.
> 
> The important issue is not that the daemon needs a *larger* dirty page
> allowance, but that it needs a *private* dirty page allowance, so that
> dirty pages for the "client" bdi that it is helping to clear (the bdi for
> an NFS filesystem or loop block device etc) do not affect the throttling
> of the deamon writing to the "final" bdi.
> 
> This patch changes the heuristic to ignore the global limits and
> consider only the limit relevant to the bdi being written to.  This
> approach is already available for BDI_CAP_STRICTLIMIT users (fuse) and
> should not introduce surprises.  This has the desired result of
> protecting the task from the consequences of large amounts of dirty data
> queued for other devices.
> 
> This approach of "only consider the target bdi" is consistent with the
> other use of PF_LESS_THROTTLE in current_may_throttle(), were it causes
> attention to be focussed only on the target bdi.
> 
> So this patch
> - renames PF_LESS_THROTTLE to PF_LOCAL_THROTTLE,
> - removes the 25% bonus that that flag gives, and
> - imposes 'strictlimit' handling for any process with PF_LOCAL_THROTTLE
>   set.
> 
> Note that previously realtime threads were treated the same as
> PF_LESS_THROTTLE threads.  This patch does *not* change the behvaiour for
> real-time threads, so it is now different from the behaviour of nfsd and
> loop tasks.  I don't know what is wanted for realtime.
> 
> Note that the worst-case situation with this patch is that the threshold
> might be calculated as zero.  In that case the daemon may block when
> there are any dirty pages for the final bdi.  These will eventually
> clear and the daemon will be able to proceed.  The writing of those
> dirty pages will increase the apparent throughput of the final bdi and
> thus increase its threshold for future calculations.
> 
> Reviewed-by: Jan Kara <jack@suse.cz>
> Signed-off-by: NeilBrown <neilb@suse.de>
> ---
> drivers/block/loop.c  |  2 +-
> fs/nfsd/vfs.c         |  9 +++++----
> include/linux/sched.h |  3 ++-
> kernel/sys.c          |  2 +-
> mm/page-writeback.c   | 10 ++++++----
> mm/vmscan.c           |  4 ++--
> 6 files changed, 17 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index a42c49e04954..0e13b9fc8dfa 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -899,7 +899,7 @@ static void loop_unprepare_queue(struct loop_device *lo)
> 
> static int loop_kthread_worker_fn(void *worker_ptr)
> {
> -	current->flags |= PF_LESS_THROTTLE | PF_MEMALLOC_NOIO;
> +	current->flags |= PF_LOCAL_THROTTLE | PF_MEMALLOC_NOIO;
> 	return kthread_worker_fn(worker_ptr);
> }
> 
> diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
> index 0aa02eb18bd3..c3fbab1753ec 100644
> --- a/fs/nfsd/vfs.c
> +++ b/fs/nfsd/vfs.c
> @@ -979,12 +979,13 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp, struct nfsd_file *nf,

Assuming these patches are not going through the NFSD tree, so this hunk is

Acked-by: Chuck Lever <chuck.lever@oracle.com>

If this isn't necessary or appropriate, then ignore me :-)


> 	if (test_bit(RQ_LOCAL, &rqstp->rq_flags))
> 		/*
> -		 * We want less throttling in balance_dirty_pages()
> -		 * and shrink_inactive_list() so that nfs to
> +		 * We want throttling in balance_dirty_pages()
> +		 * and shrink_inactive_list() to only consider
> +		 * the backingdev we are writing to, so that nfs to
> 		 * localhost doesn't cause nfsd to lock up due to all
> 		 * the client's dirty pages or its congested queue.
> 		 */
> -		current->flags |= PF_LESS_THROTTLE;
> +		current->flags |= PF_LOCAL_THROTTLE;
> 
> 	exp = fhp->fh_export;
> 	use_wgather = (rqstp->rq_vers == 2) && EX_WGATHER(exp);
> @@ -1037,7 +1038,7 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp, struct nfsd_file *nf,
> 		nfserr = nfserrno(host_err);
> 	}
> 	if (test_bit(RQ_LOCAL, &rqstp->rq_flags))
> -		current_restore_flags(pflags, PF_LESS_THROTTLE);
> +		current_restore_flags(pflags, PF_LOCAL_THROTTLE);
> 	return nfserr;
> }
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 4418f5cb8324..5955a089df32 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1481,7 +1481,8 @@ extern struct pid *cad_pid;
> #define PF_KSWAPD		0x00020000	/* I am kswapd */
> #define PF_MEMALLOC_NOFS	0x00040000	/* All allocation requests will inherit GFP_NOFS */
> #define PF_MEMALLOC_NOIO	0x00080000	/* All allocation requests will inherit GFP_NOIO */
> -#define PF_LESS_THROTTLE	0x00100000	/* Throttle me less: I clean memory */
> +#define PF_LOCAL_THROTTLE	0x00100000	/* Throttle writes only agasint the bdi I write to,
> +						 * I am cleaning dirty pages from some other bdi. */
> #define PF_KTHREAD		0x00200000	/* I am a kernel thread */
> #define PF_RANDOMIZE		0x00400000	/* Randomize virtual address space */
> #define PF_SWAPWRITE		0x00800000	/* Allowed to write to swap */
> diff --git a/kernel/sys.c b/kernel/sys.c
> index d325f3ab624a..180a2fa33f7f 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -2262,7 +2262,7 @@ int __weak arch_prctl_spec_ctrl_set(struct task_struct *t, unsigned long which,
> 	return -EINVAL;
> }
> 
> -#define PR_IO_FLUSHER (PF_MEMALLOC_NOIO | PF_LESS_THROTTLE)
> +#define PR_IO_FLUSHER (PF_MEMALLOC_NOIO | PF_LOCAL_THROTTLE)
> 
> SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
> 		unsigned long, arg4, unsigned long, arg5)
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 7326b54ab728..4c9875971de5 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -387,8 +387,7 @@ static unsigned long global_dirtyable_memory(void)
>  * Calculate @dtc->thresh and ->bg_thresh considering
>  * vm_dirty_{bytes|ratio} and dirty_background_{bytes|ratio}.  The caller
>  * must ensure that @dtc->avail is set before calling this function.  The
> - * dirty limits will be lifted by 1/4 for PF_LESS_THROTTLE (ie. nfsd) and
> - * real-time tasks.
> + * dirty limits will be lifted by 1/4 for real-time tasks.
>  */
> static void domain_dirty_limits(struct dirty_throttle_control *dtc)
> {
> @@ -436,7 +435,7 @@ static void domain_dirty_limits(struct dirty_throttle_control *dtc)
> 	if (bg_thresh >= thresh)
> 		bg_thresh = thresh / 2;
> 	tsk = current;
> -	if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) {
> +	if (rt_task(tsk)) {
> 		bg_thresh += bg_thresh / 4 + global_wb_domain.dirty_limit / 32;
> 		thresh += thresh / 4 + global_wb_domain.dirty_limit / 32;
> 	}
> @@ -486,7 +485,7 @@ static unsigned long node_dirty_limit(struct pglist_data *pgdat)
> 	else
> 		dirty = vm_dirty_ratio * node_memory / 100;
> 
> -	if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk))
> +	if (rt_task(tsk))
> 		dirty += dirty / 4;
> 
> 	return dirty;
> @@ -1580,6 +1579,9 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
> 	bool strictlimit = bdi->capabilities & BDI_CAP_STRICTLIMIT;
> 	unsigned long start_time = jiffies;
> 
> +	if (current->flags & PF_LOCAL_THROTTLE)
> +		/* This task must only be throttled by its own writeback */
> +		strictlimit = true;
> 	for (;;) {
> 		unsigned long now = jiffies;
> 		unsigned long dirty, thresh, bg_thresh;
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 2e8e690d2813..b776da4bb8c8 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1879,13 +1879,13 @@ static unsigned noinline_for_stack move_pages_to_lru(struct lruvec *lruvec,
> 
> /*
>  * If a kernel thread (such as nfsd for loop-back mounts) services
> - * a backing device by writing to the page cache it sets PF_LESS_THROTTLE.
> + * a backing device by writing to the page cache it sets PF_LOCAL_THROTTLE.
>  * In that case we should only throttle if the backing device it is
>  * writing to is congested.  In other cases it is safe to throttle.
>  */
> static int current_may_throttle(void)
> {
> -	return !(current->flags & PF_LESS_THROTTLE) ||
> +	return !(current->flags & PF_LOCAL_THROTTLE) ||
> 		current->backing_dev_info == NULL ||
> 		bdi_write_congested(current->backing_dev_info);
> }
> -- 
> 2.26.0
> 

--
Chuck Lever
chucklever@gmail.com

Writeback fixes for NFS - V3

[NeilBrown]

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This is version 3 (I think) of my patches to improve NFS writeaback.

Changes:
 - the code for adding legacy values to /proc/vmstat was broken.
   I haven't taken the approach that Michal and Jan discussed but
   a simpler (I hope) approach that just seq_puts() the lines at an
   appropriate place.

 - I've modified the handling of PF_LOCAL_THROTTLE - sufficiently that
   I dropped Jan's reviewed-by.
   Rather than invoking the same behaviour as BDI_CAP_STRICTLIMIT,
   I now just use the part I needed.
   So if the global threshold is not exceeded, PF_LOCAL_THROTTLE tasks
   are not throttled.  This is the case for normal processes, but not
   when BDI_CAP_STRICTLIMIT is in effect.
   If the global threshold *is* exceeded, only then to we check the
   local per-bdi threshold.  If that is not exceeded then
   PF_LOCAL_THROTTLE again avoid any throttling.  Only if both the
   thresholds are exceeded are these tasks throttled.
   I think this is more refletive if what we actually need.


Thanks,
NeilBrown

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[PATCH 1/2 V3] MM: replace PF_LESS_THROTTLE with PF_LOCAL_THROTTLE

[NeilBrown]

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PF_LESS_THROTTLE exists for loop-back nfsd (and a similar need in the
loop block driver and callers of prctl(PR_SET_IO_FLUSHER)), where a
daemon needs to write to one bdi (the final bdi) in order to free up
writes queued to another bdi (the client bdi).

The daemon sets PF_LESS_THROTTLE and gets a larger allowance of dirty
pages, so that it can still dirty pages after other processses have been
throttled.

This approach was designed when all threads were blocked equally,
independently on which device they were writing to, or how fast it was.
Since that time the writeback algorithm has changed substantially with
different threads getting different allowances based on non-trivial
heuristics.  This means the simple "add 25%" heuristic is no longer
reliable.

The important issue is not that the daemon needs a *larger* dirty page
allowance, but that it needs a *private* dirty page allowance, so that
dirty pages for the "client" bdi that it is helping to clear (the bdi for
an NFS filesystem or loop block device etc) do not affect the throttling
of the deamon writing to the "final" bdi.

This patch changes the heuristic so that the task is only throttled if
*both* the global threshhold *and* the per-wb threshold are exceeded.
This is similar to the effect of BDI_CAP_STRICTLIMIT which causes the
global limits to be ignored, but it isn't as strict.  A PF_LOCAL_THROTTLE
task will be allowed to proceed unthrottled if the global threshold is
not exceeded or if the local threshold is not exceeded.  They need to
both be exceeded before PF_LOCAL_THROTTLE tasks are throttled.

This approach of "only throttle when target bdi is busy" is consistent
with the other use of PF_LESS_THROTTLE in current_may_throttle(), were
it causes attention to be focussed only on the target bdi.

So this patch
 - renames PF_LESS_THROTTLE to PF_LOCAL_THROTTLE,
 - removes the 25% bonus that that flag gives, and
 - If PF_LOCAL_THROTTLE is set, don't delay at all unless both
   thresholds are exceeded.

Note that previously realtime threads were treated the same as
PF_LESS_THROTTLE threads.  This patch does *not* change the behvaiour for
real-time threads, so it is now different from the behaviour of nfsd and
loop tasks.  I don't know what is wanted for realtime.

Acked-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: NeilBrown <neilb@suse.de>
---
 drivers/block/loop.c  |  2 +-
 fs/nfsd/vfs.c         |  9 +++++----
 include/linux/sched.h |  3 ++-
 kernel/sys.c          |  2 +-
 mm/page-writeback.c   | 18 ++++++++++++++----
 mm/vmscan.c           |  4 ++--
 6 files changed, 25 insertions(+), 13 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index da693e6a834e..d89c25ba3b89 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -919,7 +919,7 @@ static void loop_unprepare_queue(struct loop_device *lo)
 
 static int loop_kthread_worker_fn(void *worker_ptr)
 {
-	current->flags |= PF_LESS_THROTTLE | PF_MEMALLOC_NOIO;
+	current->flags |= PF_LOCAL_THROTTLE | PF_MEMALLOC_NOIO;
 	return kthread_worker_fn(worker_ptr);
 }
 
diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
index 0aa02eb18bd3..c3fbab1753ec 100644
--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c
@@ -979,12 +979,13 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp, struct nfsd_file *nf,
 
 	if (test_bit(RQ_LOCAL, &rqstp->rq_flags))
 		/*
-		 * We want less throttling in balance_dirty_pages()
-		 * and shrink_inactive_list() so that nfs to
+		 * We want throttling in balance_dirty_pages()
+		 * and shrink_inactive_list() to only consider
+		 * the backingdev we are writing to, so that nfs to
 		 * localhost doesn't cause nfsd to lock up due to all
 		 * the client's dirty pages or its congested queue.
 		 */
-		current->flags |= PF_LESS_THROTTLE;
+		current->flags |= PF_LOCAL_THROTTLE;
 
 	exp = fhp->fh_export;
 	use_wgather = (rqstp->rq_vers == 2) && EX_WGATHER(exp);
@@ -1037,7 +1038,7 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp, struct nfsd_file *nf,
 		nfserr = nfserrno(host_err);
 	}
 	if (test_bit(RQ_LOCAL, &rqstp->rq_flags))
-		current_restore_flags(pflags, PF_LESS_THROTTLE);
+		current_restore_flags(pflags, PF_LOCAL_THROTTLE);
 	return nfserr;
 }
 
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 4418f5cb8324..5955a089df32 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1481,7 +1481,8 @@ extern struct pid *cad_pid;
 #define PF_KSWAPD		0x00020000	/* I am kswapd */
 #define PF_MEMALLOC_NOFS	0x00040000	/* All allocation requests will inherit GFP_NOFS */
 #define PF_MEMALLOC_NOIO	0x00080000	/* All allocation requests will inherit GFP_NOIO */
-#define PF_LESS_THROTTLE	0x00100000	/* Throttle me less: I clean memory */
+#define PF_LOCAL_THROTTLE	0x00100000	/* Throttle writes only agasint the bdi I write to,
+						 * I am cleaning dirty pages from some other bdi. */
 #define PF_KTHREAD		0x00200000	/* I am a kernel thread */
 #define PF_RANDOMIZE		0x00400000	/* Randomize virtual address space */
 #define PF_SWAPWRITE		0x00800000	/* Allowed to write to swap */
diff --git a/kernel/sys.c b/kernel/sys.c
index d325f3ab624a..180a2fa33f7f 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2262,7 +2262,7 @@ int __weak arch_prctl_spec_ctrl_set(struct task_struct *t, unsigned long which,
 	return -EINVAL;
 }
 
-#define PR_IO_FLUSHER (PF_MEMALLOC_NOIO | PF_LESS_THROTTLE)
+#define PR_IO_FLUSHER (PF_MEMALLOC_NOIO | PF_LOCAL_THROTTLE)
 
 SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 		unsigned long, arg4, unsigned long, arg5)
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 7326b54ab728..9692c553526b 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -387,8 +387,7 @@ static unsigned long global_dirtyable_memory(void)
  * Calculate @dtc->thresh and ->bg_thresh considering
  * vm_dirty_{bytes|ratio} and dirty_background_{bytes|ratio}.  The caller
  * must ensure that @dtc->avail is set before calling this function.  The
- * dirty limits will be lifted by 1/4 for PF_LESS_THROTTLE (ie. nfsd) and
- * real-time tasks.
+ * dirty limits will be lifted by 1/4 for real-time tasks.
  */
 static void domain_dirty_limits(struct dirty_throttle_control *dtc)
 {
@@ -436,7 +435,7 @@ static void domain_dirty_limits(struct dirty_throttle_control *dtc)
 	if (bg_thresh >= thresh)
 		bg_thresh = thresh / 2;
 	tsk = current;
-	if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) {
+	if (rt_task(tsk)) {
 		bg_thresh += bg_thresh / 4 + global_wb_domain.dirty_limit / 32;
 		thresh += thresh / 4 + global_wb_domain.dirty_limit / 32;
 	}
@@ -486,7 +485,7 @@ static unsigned long node_dirty_limit(struct pglist_data *pgdat)
 	else
 		dirty = vm_dirty_ratio * node_memory / 100;
 
-	if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk))
+	if (rt_task(tsk))
 		dirty += dirty / 4;
 
 	return dirty;
@@ -1700,6 +1699,17 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
 				sdtc = mdtc;
 		}
 
+		if (current->flags & PF_LOCAL_THROTTLE)
+			/* This task must only be throttled based on the bdi
+			 * it is writing to - dirty pages for other bdis might
+			 * be pages this task is trying to write out.  So it
+			 * gets a free pass unless both global and local
+			 * thresholds are exceeded.  i.e unless
+			 * "dirty_exceeded".
+			 */
+			if (!dirty_exceeded)
+				break;
+
 		if (dirty_exceeded && !wb->dirty_exceeded)
 			wb->dirty_exceeded = 1;
 
diff --git a/mm/vmscan.c b/mm/vmscan.c
index b06868fc4926..80ab523926aa 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1879,13 +1879,13 @@ static unsigned noinline_for_stack move_pages_to_lru(struct lruvec *lruvec,
 
 /*
  * If a kernel thread (such as nfsd for loop-back mounts) services
- * a backing device by writing to the page cache it sets PF_LESS_THROTTLE.
+ * a backing device by writing to the page cache it sets PF_LOCAL_THROTTLE.
  * In that case we should only throttle if the backing device it is
  * writing to is congested.  In other cases it is safe to throttle.
  */
 static int current_may_throttle(void)
 {
-	return !(current->flags & PF_LESS_THROTTLE) ||
+	return !(current->flags & PF_LOCAL_THROTTLE) ||
 		current->backing_dev_info == NULL ||
 		bdi_write_congested(current->backing_dev_info);
 }
-- 
2.26.0


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Re: [PATCH 1/2 V3] MM: replace PF_LESS_THROTTLE with PF_LOCAL_THROTTLE

[Christoph Hellwig]

> +		if (current->flags & PF_LOCAL_THROTTLE)
> +			/* This task must only be throttled based on the bdi
> +			 * it is writing to - dirty pages for other bdis might
> +			 * be pages this task is trying to write out.  So it
> +			 * gets a free pass unless both global and local
> +			 * thresholds are exceeded.  i.e unless
> +			 * "dirty_exceeded".
> +			 */

This is not our normal multi-line comment style.  The first line should
be just a

			/*

Otherwise this looks good.

Re: [PATCH 1/2 V3] MM: replace PF_LESS_THROTTLE with PF_LOCAL_THROTTLE

[Jan Kara]

On Thu 16-04-20 10:30:42, NeilBrown wrote:
> 
> PF_LESS_THROTTLE exists for loop-back nfsd (and a similar need in the
> loop block driver and callers of prctl(PR_SET_IO_FLUSHER)), where a
> daemon needs to write to one bdi (the final bdi) in order to free up
> writes queued to another bdi (the client bdi).
> 
> The daemon sets PF_LESS_THROTTLE and gets a larger allowance of dirty
> pages, so that it can still dirty pages after other processses have been
> throttled.
> 
> This approach was designed when all threads were blocked equally,
> independently on which device they were writing to, or how fast it was.
> Since that time the writeback algorithm has changed substantially with
> different threads getting different allowances based on non-trivial
> heuristics.  This means the simple "add 25%" heuristic is no longer
> reliable.
> 
> The important issue is not that the daemon needs a *larger* dirty page
> allowance, but that it needs a *private* dirty page allowance, so that
> dirty pages for the "client" bdi that it is helping to clear (the bdi for
> an NFS filesystem or loop block device etc) do not affect the throttling
> of the deamon writing to the "final" bdi.
> 
> This patch changes the heuristic so that the task is only throttled if
> *both* the global threshhold *and* the per-wb threshold are exceeded.
> This is similar to the effect of BDI_CAP_STRICTLIMIT which causes the
> global limits to be ignored, but it isn't as strict.  A PF_LOCAL_THROTTLE
> task will be allowed to proceed unthrottled if the global threshold is
> not exceeded or if the local threshold is not exceeded.  They need to
> both be exceeded before PF_LOCAL_THROTTLE tasks are throttled.
> 
> This approach of "only throttle when target bdi is busy" is consistent
> with the other use of PF_LESS_THROTTLE in current_may_throttle(), were
> it causes attention to be focussed only on the target bdi.
> 
> So this patch
>  - renames PF_LESS_THROTTLE to PF_LOCAL_THROTTLE,
>  - removes the 25% bonus that that flag gives, and
>  - If PF_LOCAL_THROTTLE is set, don't delay at all unless both
>    thresholds are exceeded.
> 
> Note that previously realtime threads were treated the same as
> PF_LESS_THROTTLE threads.  This patch does *not* change the behvaiour for
> real-time threads, so it is now different from the behaviour of nfsd and
> loop tasks.  I don't know what is wanted for realtime.
> 
> Acked-by: Chuck Lever <chuck.lever@oracle.com>
> Signed-off-by: NeilBrown <neilb@suse.de>

...

> @@ -1700,6 +1699,17 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
>  				sdtc = mdtc;
>  		}
>  
> +		if (current->flags & PF_LOCAL_THROTTLE)
> +			/* This task must only be throttled based on the bdi
> +			 * it is writing to - dirty pages for other bdis might
> +			 * be pages this task is trying to write out.  So it
> +			 * gets a free pass unless both global and local
> +			 * thresholds are exceeded.  i.e unless
> +			 * "dirty_exceeded".
> +			 */
> +			if (!dirty_exceeded)
> +				break;
> +
>  		if (dirty_exceeded && !wb->dirty_exceeded)
>  			wb->dirty_exceeded = 1;

Ok, but note that this will have one sideeffect you maybe didn't realize:
Currently we try to throttle tasks softly - the heuristic rougly works like
this: If dirty < (thresh + bg_thresh)/2, leave the task alone.
(thresh+bg_thresh)/2 is called "freerun ceiling". If dirty is greater than
this, we delay the task somewhat (the aim is to delay the task as long as
it would take to write back the pages task has dirtied) in
balance_dirty_pages() so ideally 'thresh' is never hit. Only if the
heuristic consistently underestimates the time to writeback pages, we hit
'thresh' and then block the task as long as it takes flush worker to clean
enough pages to get below 'thresh'. This all leads to task being usually
gradually slowed down in balance_dirty_pages() which generally leads to
smoother overall system behavior.

What you did makes PF_LOCAL_THROTTLE tasks ignore any limits and then when
local bdi limit is exceeded, they'll suddently hit the wall and be blocked
for a long time in balance_dirty_pages().

So I like what you suggest in principle, just I think the implementation
has undesirable sideeffects. I think it would be better to modify
wb_position_ratio() to take PF_LOCAL_THROTTLE into account. It will be
probably similar to how BDI_CAP_STRICTLIMIT is handled but different in
some ways because BDI_CAP_STRICTLIMIT takes minimum from wb_pos_ratio and
global pos_ratio, you rather want to take wb_pos_ratio only. Also there are
some early bail out conditions when we are over global dirty limit which
you need to handle differently for PF_LOCAL_THROTTLE. And then, when you
have appropriate pos_ratio computed based on your policy, you can let the
task wait for appropriate amount of time and things should just work (TM) ;).
Thinking about it, you probably also want to add 'freerun' condition for
PF_LOCAL_THROTTLE tasks like:

	if ((current->flags & PF_LOCAL_THROTTLE) &&
	    wb_dirty <= dirty_freerun_ceiling(wb_thresh, wb_bg_thresh))
		go the freerun path...

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Re: [PATCH 1/2 V3] MM: replace PF_LESS_THROTTLE with PF_LOCAL_THROTTLE

[NeilBrown]

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On Thu, Apr 16 2020, Jan Kara wrote:

> On Thu 16-04-20 10:30:42, NeilBrown wrote:
>> 
>> PF_LESS_THROTTLE exists for loop-back nfsd (and a similar need in the
>> loop block driver and callers of prctl(PR_SET_IO_FLUSHER)), where a
>> daemon needs to write to one bdi (the final bdi) in order to free up
>> writes queued to another bdi (the client bdi).
>> 
>> The daemon sets PF_LESS_THROTTLE and gets a larger allowance of dirty
>> pages, so that it can still dirty pages after other processses have been
>> throttled.
>> 
>> This approach was designed when all threads were blocked equally,
>> independently on which device they were writing to, or how fast it was.
>> Since that time the writeback algorithm has changed substantially with
>> different threads getting different allowances based on non-trivial
>> heuristics.  This means the simple "add 25%" heuristic is no longer
>> reliable.
>> 
>> The important issue is not that the daemon needs a *larger* dirty page
>> allowance, but that it needs a *private* dirty page allowance, so that
>> dirty pages for the "client" bdi that it is helping to clear (the bdi for
>> an NFS filesystem or loop block device etc) do not affect the throttling
>> of the deamon writing to the "final" bdi.
>> 
>> This patch changes the heuristic so that the task is only throttled if
>> *both* the global threshhold *and* the per-wb threshold are exceeded.
>> This is similar to the effect of BDI_CAP_STRICTLIMIT which causes the
>> global limits to be ignored, but it isn't as strict.  A PF_LOCAL_THROTTLE
>> task will be allowed to proceed unthrottled if the global threshold is
>> not exceeded or if the local threshold is not exceeded.  They need to
>> both be exceeded before PF_LOCAL_THROTTLE tasks are throttled.
>> 
>> This approach of "only throttle when target bdi is busy" is consistent
>> with the other use of PF_LESS_THROTTLE in current_may_throttle(), were
>> it causes attention to be focussed only on the target bdi.
>> 
>> So this patch
>>  - renames PF_LESS_THROTTLE to PF_LOCAL_THROTTLE,
>>  - removes the 25% bonus that that flag gives, and
>>  - If PF_LOCAL_THROTTLE is set, don't delay at all unless both
>>    thresholds are exceeded.
>> 
>> Note that previously realtime threads were treated the same as
>> PF_LESS_THROTTLE threads.  This patch does *not* change the behvaiour for
>> real-time threads, so it is now different from the behaviour of nfsd and
>> loop tasks.  I don't know what is wanted for realtime.
>> 
>> Acked-by: Chuck Lever <chuck.lever@oracle.com>
>> Signed-off-by: NeilBrown <neilb@suse.de>
>
> ...
>
>> @@ -1700,6 +1699,17 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
>>  				sdtc = mdtc;
>>  		}
>>  
>> +		if (current->flags & PF_LOCAL_THROTTLE)
>> +			/* This task must only be throttled based on the bdi
>> +			 * it is writing to - dirty pages for other bdis might
>> +			 * be pages this task is trying to write out.  So it
>> +			 * gets a free pass unless both global and local
>> +			 * thresholds are exceeded.  i.e unless
>> +			 * "dirty_exceeded".
>> +			 */
>> +			if (!dirty_exceeded)
>> +				break;
>> +
>>  		if (dirty_exceeded && !wb->dirty_exceeded)
>>  			wb->dirty_exceeded = 1;
>
> Ok, but note that this will have one sideeffect you maybe didn't realize:
> Currently we try to throttle tasks softly - the heuristic rougly works like
> this: If dirty < (thresh + bg_thresh)/2, leave the task alone.
> (thresh+bg_thresh)/2 is called "freerun ceiling". If dirty is greater than
> this, we delay the task somewhat (the aim is to delay the task as long as
> it would take to write back the pages task has dirtied) in
> balance_dirty_pages() so ideally 'thresh' is never hit. Only if the
> heuristic consistently underestimates the time to writeback pages, we hit
> 'thresh' and then block the task as long as it takes flush worker to clean
> enough pages to get below 'thresh'. This all leads to task being usually
> gradually slowed down in balance_dirty_pages() which generally leads to
> smoother overall system behavior.
>
> What you did makes PF_LOCAL_THROTTLE tasks ignore any limits and then when
> local bdi limit is exceeded, they'll suddently hit the wall and be blocked
> for a long time in balance_dirty_pages().
>
> So I like what you suggest in principle, just I think the implementation
> has undesirable sideeffects. I think it would be better to modify
> wb_position_ratio() to take PF_LOCAL_THROTTLE into account. It will be
> probably similar to how BDI_CAP_STRICTLIMIT is handled but different in
> some ways because BDI_CAP_STRICTLIMIT takes minimum from wb_pos_ratio and
> global pos_ratio, you rather want to take wb_pos_ratio only. Also there are
> some early bail out conditions when we are over global dirty limit which
> you need to handle differently for PF_LOCAL_THROTTLE. And then, when you
> have appropriate pos_ratio computed based on your policy, you can let the
> task wait for appropriate amount of time and things should just work (TM) ;).
> Thinking about it, you probably also want to add 'freerun' condition for
> PF_LOCAL_THROTTLE tasks like:
>
> 	if ((current->flags & PF_LOCAL_THROTTLE) &&
> 	    wb_dirty <= dirty_freerun_ceiling(wb_thresh, wb_bg_thresh))
> 		go the freerun path...
>

Thanks.....
I have 2 thoughts on this.
One is that I'm not sure how much it really matters.
The PF_LOCAL_THROTTLE task it always doing writeout on behalf of some
other process.  Some process writes to NFS or to a loop block device or
somewhere, then the PF_LOCAL_THROTTLE task writes those dirty pages out
to a different BDI.  So the top level task will be throttled, an the
PF_LOCAL_THROTTLE task won't get more than it can handle.
There will be starting transients of course, but I doubt it would
generally be a problem.  However it would still be nice to find the
"right" solution.

My second thought is that I really don't understand the writeback code.
I think I understand the general principle, and there are lots of big
comments that try to explain things, but it just doesn't seem to help.
I look at the code and see more questions than answers.

What are the units for "dirty_ratelimit"??  I think it is pages per
second, because it is initialized to INIT_BW which is documented as 100
MB/s.
What is the difference between dirty_ratelimit and
balanced_dirty_ratelimit?
The later is "balanced" I guess.  What does that mean?
Apparently (from backing-dev-defs.h) dirty_ratelimit moves in smaller
steps and is more smooth than balanced_dirty_ratelimit.  How is being
less smooth, more balanced??

What is pos_ratio? And what is RATELIMIT_CALC_SHIFT ???
Maybe pos_ratio is the ratio of the actual number of dirty pages to the
desired number?  And pos_ratio is calculated with fixed-point arithmetic
and RATELIMIT_CALC_SHIFT tells where the point is?

I think I understand freerun - half way between the dirty limit and the
dirty_bg limit.  Se below dirty_bg, no writeback happens.  Between there
and freerun, writeback happens, but nothing in throttled.  From free up
to the limit, tasks are progressively throttled.
"setpoint" is the midpoint of this range.  Is the goal that pos_ratio is
computed for.
(except that in the BDI_CAP_STRICTLIMIT part of wb_position_ratio)
wb_setpoint is set to the bottom of this range, same as the freerun ceiling.)

Then we have the control lines, which are cubic(?) for global counts and
linear for per-wb - but truncated at 1/4.  The comment says "so that
wb_dirty can be smoothly throttled".  It'll take me a while to work out
what a hard edge results in smooth throttling.  I suspect it makes sense
but it doesn't jump out at me.

So, you see, I don't feel at all confident changing any of this code
because I just don't get it.

So I'm inclined to stick with the patch that I have. :-(

Thanks,
NeilBrown

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Re: [PATCH 1/2 V3] MM: replace PF_LESS_THROTTLE with PF_LOCAL_THROTTLE

[Jan Kara]

On Tue 21-04-20 12:22:59, NeilBrown wrote:
> On Thu, Apr 16 2020, Jan Kara wrote:
> 
> > On Thu 16-04-20 10:30:42, NeilBrown wrote:
> >> 
> >> PF_LESS_THROTTLE exists for loop-back nfsd (and a similar need in the
> >> loop block driver and callers of prctl(PR_SET_IO_FLUSHER)), where a
> >> daemon needs to write to one bdi (the final bdi) in order to free up
> >> writes queued to another bdi (the client bdi).
> >> 
> >> The daemon sets PF_LESS_THROTTLE and gets a larger allowance of dirty
> >> pages, so that it can still dirty pages after other processses have been
> >> throttled.
> >> 
> >> This approach was designed when all threads were blocked equally,
> >> independently on which device they were writing to, or how fast it was.
> >> Since that time the writeback algorithm has changed substantially with
> >> different threads getting different allowances based on non-trivial
> >> heuristics.  This means the simple "add 25%" heuristic is no longer
> >> reliable.
> >> 
> >> The important issue is not that the daemon needs a *larger* dirty page
> >> allowance, but that it needs a *private* dirty page allowance, so that
> >> dirty pages for the "client" bdi that it is helping to clear (the bdi for
> >> an NFS filesystem or loop block device etc) do not affect the throttling
> >> of the deamon writing to the "final" bdi.
> >> 
> >> This patch changes the heuristic so that the task is only throttled if
> >> *both* the global threshhold *and* the per-wb threshold are exceeded.
> >> This is similar to the effect of BDI_CAP_STRICTLIMIT which causes the
> >> global limits to be ignored, but it isn't as strict.  A PF_LOCAL_THROTTLE
> >> task will be allowed to proceed unthrottled if the global threshold is
> >> not exceeded or if the local threshold is not exceeded.  They need to
> >> both be exceeded before PF_LOCAL_THROTTLE tasks are throttled.
> >> 
> >> This approach of "only throttle when target bdi is busy" is consistent
> >> with the other use of PF_LESS_THROTTLE in current_may_throttle(), were
> >> it causes attention to be focussed only on the target bdi.
> >> 
> >> So this patch
> >>  - renames PF_LESS_THROTTLE to PF_LOCAL_THROTTLE,
> >>  - removes the 25% bonus that that flag gives, and
> >>  - If PF_LOCAL_THROTTLE is set, don't delay at all unless both
> >>    thresholds are exceeded.
> >> 
> >> Note that previously realtime threads were treated the same as
> >> PF_LESS_THROTTLE threads.  This patch does *not* change the behvaiour for
> >> real-time threads, so it is now different from the behaviour of nfsd and
> >> loop tasks.  I don't know what is wanted for realtime.
> >> 
> >> Acked-by: Chuck Lever <chuck.lever@oracle.com>
> >> Signed-off-by: NeilBrown <neilb@suse.de>
> >
> > ...
> >
> >> @@ -1700,6 +1699,17 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
> >>  				sdtc = mdtc;
> >>  		}
> >>  
> >> +		if (current->flags & PF_LOCAL_THROTTLE)
> >> +			/* This task must only be throttled based on the bdi
> >> +			 * it is writing to - dirty pages for other bdis might
> >> +			 * be pages this task is trying to write out.  So it
> >> +			 * gets a free pass unless both global and local
> >> +			 * thresholds are exceeded.  i.e unless
> >> +			 * "dirty_exceeded".
> >> +			 */
> >> +			if (!dirty_exceeded)
> >> +				break;
> >> +
> >>  		if (dirty_exceeded && !wb->dirty_exceeded)
> >>  			wb->dirty_exceeded = 1;
> >
> > Ok, but note that this will have one sideeffect you maybe didn't realize:
> > Currently we try to throttle tasks softly - the heuristic rougly works like
> > this: If dirty < (thresh + bg_thresh)/2, leave the task alone.
> > (thresh+bg_thresh)/2 is called "freerun ceiling". If dirty is greater than
> > this, we delay the task somewhat (the aim is to delay the task as long as
> > it would take to write back the pages task has dirtied) in
> > balance_dirty_pages() so ideally 'thresh' is never hit. Only if the
> > heuristic consistently underestimates the time to writeback pages, we hit
> > 'thresh' and then block the task as long as it takes flush worker to clean
> > enough pages to get below 'thresh'. This all leads to task being usually
> > gradually slowed down in balance_dirty_pages() which generally leads to
> > smoother overall system behavior.
> >
> > What you did makes PF_LOCAL_THROTTLE tasks ignore any limits and then when
> > local bdi limit is exceeded, they'll suddently hit the wall and be blocked
> > for a long time in balance_dirty_pages().
> >
> > So I like what you suggest in principle, just I think the implementation
> > has undesirable sideeffects. I think it would be better to modify
> > wb_position_ratio() to take PF_LOCAL_THROTTLE into account. It will be
> > probably similar to how BDI_CAP_STRICTLIMIT is handled but different in
> > some ways because BDI_CAP_STRICTLIMIT takes minimum from wb_pos_ratio and
> > global pos_ratio, you rather want to take wb_pos_ratio only. Also there are
> > some early bail out conditions when we are over global dirty limit which
> > you need to handle differently for PF_LOCAL_THROTTLE. And then, when you
> > have appropriate pos_ratio computed based on your policy, you can let the
> > task wait for appropriate amount of time and things should just work (TM) ;).
> > Thinking about it, you probably also want to add 'freerun' condition for
> > PF_LOCAL_THROTTLE tasks like:
> >
> > 	if ((current->flags & PF_LOCAL_THROTTLE) &&
> > 	    wb_dirty <= dirty_freerun_ceiling(wb_thresh, wb_bg_thresh))
> > 		go the freerun path...
> >
> 
> Thanks.....
> I have 2 thoughts on this.
> One is that I'm not sure how much it really matters.
> The PF_LOCAL_THROTTLE task it always doing writeout on behalf of some
> other process.  Some process writes to NFS or to a loop block device or
> somewhere, then the PF_LOCAL_THROTTLE task writes those dirty pages out
> to a different BDI.  So the top level task will be throttled, an the
> PF_LOCAL_THROTTLE task won't get more than it can handle.
> There will be starting transients of course, but I doubt it would
> generally be a problem.  However it would still be nice to find the
> "right" solution.

I'm not sure PF_LOCAL_THROTTLE "won't get more than it can handle". Once
dirty pages on NFS BDI accumulate, flush worker will start to push them out
as fast as it can. So the only thing that's limitting this is the dirty
throttling on the receiving (NFS server - thus underlying BDI) side. When
underlying BDI throttling triggers depends on that BDI dirty limits and
those are proportional part of global dirty limits scaled by writeback
throughput on underlying BDI compared to other BDIs. So depending on which
BDIs are in the system and how active they are in dirtying pages
'underlying BDI' will get different dirty limits set. It's quite imaginable
that in some configurations it will be easy to push NFS server to hit its
dirty limit even with PF_LOCAL_THROTTLE. And then having NFS server
undersponsive for couple seconds because it is blocked in
balance_dirty_pages() just is not nice...

> My second thought is that I really don't understand the writeback code.
> I think I understand the general principle, and there are lots of big
> comments that try to explain things, but it just doesn't seem to help.
> I look at the code and see more questions than answers.

I fully understand what you mean :). The logic is complex and while
Fengguang wrote a lot of comments it is still rather hard to follow.

> What are the units for "dirty_ratelimit"??  I think it is pages per
> second, because it is initialized to INIT_BW which is documented as 100
> MB/s.

Yes, that's what I think as well.

> What is the difference between dirty_ratelimit and
> balanced_dirty_ratelimit?
> The later is "balanced" I guess.  What does that mean?
> Apparently (from backing-dev-defs.h) dirty_ratelimit moves in smaller
> steps and is more smooth than balanced_dirty_ratelimit.  How is being
> less smooth, more balanced??

Yeah. So I cannot really explain the naming to you (not sure why Fengguang
chose these names). But 'balanced_dirty_ratelimit' is pages/second value we
want to limit task to based on the events in the most current time slice.
'dirty_ratelimit' is smoothed version of 'balanced_dirty_ratelimit' taking
more of history into account.

> What is pos_ratio? And what is RATELIMIT_CALC_SHIFT ???
> Maybe pos_ratio is the ratio of the actual number of dirty pages to the
> desired number?  And pos_ratio is calculated with fixed-point arithmetic
> and RATELIMIT_CALC_SHIFT tells where the point is?

So RATELIMIT_CALC_SHIFT is indeed the shift of fixed point arithmetic used
in the computations. Pos_ratio is the multiplicative "correction" factor we
apply to computed dirty_ratelimit (i.e., task_ratelimit = dirty_ratelimit *
pos_ratio) - so if we see we are able to writeout say 100 MB/s but we are
still relatively far from dirty limits, we let tasks dirty 200 MB/s
(pos_ratio is 2). As we are nearing dirty limits, pos_ratio is dropping
(it's appropriately scaled and shifted third order polynomial) so very
close to dirty limits, we let tasks dirty only say 10 MB/s even though we
are still able to write out 100 MB/s.

> I think I understand freerun - half way between the dirty limit and the
> dirty_bg limit.  Se below dirty_bg, no writeback happens.  Between there
> and freerun, writeback happens, but nothing in throttled.  From free up
> to the limit, tasks are progressively throttled.

Correct.

> "setpoint" is the midpoint of this range.  Is the goal that pos_ratio is
> computed for.
> (except that in the BDI_CAP_STRICTLIMIT part of wb_position_ratio)
> wb_setpoint is set to the bottom of this range, same as the freerun ceiling.)

Correct.

> Then we have the control lines, which are cubic(?) for global counts and
> linear for per-wb - but truncated at 1/4.  The comment says "so that
> wb_dirty can be smoothly throttled".  It'll take me a while to work out
> what a hard edge results in smooth throttling.  I suspect it makes sense
> but it doesn't jump out at me.
> 
> So, you see, I don't feel at all confident changing any of this code
> because I just don't get it.
> 
> So I'm inclined to stick with the patch that I have. :-(

OK, I'll try to write something and we'll see if it will work :)

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Re: [PATCH 1/2 V3] MM: replace PF_LESS_THROTTLE with PF_LOCAL_THROTTLE

[NeilBrown]

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I thought about this some more and come up with another "simple"
approach that didn't require me understanding too much code, but does -
I think - address your concerns.

I've changed the heuristic to avoid any throttling on PF_LOCAL_THROTTLE
task if:
 - the global dirty count is below the global free-run threshold.  The
   code did this already.
 - (or) the per-wb dirty count is below the per-wb free-run threshold.
   This is the change.

This means that:
 - in a steady stated, all bdis will be throttled based on their (steady
   state) throughput, which is equally appropriate for PF_LOCAL_THROTTLE
   tasks.
 - a PF_LOCAL_THROTTLE task will never be *completely* blocked by dirty
   pages queued for other devices.  This means no deadlock, and that is
   the primary purpose of PF_LOCAL_THROTTLE.
 - when writes through the PF_LOCAL_THROTTLE task start up from idle -
   when there is no current throughput estimate - the PF_LOCAL_THROTTLE
   can be expected to get a fair share of the available memory, just as
   much as any other writer.  This was the possible problem with
   treating PF_LOCAL_THROTTLE just like BDI_CAP_STRICTLIMIT.

So I think this is a good solution.  Thoughts?
Patches follow - I've address the comment formatting issue.

Thanks,
NeilBrown

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[PATCH 1/2 V4] MM: replace PF_LESS_THROTTLE with PF_LOCAL_THROTTLE

[NeilBrown]

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PF_LESS_THROTTLE exists for loop-back nfsd (and a similar need in the
loop block driver and callers of prctl(PR_SET_IO_FLUSHER)), where a
daemon needs to write to one bdi (the final bdi) in order to free up
writes queued to another bdi (the client bdi).

The daemon sets PF_LESS_THROTTLE and gets a larger allowance of dirty
pages, so that it can still dirty pages after other processses have been
throttled.  The purpose of this is to avoid deadlock that happen when
the PF_LESS_THROTTLE process must write for any dirty pages to be freed,
but it is being thottled and cannot write.

This approach was designed when all threads were blocked equally,
independently on which device they were writing to, or how fast it was.
Since that time the writeback algorithm has changed substantially with
different threads getting different allowances based on non-trivial
heuristics.  This means the simple "add 25%" heuristic is no longer
reliable.

The important issue is not that the daemon needs a *larger* dirty page
allowance, but that it needs a *private* dirty page allowance, so that
dirty pages for the "client" bdi that it is helping to clear (the bdi for
an NFS filesystem or loop block device etc) do not affect the throttling
of the deamon writing to the "final" bdi.

This patch changes the heuristic so that the task is not throttled when
the bdi it is writing to has a dirty page count below below (or equal
to) the free-run threshold for that bdi.  This ensures it will always be
able to have some pages in flight, and so will not deadlock.

In a steady-state, it is expected that PF_LOCAL_THROTTLE tasks might
still be throttled by global threshold, but that is acceptable as it is
only the deadlock state that is interesting for this flag.

This approach of "only throttle when target bdi is busy" is consistent
with the other use of PF_LESS_THROTTLE in current_may_throttle(), were
it causes attention to be focussed only on the target bdi.

So this patch
 - renames PF_LESS_THROTTLE to PF_LOCAL_THROTTLE,
 - removes the 25% bonus that that flag gives, and
 - If PF_LOCAL_THROTTLE is set, don't delay at all unless the
   global and the local free-run thresholds are exceeded.

Note that previously realtime threads were treated the same as
PF_LESS_THROTTLE threads.  This patch does *not* change the behvaiour for
real-time threads, so it is now different from the behaviour of nfsd and
loop tasks.  I don't know what is wanted for realtime.

Acked-by: Chuck Lever <chuck.lever@oracle.com> (for nfsd change)
Signed-off-by: NeilBrown <neilb@suse.de>
---
 drivers/block/loop.c  |  2 +-
 fs/nfsd/vfs.c         |  9 +++++----
 include/linux/sched.h |  3 ++-
 kernel/sys.c          |  2 +-
 mm/page-writeback.c   | 41 +++++++++++++++++++++++++++++++++--------
 mm/vmscan.c           |  4 ++--
 6 files changed, 44 insertions(+), 17 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index da693e6a834e..d89c25ba3b89 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -919,7 +919,7 @@ static void loop_unprepare_queue(struct loop_device *lo)
 
 static int loop_kthread_worker_fn(void *worker_ptr)
 {
-	current->flags |= PF_LESS_THROTTLE | PF_MEMALLOC_NOIO;
+	current->flags |= PF_LOCAL_THROTTLE | PF_MEMALLOC_NOIO;
 	return kthread_worker_fn(worker_ptr);
 }
 
diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
index 0aa02eb18bd3..c3fbab1753ec 100644
--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c
@@ -979,12 +979,13 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp, struct nfsd_file *nf,
 
 	if (test_bit(RQ_LOCAL, &rqstp->rq_flags))
 		/*
-		 * We want less throttling in balance_dirty_pages()
-		 * and shrink_inactive_list() so that nfs to
+		 * We want throttling in balance_dirty_pages()
+		 * and shrink_inactive_list() to only consider
+		 * the backingdev we are writing to, so that nfs to
 		 * localhost doesn't cause nfsd to lock up due to all
 		 * the client's dirty pages or its congested queue.
 		 */
-		current->flags |= PF_LESS_THROTTLE;
+		current->flags |= PF_LOCAL_THROTTLE;
 
 	exp = fhp->fh_export;
 	use_wgather = (rqstp->rq_vers == 2) && EX_WGATHER(exp);
@@ -1037,7 +1038,7 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp, struct nfsd_file *nf,
 		nfserr = nfserrno(host_err);
 	}
 	if (test_bit(RQ_LOCAL, &rqstp->rq_flags))
-		current_restore_flags(pflags, PF_LESS_THROTTLE);
+		current_restore_flags(pflags, PF_LOCAL_THROTTLE);
 	return nfserr;
 }
 
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 4418f5cb8324..5955a089df32 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1481,7 +1481,8 @@ extern struct pid *cad_pid;
 #define PF_KSWAPD		0x00020000	/* I am kswapd */
 #define PF_MEMALLOC_NOFS	0x00040000	/* All allocation requests will inherit GFP_NOFS */
 #define PF_MEMALLOC_NOIO	0x00080000	/* All allocation requests will inherit GFP_NOIO */
-#define PF_LESS_THROTTLE	0x00100000	/* Throttle me less: I clean memory */
+#define PF_LOCAL_THROTTLE	0x00100000	/* Throttle writes only agasint the bdi I write to,
+						 * I am cleaning dirty pages from some other bdi. */
 #define PF_KTHREAD		0x00200000	/* I am a kernel thread */
 #define PF_RANDOMIZE		0x00400000	/* Randomize virtual address space */
 #define PF_SWAPWRITE		0x00800000	/* Allowed to write to swap */
diff --git a/kernel/sys.c b/kernel/sys.c
index d325f3ab624a..180a2fa33f7f 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2262,7 +2262,7 @@ int __weak arch_prctl_spec_ctrl_set(struct task_struct *t, unsigned long which,
 	return -EINVAL;
 }
 
-#define PR_IO_FLUSHER (PF_MEMALLOC_NOIO | PF_LESS_THROTTLE)
+#define PR_IO_FLUSHER (PF_MEMALLOC_NOIO | PF_LOCAL_THROTTLE)
 
 SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 		unsigned long, arg4, unsigned long, arg5)
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 7326b54ab728..f02a71797781 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -387,8 +387,7 @@ static unsigned long global_dirtyable_memory(void)
  * Calculate @dtc->thresh and ->bg_thresh considering
  * vm_dirty_{bytes|ratio} and dirty_background_{bytes|ratio}.  The caller
  * must ensure that @dtc->avail is set before calling this function.  The
- * dirty limits will be lifted by 1/4 for PF_LESS_THROTTLE (ie. nfsd) and
- * real-time tasks.
+ * dirty limits will be lifted by 1/4 for real-time tasks.
  */
 static void domain_dirty_limits(struct dirty_throttle_control *dtc)
 {
@@ -436,7 +435,7 @@ static void domain_dirty_limits(struct dirty_throttle_control *dtc)
 	if (bg_thresh >= thresh)
 		bg_thresh = thresh / 2;
 	tsk = current;
-	if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) {
+	if (rt_task(tsk)) {
 		bg_thresh += bg_thresh / 4 + global_wb_domain.dirty_limit / 32;
 		thresh += thresh / 4 + global_wb_domain.dirty_limit / 32;
 	}
@@ -486,7 +485,7 @@ static unsigned long node_dirty_limit(struct pglist_data *pgdat)
 	else
 		dirty = vm_dirty_ratio * node_memory / 100;
 
-	if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk))
+	if (rt_task(tsk))
 		dirty += dirty / 4;
 
 	return dirty;
@@ -1653,8 +1652,12 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
 		if (dirty <= dirty_freerun_ceiling(thresh, bg_thresh) &&
 		    (!mdtc ||
 		     m_dirty <= dirty_freerun_ceiling(m_thresh, m_bg_thresh))) {
-			unsigned long intv = dirty_poll_interval(dirty, thresh);
-			unsigned long m_intv = ULONG_MAX;
+			unsigned long intv;
+			unsigned long m_intv;
+
+		free_running:
+			intv = dirty_poll_interval(dirty, thresh);
+			m_intv = ULONG_MAX;
 
 			current->dirty_paused_when = now;
 			current->nr_dirtied = 0;
@@ -1673,9 +1676,20 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
 		 * Calculate global domain's pos_ratio and select the
 		 * global dtc by default.
 		 */
-		if (!strictlimit)
+		if (!strictlimit) {
 			wb_dirty_limits(gdtc);
 
+			if ((current->flags & PF_LOCAL_THROTTLE) &&
+			    gdtc->wb_dirty <
+			    dirty_freerun_ceiling(gdtc->wb_thresh,
+						  gdtc->wb_bg_thresh))
+				/*
+				 * LOCAL_THROTTLE tasks must not be throttled
+				 * when below the per-wb freerun ceiling.
+				 */
+				goto free_running;
+		}
+
 		dirty_exceeded = (gdtc->wb_dirty > gdtc->wb_thresh) &&
 			((gdtc->dirty > gdtc->thresh) || strictlimit);
 
@@ -1689,9 +1703,20 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
 			 * both global and memcg domains.  Choose the one
 			 * w/ lower pos_ratio.
 			 */
-			if (!strictlimit)
+			if (!strictlimit) {
 				wb_dirty_limits(mdtc);
 
+				if ((current->flags & PF_LOCAL_THROTTLE) &&
+				    mdtc->wb_dirty <
+				    dirty_freerun_ceiling(mdtc->wb_thresh,
+							  mdtc->wb_bg_thresh))
+					/*
+					 * LOCAL_THROTTLE tasks must not be
+					 * throttled when below the per-wb
+					 * freerun ceiling.
+					 */
+					goto free_running;
+			}
 			dirty_exceeded |= (mdtc->wb_dirty > mdtc->wb_thresh) &&
 				((mdtc->dirty > mdtc->thresh) || strictlimit);
 
diff --git a/mm/vmscan.c b/mm/vmscan.c
index a37c87b5aee2..b2f5deb3603c 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1878,13 +1878,13 @@ static unsigned noinline_for_stack move_pages_to_lru(struct lruvec *lruvec,
 
 /*
  * If a kernel thread (such as nfsd for loop-back mounts) services
- * a backing device by writing to the page cache it sets PF_LESS_THROTTLE.
+ * a backing device by writing to the page cache it sets PF_LOCAL_THROTTLE.
  * In that case we should only throttle if the backing device it is
  * writing to is congested.  In other cases it is safe to throttle.
  */
 static int current_may_throttle(void)
 {
-	return !(current->flags & PF_LESS_THROTTLE) ||
+	return !(current->flags & PF_LOCAL_THROTTLE) ||
 		current->backing_dev_info == NULL ||
 		bdi_write_congested(current->backing_dev_info);
 }
-- 
2.26.2


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Re: [PATCH 1/2 V4] MM: replace PF_LESS_THROTTLE with PF_LOCAL_THROTTLE

[Jan Kara]

On Wed 13-05-20 17:17:20, NeilBrown wrote:
> 
> PF_LESS_THROTTLE exists for loop-back nfsd (and a similar need in the
> loop block driver and callers of prctl(PR_SET_IO_FLUSHER)), where a
> daemon needs to write to one bdi (the final bdi) in order to free up
> writes queued to another bdi (the client bdi).
> 
> The daemon sets PF_LESS_THROTTLE and gets a larger allowance of dirty
> pages, so that it can still dirty pages after other processses have been
> throttled.  The purpose of this is to avoid deadlock that happen when
> the PF_LESS_THROTTLE process must write for any dirty pages to be freed,
> but it is being thottled and cannot write.
> 
> This approach was designed when all threads were blocked equally,
> independently on which device they were writing to, or how fast it was.
> Since that time the writeback algorithm has changed substantially with
> different threads getting different allowances based on non-trivial
> heuristics.  This means the simple "add 25%" heuristic is no longer
> reliable.
> 
> The important issue is not that the daemon needs a *larger* dirty page
> allowance, but that it needs a *private* dirty page allowance, so that
> dirty pages for the "client" bdi that it is helping to clear (the bdi for
> an NFS filesystem or loop block device etc) do not affect the throttling
> of the deamon writing to the "final" bdi.
> 
> This patch changes the heuristic so that the task is not throttled when
> the bdi it is writing to has a dirty page count below below (or equal
> to) the free-run threshold for that bdi.  This ensures it will always be
> able to have some pages in flight, and so will not deadlock.
> 
> In a steady-state, it is expected that PF_LOCAL_THROTTLE tasks might
> still be throttled by global threshold, but that is acceptable as it is
> only the deadlock state that is interesting for this flag.
> 
> This approach of "only throttle when target bdi is busy" is consistent
> with the other use of PF_LESS_THROTTLE in current_may_throttle(), were
> it causes attention to be focussed only on the target bdi.
> 
> So this patch
>  - renames PF_LESS_THROTTLE to PF_LOCAL_THROTTLE,
>  - removes the 25% bonus that that flag gives, and
>  - If PF_LOCAL_THROTTLE is set, don't delay at all unless the
>    global and the local free-run thresholds are exceeded.
> 
> Note that previously realtime threads were treated the same as
> PF_LESS_THROTTLE threads.  This patch does *not* change the behvaiour for
> real-time threads, so it is now different from the behaviour of nfsd and
> loop tasks.  I don't know what is wanted for realtime.
> 
> Acked-by: Chuck Lever <chuck.lever@oracle.com> (for nfsd change)
> Signed-off-by: NeilBrown <neilb@suse.de>

The idea looks good to me. It will still have the "threads may hit hard
wall" behavior when BDI freerun threshold is crossed at the moment we are
very close to the global limit but that should be rare. So I think for now
this good enough.

The patch looks good to me so feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

Throttling patches usually go through mm tree so ask Andrew to pick them
up.


								Honza

> @@ -1653,8 +1652,12 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
>  		if (dirty <= dirty_freerun_ceiling(thresh, bg_thresh) &&
>  		    (!mdtc ||
>  		     m_dirty <= dirty_freerun_ceiling(m_thresh, m_bg_thresh))) {
> -			unsigned long intv = dirty_poll_interval(dirty, thresh);
> -			unsigned long m_intv = ULONG_MAX;
> +			unsigned long intv;
> +			unsigned long m_intv;
> +
> +		free_running:
> +			intv = dirty_poll_interval(dirty, thresh);
> +			m_intv = ULONG_MAX;
>  
>  			current->dirty_paused_when = now;
>  			current->nr_dirtied = 0;
> @@ -1673,9 +1676,20 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
>  		 * Calculate global domain's pos_ratio and select the
>  		 * global dtc by default.
>  		 */
> -		if (!strictlimit)
> +		if (!strictlimit) {
>  			wb_dirty_limits(gdtc);
>  
> +			if ((current->flags & PF_LOCAL_THROTTLE) &&
> +			    gdtc->wb_dirty <
> +			    dirty_freerun_ceiling(gdtc->wb_thresh,
> +						  gdtc->wb_bg_thresh))
> +				/*
> +				 * LOCAL_THROTTLE tasks must not be throttled
> +				 * when below the per-wb freerun ceiling.
> +				 */
> +				goto free_running;
> +		}
> +
>  		dirty_exceeded = (gdtc->wb_dirty > gdtc->wb_thresh) &&
>  			((gdtc->dirty > gdtc->thresh) || strictlimit);
>  
> @@ -1689,9 +1703,20 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
>  			 * both global and memcg domains.  Choose the one
>  			 * w/ lower pos_ratio.
>  			 */
> -			if (!strictlimit)
> +			if (!strictlimit) {
>  				wb_dirty_limits(mdtc);
>  
> +				if ((current->flags & PF_LOCAL_THROTTLE) &&
> +				    mdtc->wb_dirty <
> +				    dirty_freerun_ceiling(mdtc->wb_thresh,
> +							  mdtc->wb_bg_thresh))
> +					/*
> +					 * LOCAL_THROTTLE tasks must not be
> +					 * throttled when below the per-wb
> +					 * freerun ceiling.
> +					 */
> +					goto free_running;
> +			}
>  			dirty_exceeded |= (mdtc->wb_dirty > mdtc->wb_thresh) &&
>  				((mdtc->dirty > mdtc->thresh) || strictlimit);
>  
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index a37c87b5aee2..b2f5deb3603c 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1878,13 +1878,13 @@ static unsigned noinline_for_stack move_pages_to_lru(struct lruvec *lruvec,
>  
>  /*
>   * If a kernel thread (such as nfsd for loop-back mounts) services
> - * a backing device by writing to the page cache it sets PF_LESS_THROTTLE.
> + * a backing device by writing to the page cache it sets PF_LOCAL_THROTTLE.
>   * In that case we should only throttle if the backing device it is
>   * writing to is congested.  In other cases it is safe to throttle.
>   */
>  static int current_may_throttle(void)
>  {
> -	return !(current->flags & PF_LESS_THROTTLE) ||
> +	return !(current->flags & PF_LOCAL_THROTTLE) ||
>  		current->backing_dev_info == NULL ||
>  		bdi_write_congested(current->backing_dev_info);
>  }
> -- 
> 2.26.2
> 


-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Writeback fixes for NFS

[NeilBrown]

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Hi Andrew,
 could you please queue these two patches (following).
 I think they have sufficient review and no remaining complaints.

Thanks,
NeilBrown

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[PATCH 1/2] MM: replace PF_LESS_THROTTLE with PF_LOCAL_THROTTLE

[NeilBrown]

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PF_LESS_THROTTLE exists for loop-back nfsd (and a similar need in the
loop block driver and callers of prctl(PR_SET_IO_FLUSHER)), where a
daemon needs to write to one bdi (the final bdi) in order to free up
writes queued to another bdi (the client bdi).

The daemon sets PF_LESS_THROTTLE and gets a larger allowance of dirty
pages, so that it can still dirty pages after other processses have been
throttled.  The purpose of this is to avoid deadlock that happen when
the PF_LESS_THROTTLE process must write for any dirty pages to be freed,
but it is being thottled and cannot write.

This approach was designed when all threads were blocked equally,
independently on which device they were writing to, or how fast it was.
Since that time the writeback algorithm has changed substantially with
different threads getting different allowances based on non-trivial
heuristics.  This means the simple "add 25%" heuristic is no longer
reliable.

The important issue is not that the daemon needs a *larger* dirty page
allowance, but that it needs a *private* dirty page allowance, so that
dirty pages for the "client" bdi that it is helping to clear (the bdi for
an NFS filesystem or loop block device etc) do not affect the throttling
of the deamon writing to the "final" bdi.

This patch changes the heuristic so that the task is not throttled when
the bdi it is writing to has a dirty page count below below (or equal
to) the free-run threshold for that bdi.  This ensures it will always be
able to have some pages in flight, and so will not deadlock.

In a steady-state, it is expected that PF_LOCAL_THROTTLE tasks might
still be throttled by global threshold, but that is acceptable as it is
only the deadlock state that is interesting for this flag.

This approach of "only throttle when target bdi is busy" is consistent
with the other use of PF_LESS_THROTTLE in current_may_throttle(), were
it causes attention to be focussed only on the target bdi.

So this patch
 - renames PF_LESS_THROTTLE to PF_LOCAL_THROTTLE,
 - removes the 25% bonus that that flag gives, and
 - If PF_LOCAL_THROTTLE is set, don't delay at all unless the
   global and the local free-run thresholds are exceeded.

Note that previously realtime threads were treated the same as
PF_LESS_THROTTLE threads.  This patch does *not* change the behvaiour for
real-time threads, so it is now different from the behaviour of nfsd and
loop tasks.  I don't know what is wanted for realtime.

Reviewed-by: Jan Kara <jack@suse.cz>
Acked-by: Chuck Lever <chuck.lever@oracle.com> (for nfsd change)
Signed-off-by: NeilBrown <neilb@suse.de>
---
 drivers/block/loop.c  |  2 +-
 fs/nfsd/vfs.c         |  9 +++++----
 include/linux/sched.h |  3 ++-
 kernel/sys.c          |  2 +-
 mm/page-writeback.c   | 41 +++++++++++++++++++++++++++++++++--------
 mm/vmscan.c           |  4 ++--
 6 files changed, 44 insertions(+), 17 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index da693e6a834e..d89c25ba3b89 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -919,7 +919,7 @@ static void loop_unprepare_queue(struct loop_device *lo)
 
 static int loop_kthread_worker_fn(void *worker_ptr)
 {
-	current->flags |= PF_LESS_THROTTLE | PF_MEMALLOC_NOIO;
+	current->flags |= PF_LOCAL_THROTTLE | PF_MEMALLOC_NOIO;
 	return kthread_worker_fn(worker_ptr);
 }
 
diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
index 0aa02eb18bd3..c3fbab1753ec 100644
--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c
@@ -979,12 +979,13 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp, struct nfsd_file *nf,
 
 	if (test_bit(RQ_LOCAL, &rqstp->rq_flags))
 		/*
-		 * We want less throttling in balance_dirty_pages()
-		 * and shrink_inactive_list() so that nfs to
+		 * We want throttling in balance_dirty_pages()
+		 * and shrink_inactive_list() to only consider
+		 * the backingdev we are writing to, so that nfs to
 		 * localhost doesn't cause nfsd to lock up due to all
 		 * the client's dirty pages or its congested queue.
 		 */
-		current->flags |= PF_LESS_THROTTLE;
+		current->flags |= PF_LOCAL_THROTTLE;
 
 	exp = fhp->fh_export;
 	use_wgather = (rqstp->rq_vers == 2) && EX_WGATHER(exp);
@@ -1037,7 +1038,7 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp, struct nfsd_file *nf,
 		nfserr = nfserrno(host_err);
 	}
 	if (test_bit(RQ_LOCAL, &rqstp->rq_flags))
-		current_restore_flags(pflags, PF_LESS_THROTTLE);
+		current_restore_flags(pflags, PF_LOCAL_THROTTLE);
 	return nfserr;
 }
 
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 4418f5cb8324..5955a089df32 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1481,7 +1481,8 @@ extern struct pid *cad_pid;
 #define PF_KSWAPD		0x00020000	/* I am kswapd */
 #define PF_MEMALLOC_NOFS	0x00040000	/* All allocation requests will inherit GFP_NOFS */
 #define PF_MEMALLOC_NOIO	0x00080000	/* All allocation requests will inherit GFP_NOIO */
-#define PF_LESS_THROTTLE	0x00100000	/* Throttle me less: I clean memory */
+#define PF_LOCAL_THROTTLE	0x00100000	/* Throttle writes only agasint the bdi I write to,
+						 * I am cleaning dirty pages from some other bdi. */
 #define PF_KTHREAD		0x00200000	/* I am a kernel thread */
 #define PF_RANDOMIZE		0x00400000	/* Randomize virtual address space */
 #define PF_SWAPWRITE		0x00800000	/* Allowed to write to swap */
diff --git a/kernel/sys.c b/kernel/sys.c
index d325f3ab624a..180a2fa33f7f 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2262,7 +2262,7 @@ int __weak arch_prctl_spec_ctrl_set(struct task_struct *t, unsigned long which,
 	return -EINVAL;
 }
 
-#define PR_IO_FLUSHER (PF_MEMALLOC_NOIO | PF_LESS_THROTTLE)
+#define PR_IO_FLUSHER (PF_MEMALLOC_NOIO | PF_LOCAL_THROTTLE)
 
 SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 		unsigned long, arg4, unsigned long, arg5)
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 7326b54ab728..f02a71797781 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -387,8 +387,7 @@ static unsigned long global_dirtyable_memory(void)
  * Calculate @dtc->thresh and ->bg_thresh considering
  * vm_dirty_{bytes|ratio} and dirty_background_{bytes|ratio}.  The caller
  * must ensure that @dtc->avail is set before calling this function.  The
- * dirty limits will be lifted by 1/4 for PF_LESS_THROTTLE (ie. nfsd) and
- * real-time tasks.
+ * dirty limits will be lifted by 1/4 for real-time tasks.
  */
 static void domain_dirty_limits(struct dirty_throttle_control *dtc)
 {
@@ -436,7 +435,7 @@ static void domain_dirty_limits(struct dirty_throttle_control *dtc)
 	if (bg_thresh >= thresh)
 		bg_thresh = thresh / 2;
 	tsk = current;
-	if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) {
+	if (rt_task(tsk)) {
 		bg_thresh += bg_thresh / 4 + global_wb_domain.dirty_limit / 32;
 		thresh += thresh / 4 + global_wb_domain.dirty_limit / 32;
 	}
@@ -486,7 +485,7 @@ static unsigned long node_dirty_limit(struct pglist_data *pgdat)
 	else
 		dirty = vm_dirty_ratio * node_memory / 100;
 
-	if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk))
+	if (rt_task(tsk))
 		dirty += dirty / 4;
 
 	return dirty;
@@ -1653,8 +1652,12 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
 		if (dirty <= dirty_freerun_ceiling(thresh, bg_thresh) &&
 		    (!mdtc ||
 		     m_dirty <= dirty_freerun_ceiling(m_thresh, m_bg_thresh))) {
-			unsigned long intv = dirty_poll_interval(dirty, thresh);
-			unsigned long m_intv = ULONG_MAX;
+			unsigned long intv;
+			unsigned long m_intv;
+
+		free_running:
+			intv = dirty_poll_interval(dirty, thresh);
+			m_intv = ULONG_MAX;
 
 			current->dirty_paused_when = now;
 			current->nr_dirtied = 0;
@@ -1673,9 +1676,20 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
 		 * Calculate global domain's pos_ratio and select the
 		 * global dtc by default.
 		 */
-		if (!strictlimit)
+		if (!strictlimit) {
 			wb_dirty_limits(gdtc);
 
+			if ((current->flags & PF_LOCAL_THROTTLE) &&
+			    gdtc->wb_dirty <
+			    dirty_freerun_ceiling(gdtc->wb_thresh,
+						  gdtc->wb_bg_thresh))
+				/*
+				 * LOCAL_THROTTLE tasks must not be throttled
+				 * when below the per-wb freerun ceiling.
+				 */
+				goto free_running;
+		}
+
 		dirty_exceeded = (gdtc->wb_dirty > gdtc->wb_thresh) &&
 			((gdtc->dirty > gdtc->thresh) || strictlimit);
 
@@ -1689,9 +1703,20 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
 			 * both global and memcg domains.  Choose the one
 			 * w/ lower pos_ratio.
 			 */
-			if (!strictlimit)
+			if (!strictlimit) {
 				wb_dirty_limits(mdtc);
 
+				if ((current->flags & PF_LOCAL_THROTTLE) &&
+				    mdtc->wb_dirty <
+				    dirty_freerun_ceiling(mdtc->wb_thresh,
+							  mdtc->wb_bg_thresh))
+					/*
+					 * LOCAL_THROTTLE tasks must not be
+					 * throttled when below the per-wb
+					 * freerun ceiling.
+					 */
+					goto free_running;
+			}
 			dirty_exceeded |= (mdtc->wb_dirty > mdtc->wb_thresh) &&
 				((mdtc->dirty > mdtc->thresh) || strictlimit);
 
diff --git a/mm/vmscan.c b/mm/vmscan.c
index a37c87b5aee2..b2f5deb3603c 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1878,13 +1878,13 @@ static unsigned noinline_for_stack move_pages_to_lru(struct lruvec *lruvec,
 
 /*
  * If a kernel thread (such as nfsd for loop-back mounts) services
- * a backing device by writing to the page cache it sets PF_LESS_THROTTLE.
+ * a backing device by writing to the page cache it sets PF_LOCAL_THROTTLE.
  * In that case we should only throttle if the backing device it is
  * writing to is congested.  In other cases it is safe to throttle.
  */
 static int current_may_throttle(void)
 {
-	return !(current->flags & PF_LESS_THROTTLE) ||
+	return !(current->flags & PF_LOCAL_THROTTLE) ||
 		current->backing_dev_info == NULL ||
 		bdi_write_congested(current->backing_dev_info);
 }
-- 
2.26.2


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[PATCH 2/2] MM: Discard NR_UNSTABLE_NFS, use NR_WRITEBACK instead.

[NeilBrown]

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After an NFS page has been written it is considered "unstable" until a
COMMIT request succeeds.  If the COMMIT fails, the page will be
re-written.

These "unstable" pages are currently accounted as "reclaimable", either
in WB_RECLAIMABLE, or in NR_UNSTABLE_NFS which is included in a
'reclaimable' count.  This might have made sense when sending the COMMIT
required a separate action by the VFS/MM (e.g.  releasepage() used to
send a COMMIT).  However now that all writes generated by ->writepages()
will automatically be followed by a COMMIT (since commit 919e3bd9a875
("NFS: Ensure we commit after writeback is complete")) it makes more
sense to treat them as writeback pages.

So this patch removes NR_UNSTABLE_NFS and accounts unstable pages in
NR_WRITEBACK and WB_WRITEBACK.

A particular effect of this change is that when
wb_check_background_flush() calls wb_over_bg_threshold(), the latter
will report 'true' a lot less often as the 'unstable' pages are no
longer considered 'dirty' (as there is nothing that writeback can do
about them anyway).

Currently wb_check_background_flush() will trigger writeback to NFS even
when there are relatively few dirty pages (if there are lots of unstable
pages), this can result in small writes going to the server (10s of
Kilobytes rather than a Megabyte) which hurts throughput.
With this patch, there are fewer writes which are each larger on average.

Where the NR_UNSTABLE_NFS count was included in statistics
virtual-files, the entry is retained, but the value is hard-coded as
zero.  static trace points and warning printks which mentioned this
counter no longer report it.

Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Acked-by: Michal Hocko <mhocko@suse.com> # for MM parts
Signed-off-by: NeilBrown <neilb@suse.de>
---
 Documentation/filesystems/proc.rst |  4 ++--
 drivers/base/node.c                |  2 +-
 fs/fs-writeback.c                  |  1 -
 fs/nfs/internal.h                  | 10 +++++++---
 fs/nfs/write.c                     |  4 ++--
 fs/proc/meminfo.c                  |  3 +--
 include/linux/mmzone.h             |  1 -
 include/trace/events/writeback.h   |  5 +----
 mm/memcontrol.c                    |  1 -
 mm/page-writeback.c                | 17 ++++-------------
 mm/page_alloc.c                    |  5 +----
 mm/vmstat.c                        | 11 +++++++++--
 12 files changed, 28 insertions(+), 36 deletions(-)

diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
index 38b606991065..092b7b44d158 100644
--- a/Documentation/filesystems/proc.rst
+++ b/Documentation/filesystems/proc.rst
@@ -1042,8 +1042,8 @@ PageTables
               amount of memory dedicated to the lowest level of page
               tables.
 NFS_Unstable
-              NFS pages sent to the server, but not yet committed to stable
-	      storage
+              Always zero. Previous counted pages which had been written to
+              the server, but has not been committed to stable storage.
 Bounce
               Memory used for block device "bounce buffers"
 WritebackTmp
diff --git a/drivers/base/node.c b/drivers/base/node.c
index 10d7e818e118..15f5ed6a8830 100644
--- a/drivers/base/node.c
+++ b/drivers/base/node.c
@@ -439,7 +439,7 @@ static ssize_t node_read_meminfo(struct device *dev,
 		       nid, K(i.sharedram),
 		       nid, sum_zone_node_page_state(nid, NR_KERNEL_STACK_KB),
 		       nid, K(sum_zone_node_page_state(nid, NR_PAGETABLE)),
-		       nid, K(node_page_state(pgdat, NR_UNSTABLE_NFS)),
+		       nid, 0,
 		       nid, K(sum_zone_node_page_state(nid, NR_BOUNCE)),
 		       nid, K(node_page_state(pgdat, NR_WRITEBACK_TEMP)),
 		       nid, K(sreclaimable +
diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 76ac9c7d32ec..c5bdf46e3b4b 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -1070,7 +1070,6 @@ static void bdi_split_work_to_wbs(struct backing_dev_info *bdi,
 static unsigned long get_nr_dirty_pages(void)
 {
 	return global_node_page_state(NR_FILE_DIRTY) +
-		global_node_page_state(NR_UNSTABLE_NFS) +
 		get_nr_dirty_inodes();
 }
 
diff --git a/fs/nfs/internal.h b/fs/nfs/internal.h
index 1f32a9fbfdaf..6673a77884d9 100644
--- a/fs/nfs/internal.h
+++ b/fs/nfs/internal.h
@@ -668,7 +668,8 @@ void nfs_super_set_maxbytes(struct super_block *sb, __u64 maxfilesize)
 }
 
 /*
- * Record the page as unstable and mark its inode as dirty.
+ * Record the page as unstable (an extra writeback period) and mark its
+ * inode as dirty.
  */
 static inline
 void nfs_mark_page_unstable(struct page *page, struct nfs_commit_info *cinfo)
@@ -676,8 +677,11 @@ void nfs_mark_page_unstable(struct page *page, struct nfs_commit_info *cinfo)
 	if (!cinfo->dreq) {
 		struct inode *inode = page_file_mapping(page)->host;
 
-		inc_node_page_state(page, NR_UNSTABLE_NFS);
-		inc_wb_stat(&inode_to_bdi(inode)->wb, WB_RECLAIMABLE);
+		/* This page is really still in write-back - just that the
+		 * writeback is happening on the server now.
+		 */
+		inc_node_page_state(page, NR_WRITEBACK);
+		inc_wb_stat(&inode_to_bdi(inode)->wb, WB_WRITEBACK);
 		__mark_inode_dirty(inode, I_DIRTY_DATASYNC);
 	}
 }
diff --git a/fs/nfs/write.c b/fs/nfs/write.c
index 1e767f779c49..639c34fec04a 100644
--- a/fs/nfs/write.c
+++ b/fs/nfs/write.c
@@ -946,9 +946,9 @@ nfs_mark_request_commit(struct nfs_page *req, struct pnfs_layout_segment *lseg,
 static void
 nfs_clear_page_commit(struct page *page)
 {
-	dec_node_page_state(page, NR_UNSTABLE_NFS);
+	dec_node_page_state(page, NR_WRITEBACK);
 	dec_wb_stat(&inode_to_bdi(page_file_mapping(page)->host)->wb,
-		    WB_RECLAIMABLE);
+		    WB_WRITEBACK);
 }
 
 /* Called holding the request lock on @req */
diff --git a/fs/proc/meminfo.c b/fs/proc/meminfo.c
index 8c1f1bb1a5ce..9bd94b5a9658 100644
--- a/fs/proc/meminfo.c
+++ b/fs/proc/meminfo.c
@@ -106,8 +106,7 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
 	show_val_kb(m, "PageTables:     ",
 		    global_zone_page_state(NR_PAGETABLE));
 
-	show_val_kb(m, "NFS_Unstable:   ",
-		    global_node_page_state(NR_UNSTABLE_NFS));
+	show_val_kb(m, "NFS_Unstable:   ", 0);
 	show_val_kb(m, "Bounce:         ",
 		    global_zone_page_state(NR_BOUNCE));
 	show_val_kb(m, "WritebackTmp:   ",
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 1b9de7d220fb..a89f47515eb1 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -193,7 +193,6 @@ enum node_stat_item {
 	NR_FILE_THPS,
 	NR_FILE_PMDMAPPED,
 	NR_ANON_THPS,
-	NR_UNSTABLE_NFS,	/* NFS unstable pages */
 	NR_VMSCAN_WRITE,
 	NR_VMSCAN_IMMEDIATE,	/* Prioritise for reclaim when writeback ends */
 	NR_DIRTIED,		/* page dirtyings since bootup */
diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
index 85a33bea76f1..10f5d1fa7347 100644
--- a/include/trace/events/writeback.h
+++ b/include/trace/events/writeback.h
@@ -541,7 +541,6 @@ TRACE_EVENT(global_dirty_state,
 	TP_STRUCT__entry(
 		__field(unsigned long,	nr_dirty)
 		__field(unsigned long,	nr_writeback)
-		__field(unsigned long,	nr_unstable)
 		__field(unsigned long,	background_thresh)
 		__field(unsigned long,	dirty_thresh)
 		__field(unsigned long,	dirty_limit)
@@ -552,7 +551,6 @@ TRACE_EVENT(global_dirty_state,
 	TP_fast_assign(
 		__entry->nr_dirty	= global_node_page_state(NR_FILE_DIRTY);
 		__entry->nr_writeback	= global_node_page_state(NR_WRITEBACK);
-		__entry->nr_unstable	= global_node_page_state(NR_UNSTABLE_NFS);
 		__entry->nr_dirtied	= global_node_page_state(NR_DIRTIED);
 		__entry->nr_written	= global_node_page_state(NR_WRITTEN);
 		__entry->background_thresh = background_thresh;
@@ -560,12 +558,11 @@ TRACE_EVENT(global_dirty_state,
 		__entry->dirty_limit	= global_wb_domain.dirty_limit;
 	),
 
-	TP_printk("dirty=%lu writeback=%lu unstable=%lu "
+	TP_printk("dirty=%lu writeback=%lu "
 		  "bg_thresh=%lu thresh=%lu limit=%lu "
 		  "dirtied=%lu written=%lu",
 		  __entry->nr_dirty,
 		  __entry->nr_writeback,
-		  __entry->nr_unstable,
 		  __entry->background_thresh,
 		  __entry->dirty_thresh,
 		  __entry->dirty_limit,
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index a3b97f103966..1db4b285c407 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4330,7 +4330,6 @@ void mem_cgroup_wb_stats(struct bdi_writeback *wb, unsigned long *pfilepages,
 
 	*pdirty = memcg_exact_page_state(memcg, NR_FILE_DIRTY);
 
-	/* this should eventually include NR_UNSTABLE_NFS */
 	*pwriteback = memcg_exact_page_state(memcg, NR_WRITEBACK);
 	*pfilepages = memcg_exact_page_state(memcg, NR_INACTIVE_FILE) +
 			memcg_exact_page_state(memcg, NR_ACTIVE_FILE);
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index f02a71797781..d11b097c8002 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -504,7 +504,6 @@ bool node_dirty_ok(struct pglist_data *pgdat)
 	unsigned long nr_pages = 0;
 
 	nr_pages += node_page_state(pgdat, NR_FILE_DIRTY);
-	nr_pages += node_page_state(pgdat, NR_UNSTABLE_NFS);
 	nr_pages += node_page_state(pgdat, NR_WRITEBACK);
 
 	return nr_pages <= limit;
@@ -758,7 +757,7 @@ static void mdtc_calc_avail(struct dirty_throttle_control *mdtc,
  * bounded by the bdi->min_ratio and/or bdi->max_ratio parameters, if set.
  *
  * Return: @wb's dirty limit in pages. The term "dirty" in the context of
- * dirty balancing includes all PG_dirty, PG_writeback and NFS unstable pages.
+ * dirty balancing includes all PG_dirty and PG_writeback pages.
  */
 static unsigned long __wb_calc_thresh(struct dirty_throttle_control *dtc)
 {
@@ -1566,7 +1565,7 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
 	struct dirty_throttle_control * const mdtc = mdtc_valid(&mdtc_stor) ?
 						     &mdtc_stor : NULL;
 	struct dirty_throttle_control *sdtc;
-	unsigned long nr_reclaimable;	/* = file_dirty + unstable_nfs */
+	unsigned long nr_reclaimable;	/* = file_dirty */
 	long period;
 	long pause;
 	long max_pause;
@@ -1586,14 +1585,7 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
 		unsigned long m_thresh = 0;
 		unsigned long m_bg_thresh = 0;
 
-		/*
-		 * Unstable writes are a feature of certain networked
-		 * filesystems (i.e. NFS) in which data may have been
-		 * written to the server's write cache, but has not yet
-		 * been flushed to permanent storage.
-		 */
-		nr_reclaimable = global_node_page_state(NR_FILE_DIRTY) +
-					global_node_page_state(NR_UNSTABLE_NFS);
+		nr_reclaimable = global_node_page_state(NR_FILE_DIRTY);
 		gdtc->avail = global_dirtyable_memory();
 		gdtc->dirty = nr_reclaimable + global_node_page_state(NR_WRITEBACK);
 
@@ -1963,8 +1955,7 @@ bool wb_over_bg_thresh(struct bdi_writeback *wb)
 	 * as we're trying to decide whether to put more under writeback.
 	 */
 	gdtc->avail = global_dirtyable_memory();
-	gdtc->dirty = global_node_page_state(NR_FILE_DIRTY) +
-		      global_node_page_state(NR_UNSTABLE_NFS);
+	gdtc->dirty = global_node_page_state(NR_FILE_DIRTY);
 	domain_dirty_limits(gdtc);
 
 	if (gdtc->dirty > gdtc->bg_thresh)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 13cc653122b7..cc406ee17ad9 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5319,7 +5319,7 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
 
 	printk("active_anon:%lu inactive_anon:%lu isolated_anon:%lu\n"
 		" active_file:%lu inactive_file:%lu isolated_file:%lu\n"
-		" unevictable:%lu dirty:%lu writeback:%lu unstable:%lu\n"
+		" unevictable:%lu dirty:%lu writeback:%lu\n"
 		" slab_reclaimable:%lu slab_unreclaimable:%lu\n"
 		" mapped:%lu shmem:%lu pagetables:%lu bounce:%lu\n"
 		" free:%lu free_pcp:%lu free_cma:%lu\n",
@@ -5332,7 +5332,6 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
 		global_node_page_state(NR_UNEVICTABLE),
 		global_node_page_state(NR_FILE_DIRTY),
 		global_node_page_state(NR_WRITEBACK),
-		global_node_page_state(NR_UNSTABLE_NFS),
 		global_node_page_state(NR_SLAB_RECLAIMABLE),
 		global_node_page_state(NR_SLAB_UNRECLAIMABLE),
 		global_node_page_state(NR_FILE_MAPPED),
@@ -5365,7 +5364,6 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
 			" anon_thp: %lukB"
 #endif
 			" writeback_tmp:%lukB"
-			" unstable:%lukB"
 			" all_unreclaimable? %s"
 			"\n",
 			pgdat->node_id,
@@ -5387,7 +5385,6 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
 			K(node_page_state(pgdat, NR_ANON_THPS) * HPAGE_PMD_NR),
 #endif
 			K(node_page_state(pgdat, NR_WRITEBACK_TEMP)),
-			K(node_page_state(pgdat, NR_UNSTABLE_NFS)),
 			pgdat->kswapd_failures >= MAX_RECLAIM_RETRIES ?
 				"yes" : "no");
 	}
diff --git a/mm/vmstat.c b/mm/vmstat.c
index 96d21a792b57..6c719f184843 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1108,7 +1108,7 @@ int fragmentation_index(struct zone *zone, unsigned int order)
 					TEXT_FOR_HIGHMEM(xx) xx "_movable",
 
 const char * const vmstat_text[] = {
-	/* enum zone_stat_item countes */
+	/* enum zone_stat_item counters */
 	"nr_free_pages",
 	"nr_zone_inactive_anon",
 	"nr_zone_active_anon",
@@ -1162,7 +1162,6 @@ const char * const vmstat_text[] = {
 	"nr_file_hugepages",
 	"nr_file_pmdmapped",
 	"nr_anon_transparent_hugepages",
-	"nr_unstable",
 	"nr_vmscan_write",
 	"nr_vmscan_immediate_reclaim",
 	"nr_dirtied",
@@ -1723,6 +1722,14 @@ static int vmstat_show(struct seq_file *m, void *arg)
 	seq_puts(m, vmstat_text[off]);
 	seq_put_decimal_ull(m, " ", *l);
 	seq_putc(m, '\n');
+
+	if (off == NR_VMSTAT_ITEMS - 1) {
+		/* We've come to the end - add any deprecated counters
+		 * to avoid breaking userspace which might depend on
+		 * them being present.
+		 */
+		seq_puts(m, "nr_unstable 0\n");
+	}
 	return 0;
 }
 
-- 
2.26.2


[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 832 bytes --]

[PATCH 2/2 V4] MM: Discard NR_UNSTABLE_NFS, use NR_WRITEBACK instead.

[NeilBrown]

[-- Attachment #1: Type: text/plain, Size: 13963 bytes --]


After an NFS page has been written it is considered "unstable" until a
COMMIT request succeeds.  If the COMMIT fails, the page will be
re-written.

These "unstable" pages are currently accounted as "reclaimable", either
in WB_RECLAIMABLE, or in NR_UNSTABLE_NFS which is included in a
'reclaimable' count.  This might have made sense when sending the COMMIT
required a separate action by the VFS/MM (e.g.  releasepage() used to
send a COMMIT).  However now that all writes generated by ->writepages()
will automatically be followed by a COMMIT (since commit 919e3bd9a875
("NFS: Ensure we commit after writeback is complete")) it makes more
sense to treat them as writeback pages.

So this patch removes NR_UNSTABLE_NFS and accounts unstable pages in
NR_WRITEBACK and WB_WRITEBACK.

A particular effect of this change is that when
wb_check_background_flush() calls wb_over_bg_threshold(), the latter
will report 'true' a lot less often as the 'unstable' pages are no
longer considered 'dirty' (as there is nothing that writeback can do
about them anyway).

Currently wb_check_background_flush() will trigger writeback to NFS even
when there are relatively few dirty pages (if there are lots of unstable
pages), this can result in small writes going to the server (10s of
Kilobytes rather than a Megabyte) which hurts throughput.
With this patch, there are fewer writes which are each larger on average.

Where the NR_UNSTABLE_NFS count was included in statistics
virtual-files, the entry is retained, but the value is hard-coded as
zero.  static trace points and warning printks which mentioned this
counter no longer report it.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Acked-by: Michal Hocko <mhocko@suse.com> # for MM parts
Signed-off-by: NeilBrown <neilb@suse.de>
---
 Documentation/filesystems/proc.rst |  4 ++--
 drivers/base/node.c                |  2 +-
 fs/fs-writeback.c                  |  1 -
 fs/nfs/internal.h                  | 10 +++++++---
 fs/nfs/write.c                     |  4 ++--
 fs/proc/meminfo.c                  |  3 +--
 include/linux/mmzone.h             |  1 -
 include/trace/events/writeback.h   |  5 +----
 mm/memcontrol.c                    |  1 -
 mm/page-writeback.c                | 17 ++++-------------
 mm/page_alloc.c                    |  5 +----
 mm/vmstat.c                        | 11 +++++++++--
 12 files changed, 28 insertions(+), 36 deletions(-)

diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
index 38b606991065..092b7b44d158 100644
--- a/Documentation/filesystems/proc.rst
+++ b/Documentation/filesystems/proc.rst
@@ -1042,8 +1042,8 @@ PageTables
               amount of memory dedicated to the lowest level of page
               tables.
 NFS_Unstable
-              NFS pages sent to the server, but not yet committed to stable
-	      storage
+              Always zero. Previous counted pages which had been written to
+              the server, but has not been committed to stable storage.
 Bounce
               Memory used for block device "bounce buffers"
 WritebackTmp
diff --git a/drivers/base/node.c b/drivers/base/node.c
index 10d7e818e118..15f5ed6a8830 100644
--- a/drivers/base/node.c
+++ b/drivers/base/node.c
@@ -439,7 +439,7 @@ static ssize_t node_read_meminfo(struct device *dev,
 		       nid, K(i.sharedram),
 		       nid, sum_zone_node_page_state(nid, NR_KERNEL_STACK_KB),
 		       nid, K(sum_zone_node_page_state(nid, NR_PAGETABLE)),
-		       nid, K(node_page_state(pgdat, NR_UNSTABLE_NFS)),
+		       nid, 0,
 		       nid, K(sum_zone_node_page_state(nid, NR_BOUNCE)),
 		       nid, K(node_page_state(pgdat, NR_WRITEBACK_TEMP)),
 		       nid, K(sreclaimable +
diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 76ac9c7d32ec..c5bdf46e3b4b 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -1070,7 +1070,6 @@ static void bdi_split_work_to_wbs(struct backing_dev_info *bdi,
 static unsigned long get_nr_dirty_pages(void)
 {
 	return global_node_page_state(NR_FILE_DIRTY) +
-		global_node_page_state(NR_UNSTABLE_NFS) +
 		get_nr_dirty_inodes();
 }
 
diff --git a/fs/nfs/internal.h b/fs/nfs/internal.h
index 1f32a9fbfdaf..6673a77884d9 100644
--- a/fs/nfs/internal.h
+++ b/fs/nfs/internal.h
@@ -668,7 +668,8 @@ void nfs_super_set_maxbytes(struct super_block *sb, __u64 maxfilesize)
 }
 
 /*
- * Record the page as unstable and mark its inode as dirty.
+ * Record the page as unstable (an extra writeback period) and mark its
+ * inode as dirty.
  */
 static inline
 void nfs_mark_page_unstable(struct page *page, struct nfs_commit_info *cinfo)
@@ -676,8 +677,11 @@ void nfs_mark_page_unstable(struct page *page, struct nfs_commit_info *cinfo)
 	if (!cinfo->dreq) {
 		struct inode *inode = page_file_mapping(page)->host;
 
-		inc_node_page_state(page, NR_UNSTABLE_NFS);
-		inc_wb_stat(&inode_to_bdi(inode)->wb, WB_RECLAIMABLE);
+		/* This page is really still in write-back - just that the
+		 * writeback is happening on the server now.
+		 */
+		inc_node_page_state(page, NR_WRITEBACK);
+		inc_wb_stat(&inode_to_bdi(inode)->wb, WB_WRITEBACK);
 		__mark_inode_dirty(inode, I_DIRTY_DATASYNC);
 	}
 }
diff --git a/fs/nfs/write.c b/fs/nfs/write.c
index df4b87c30ac9..d9ea824accb7 100644
--- a/fs/nfs/write.c
+++ b/fs/nfs/write.c
@@ -946,9 +946,9 @@ nfs_mark_request_commit(struct nfs_page *req, struct pnfs_layout_segment *lseg,
 static void
 nfs_clear_page_commit(struct page *page)
 {
-	dec_node_page_state(page, NR_UNSTABLE_NFS);
+	dec_node_page_state(page, NR_WRITEBACK);
 	dec_wb_stat(&inode_to_bdi(page_file_mapping(page)->host)->wb,
-		    WB_RECLAIMABLE);
+		    WB_WRITEBACK);
 }
 
 /* Called holding the request lock on @req */
diff --git a/fs/proc/meminfo.c b/fs/proc/meminfo.c
index 8c1f1bb1a5ce..9bd94b5a9658 100644
--- a/fs/proc/meminfo.c
+++ b/fs/proc/meminfo.c
@@ -106,8 +106,7 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
 	show_val_kb(m, "PageTables:     ",
 		    global_zone_page_state(NR_PAGETABLE));
 
-	show_val_kb(m, "NFS_Unstable:   ",
-		    global_node_page_state(NR_UNSTABLE_NFS));
+	show_val_kb(m, "NFS_Unstable:   ", 0);
 	show_val_kb(m, "Bounce:         ",
 		    global_zone_page_state(NR_BOUNCE));
 	show_val_kb(m, "WritebackTmp:   ",
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 1b9de7d220fb..a89f47515eb1 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -193,7 +193,6 @@ enum node_stat_item {
 	NR_FILE_THPS,
 	NR_FILE_PMDMAPPED,
 	NR_ANON_THPS,
-	NR_UNSTABLE_NFS,	/* NFS unstable pages */
 	NR_VMSCAN_WRITE,
 	NR_VMSCAN_IMMEDIATE,	/* Prioritise for reclaim when writeback ends */
 	NR_DIRTIED,		/* page dirtyings since bootup */
diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
index 85a33bea76f1..10f5d1fa7347 100644
--- a/include/trace/events/writeback.h
+++ b/include/trace/events/writeback.h
@@ -541,7 +541,6 @@ TRACE_EVENT(global_dirty_state,
 	TP_STRUCT__entry(
 		__field(unsigned long,	nr_dirty)
 		__field(unsigned long,	nr_writeback)
-		__field(unsigned long,	nr_unstable)
 		__field(unsigned long,	background_thresh)
 		__field(unsigned long,	dirty_thresh)
 		__field(unsigned long,	dirty_limit)
@@ -552,7 +551,6 @@ TRACE_EVENT(global_dirty_state,
 	TP_fast_assign(
 		__entry->nr_dirty	= global_node_page_state(NR_FILE_DIRTY);
 		__entry->nr_writeback	= global_node_page_state(NR_WRITEBACK);
-		__entry->nr_unstable	= global_node_page_state(NR_UNSTABLE_NFS);
 		__entry->nr_dirtied	= global_node_page_state(NR_DIRTIED);
 		__entry->nr_written	= global_node_page_state(NR_WRITTEN);
 		__entry->background_thresh = background_thresh;
@@ -560,12 +558,11 @@ TRACE_EVENT(global_dirty_state,
 		__entry->dirty_limit	= global_wb_domain.dirty_limit;
 	),
 
-	TP_printk("dirty=%lu writeback=%lu unstable=%lu "
+	TP_printk("dirty=%lu writeback=%lu "
 		  "bg_thresh=%lu thresh=%lu limit=%lu "
 		  "dirtied=%lu written=%lu",
 		  __entry->nr_dirty,
 		  __entry->nr_writeback,
-		  __entry->nr_unstable,
 		  __entry->background_thresh,
 		  __entry->dirty_thresh,
 		  __entry->dirty_limit,
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index a3b97f103966..1db4b285c407 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4330,7 +4330,6 @@ void mem_cgroup_wb_stats(struct bdi_writeback *wb, unsigned long *pfilepages,
 
 	*pdirty = memcg_exact_page_state(memcg, NR_FILE_DIRTY);
 
-	/* this should eventually include NR_UNSTABLE_NFS */
 	*pwriteback = memcg_exact_page_state(memcg, NR_WRITEBACK);
 	*pfilepages = memcg_exact_page_state(memcg, NR_INACTIVE_FILE) +
 			memcg_exact_page_state(memcg, NR_ACTIVE_FILE);
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index f02a71797781..d11b097c8002 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -504,7 +504,6 @@ bool node_dirty_ok(struct pglist_data *pgdat)
 	unsigned long nr_pages = 0;
 
 	nr_pages += node_page_state(pgdat, NR_FILE_DIRTY);
-	nr_pages += node_page_state(pgdat, NR_UNSTABLE_NFS);
 	nr_pages += node_page_state(pgdat, NR_WRITEBACK);
 
 	return nr_pages <= limit;
@@ -758,7 +757,7 @@ static void mdtc_calc_avail(struct dirty_throttle_control *mdtc,
  * bounded by the bdi->min_ratio and/or bdi->max_ratio parameters, if set.
  *
  * Return: @wb's dirty limit in pages. The term "dirty" in the context of
- * dirty balancing includes all PG_dirty, PG_writeback and NFS unstable pages.
+ * dirty balancing includes all PG_dirty and PG_writeback pages.
  */
 static unsigned long __wb_calc_thresh(struct dirty_throttle_control *dtc)
 {
@@ -1566,7 +1565,7 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
 	struct dirty_throttle_control * const mdtc = mdtc_valid(&mdtc_stor) ?
 						     &mdtc_stor : NULL;
 	struct dirty_throttle_control *sdtc;
-	unsigned long nr_reclaimable;	/* = file_dirty + unstable_nfs */
+	unsigned long nr_reclaimable;	/* = file_dirty */
 	long period;
 	long pause;
 	long max_pause;
@@ -1586,14 +1585,7 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
 		unsigned long m_thresh = 0;
 		unsigned long m_bg_thresh = 0;
 
-		/*
-		 * Unstable writes are a feature of certain networked
-		 * filesystems (i.e. NFS) in which data may have been
-		 * written to the server's write cache, but has not yet
-		 * been flushed to permanent storage.
-		 */
-		nr_reclaimable = global_node_page_state(NR_FILE_DIRTY) +
-					global_node_page_state(NR_UNSTABLE_NFS);
+		nr_reclaimable = global_node_page_state(NR_FILE_DIRTY);
 		gdtc->avail = global_dirtyable_memory();
 		gdtc->dirty = nr_reclaimable + global_node_page_state(NR_WRITEBACK);
 
@@ -1963,8 +1955,7 @@ bool wb_over_bg_thresh(struct bdi_writeback *wb)
 	 * as we're trying to decide whether to put more under writeback.
 	 */
 	gdtc->avail = global_dirtyable_memory();
-	gdtc->dirty = global_node_page_state(NR_FILE_DIRTY) +
-		      global_node_page_state(NR_UNSTABLE_NFS);
+	gdtc->dirty = global_node_page_state(NR_FILE_DIRTY);
 	domain_dirty_limits(gdtc);
 
 	if (gdtc->dirty > gdtc->bg_thresh)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 13cc653122b7..cc406ee17ad9 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5319,7 +5319,7 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
 
 	printk("active_anon:%lu inactive_anon:%lu isolated_anon:%lu\n"
 		" active_file:%lu inactive_file:%lu isolated_file:%lu\n"
-		" unevictable:%lu dirty:%lu writeback:%lu unstable:%lu\n"
+		" unevictable:%lu dirty:%lu writeback:%lu\n"
 		" slab_reclaimable:%lu slab_unreclaimable:%lu\n"
 		" mapped:%lu shmem:%lu pagetables:%lu bounce:%lu\n"
 		" free:%lu free_pcp:%lu free_cma:%lu\n",
@@ -5332,7 +5332,6 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
 		global_node_page_state(NR_UNEVICTABLE),
 		global_node_page_state(NR_FILE_DIRTY),
 		global_node_page_state(NR_WRITEBACK),
-		global_node_page_state(NR_UNSTABLE_NFS),
 		global_node_page_state(NR_SLAB_RECLAIMABLE),
 		global_node_page_state(NR_SLAB_UNRECLAIMABLE),
 		global_node_page_state(NR_FILE_MAPPED),
@@ -5365,7 +5364,6 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
 			" anon_thp: %lukB"
 #endif
 			" writeback_tmp:%lukB"
-			" unstable:%lukB"
 			" all_unreclaimable? %s"
 			"\n",
 			pgdat->node_id,
@@ -5387,7 +5385,6 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
 			K(node_page_state(pgdat, NR_ANON_THPS) * HPAGE_PMD_NR),
 #endif
 			K(node_page_state(pgdat, NR_WRITEBACK_TEMP)),
-			K(node_page_state(pgdat, NR_UNSTABLE_NFS)),
 			pgdat->kswapd_failures >= MAX_RECLAIM_RETRIES ?
 				"yes" : "no");
 	}
diff --git a/mm/vmstat.c b/mm/vmstat.c
index 96d21a792b57..6c719f184843 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1108,7 +1108,7 @@ int fragmentation_index(struct zone *zone, unsigned int order)
 					TEXT_FOR_HIGHMEM(xx) xx "_movable",
 
 const char * const vmstat_text[] = {
-	/* enum zone_stat_item countes */
+	/* enum zone_stat_item counters */
 	"nr_free_pages",
 	"nr_zone_inactive_anon",
 	"nr_zone_active_anon",
@@ -1162,7 +1162,6 @@ const char * const vmstat_text[] = {
 	"nr_file_hugepages",
 	"nr_file_pmdmapped",
 	"nr_anon_transparent_hugepages",
-	"nr_unstable",
 	"nr_vmscan_write",
 	"nr_vmscan_immediate_reclaim",
 	"nr_dirtied",
@@ -1723,6 +1722,14 @@ static int vmstat_show(struct seq_file *m, void *arg)
 	seq_puts(m, vmstat_text[off]);
 	seq_put_decimal_ull(m, " ", *l);
 	seq_putc(m, '\n');
+
+	if (off == NR_VMSTAT_ITEMS - 1) {
+		/* We've come to the end - add any deprecated counters
+		 * to avoid breaking userspace which might depend on
+		 * them being present.
+		 */
+		seq_puts(m, "nr_unstable 0\n");
+	}
 	return 0;
 }
 
-- 
2.26.2


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Re: [PATCH 2/2 V4] MM: Discard NR_UNSTABLE_NFS, use NR_WRITEBACK instead.

[Jan Kara]

On Wed 13-05-20 17:18:29, NeilBrown wrote:
> 
> After an NFS page has been written it is considered "unstable" until a
> COMMIT request succeeds.  If the COMMIT fails, the page will be
> re-written.
> 
> These "unstable" pages are currently accounted as "reclaimable", either
> in WB_RECLAIMABLE, or in NR_UNSTABLE_NFS which is included in a
> 'reclaimable' count.  This might have made sense when sending the COMMIT
> required a separate action by the VFS/MM (e.g.  releasepage() used to
> send a COMMIT).  However now that all writes generated by ->writepages()
> will automatically be followed by a COMMIT (since commit 919e3bd9a875
> ("NFS: Ensure we commit after writeback is complete")) it makes more
> sense to treat them as writeback pages.
> 
> So this patch removes NR_UNSTABLE_NFS and accounts unstable pages in
> NR_WRITEBACK and WB_WRITEBACK.
> 
> A particular effect of this change is that when
> wb_check_background_flush() calls wb_over_bg_threshold(), the latter
> will report 'true' a lot less often as the 'unstable' pages are no
> longer considered 'dirty' (as there is nothing that writeback can do
> about them anyway).
> 
> Currently wb_check_background_flush() will trigger writeback to NFS even
> when there are relatively few dirty pages (if there are lots of unstable
> pages), this can result in small writes going to the server (10s of
> Kilobytes rather than a Megabyte) which hurts throughput.
> With this patch, there are fewer writes which are each larger on average.
> 
> Where the NR_UNSTABLE_NFS count was included in statistics
> virtual-files, the entry is retained, but the value is hard-coded as
> zero.  static trace points and warning printks which mentioned this
> counter no longer report it.
> 
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> Acked-by: Trond Myklebust <trond.myklebust@hammerspace.com>
> Acked-by: Michal Hocko <mhocko@suse.com> # for MM parts
> Signed-off-by: NeilBrown <neilb@suse.de>

The patch looks good to me. You can add:

Reviewed-by: Jan Kara <jack@suse.cz>

									Honza

> ---
>  Documentation/filesystems/proc.rst |  4 ++--
>  drivers/base/node.c                |  2 +-
>  fs/fs-writeback.c                  |  1 -
>  fs/nfs/internal.h                  | 10 +++++++---
>  fs/nfs/write.c                     |  4 ++--
>  fs/proc/meminfo.c                  |  3 +--
>  include/linux/mmzone.h             |  1 -
>  include/trace/events/writeback.h   |  5 +----
>  mm/memcontrol.c                    |  1 -
>  mm/page-writeback.c                | 17 ++++-------------
>  mm/page_alloc.c                    |  5 +----
>  mm/vmstat.c                        | 11 +++++++++--
>  12 files changed, 28 insertions(+), 36 deletions(-)
> 
> diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
> index 38b606991065..092b7b44d158 100644
> --- a/Documentation/filesystems/proc.rst
> +++ b/Documentation/filesystems/proc.rst
> @@ -1042,8 +1042,8 @@ PageTables
>                amount of memory dedicated to the lowest level of page
>                tables.
>  NFS_Unstable
> -              NFS pages sent to the server, but not yet committed to stable
> -	      storage
> +              Always zero. Previous counted pages which had been written to
> +              the server, but has not been committed to stable storage.
>  Bounce
>                Memory used for block device "bounce buffers"
>  WritebackTmp
> diff --git a/drivers/base/node.c b/drivers/base/node.c
> index 10d7e818e118..15f5ed6a8830 100644
> --- a/drivers/base/node.c
> +++ b/drivers/base/node.c
> @@ -439,7 +439,7 @@ static ssize_t node_read_meminfo(struct device *dev,
>  		       nid, K(i.sharedram),
>  		       nid, sum_zone_node_page_state(nid, NR_KERNEL_STACK_KB),
>  		       nid, K(sum_zone_node_page_state(nid, NR_PAGETABLE)),
> -		       nid, K(node_page_state(pgdat, NR_UNSTABLE_NFS)),
> +		       nid, 0,
>  		       nid, K(sum_zone_node_page_state(nid, NR_BOUNCE)),
>  		       nid, K(node_page_state(pgdat, NR_WRITEBACK_TEMP)),
>  		       nid, K(sreclaimable +
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 76ac9c7d32ec..c5bdf46e3b4b 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -1070,7 +1070,6 @@ static void bdi_split_work_to_wbs(struct backing_dev_info *bdi,
>  static unsigned long get_nr_dirty_pages(void)
>  {
>  	return global_node_page_state(NR_FILE_DIRTY) +
> -		global_node_page_state(NR_UNSTABLE_NFS) +
>  		get_nr_dirty_inodes();
>  }
>  
> diff --git a/fs/nfs/internal.h b/fs/nfs/internal.h
> index 1f32a9fbfdaf..6673a77884d9 100644
> --- a/fs/nfs/internal.h
> +++ b/fs/nfs/internal.h
> @@ -668,7 +668,8 @@ void nfs_super_set_maxbytes(struct super_block *sb, __u64 maxfilesize)
>  }
>  
>  /*
> - * Record the page as unstable and mark its inode as dirty.
> + * Record the page as unstable (an extra writeback period) and mark its
> + * inode as dirty.
>   */
>  static inline
>  void nfs_mark_page_unstable(struct page *page, struct nfs_commit_info *cinfo)
> @@ -676,8 +677,11 @@ void nfs_mark_page_unstable(struct page *page, struct nfs_commit_info *cinfo)
>  	if (!cinfo->dreq) {
>  		struct inode *inode = page_file_mapping(page)->host;
>  
> -		inc_node_page_state(page, NR_UNSTABLE_NFS);
> -		inc_wb_stat(&inode_to_bdi(inode)->wb, WB_RECLAIMABLE);
> +		/* This page is really still in write-back - just that the
> +		 * writeback is happening on the server now.
> +		 */
> +		inc_node_page_state(page, NR_WRITEBACK);
> +		inc_wb_stat(&inode_to_bdi(inode)->wb, WB_WRITEBACK);
>  		__mark_inode_dirty(inode, I_DIRTY_DATASYNC);
>  	}
>  }
> diff --git a/fs/nfs/write.c b/fs/nfs/write.c
> index df4b87c30ac9..d9ea824accb7 100644
> --- a/fs/nfs/write.c
> +++ b/fs/nfs/write.c
> @@ -946,9 +946,9 @@ nfs_mark_request_commit(struct nfs_page *req, struct pnfs_layout_segment *lseg,
>  static void
>  nfs_clear_page_commit(struct page *page)
>  {
> -	dec_node_page_state(page, NR_UNSTABLE_NFS);
> +	dec_node_page_state(page, NR_WRITEBACK);
>  	dec_wb_stat(&inode_to_bdi(page_file_mapping(page)->host)->wb,
> -		    WB_RECLAIMABLE);
> +		    WB_WRITEBACK);
>  }
>  
>  /* Called holding the request lock on @req */
> diff --git a/fs/proc/meminfo.c b/fs/proc/meminfo.c
> index 8c1f1bb1a5ce..9bd94b5a9658 100644
> --- a/fs/proc/meminfo.c
> +++ b/fs/proc/meminfo.c
> @@ -106,8 +106,7 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
>  	show_val_kb(m, "PageTables:     ",
>  		    global_zone_page_state(NR_PAGETABLE));
>  
> -	show_val_kb(m, "NFS_Unstable:   ",
> -		    global_node_page_state(NR_UNSTABLE_NFS));
> +	show_val_kb(m, "NFS_Unstable:   ", 0);
>  	show_val_kb(m, "Bounce:         ",
>  		    global_zone_page_state(NR_BOUNCE));
>  	show_val_kb(m, "WritebackTmp:   ",
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 1b9de7d220fb..a89f47515eb1 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -193,7 +193,6 @@ enum node_stat_item {
>  	NR_FILE_THPS,
>  	NR_FILE_PMDMAPPED,
>  	NR_ANON_THPS,
> -	NR_UNSTABLE_NFS,	/* NFS unstable pages */
>  	NR_VMSCAN_WRITE,
>  	NR_VMSCAN_IMMEDIATE,	/* Prioritise for reclaim when writeback ends */
>  	NR_DIRTIED,		/* page dirtyings since bootup */
> diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
> index 85a33bea76f1..10f5d1fa7347 100644
> --- a/include/trace/events/writeback.h
> +++ b/include/trace/events/writeback.h
> @@ -541,7 +541,6 @@ TRACE_EVENT(global_dirty_state,
>  	TP_STRUCT__entry(
>  		__field(unsigned long,	nr_dirty)
>  		__field(unsigned long,	nr_writeback)
> -		__field(unsigned long,	nr_unstable)
>  		__field(unsigned long,	background_thresh)
>  		__field(unsigned long,	dirty_thresh)
>  		__field(unsigned long,	dirty_limit)
> @@ -552,7 +551,6 @@ TRACE_EVENT(global_dirty_state,
>  	TP_fast_assign(
>  		__entry->nr_dirty	= global_node_page_state(NR_FILE_DIRTY);
>  		__entry->nr_writeback	= global_node_page_state(NR_WRITEBACK);
> -		__entry->nr_unstable	= global_node_page_state(NR_UNSTABLE_NFS);
>  		__entry->nr_dirtied	= global_node_page_state(NR_DIRTIED);
>  		__entry->nr_written	= global_node_page_state(NR_WRITTEN);
>  		__entry->background_thresh = background_thresh;
> @@ -560,12 +558,11 @@ TRACE_EVENT(global_dirty_state,
>  		__entry->dirty_limit	= global_wb_domain.dirty_limit;
>  	),
>  
> -	TP_printk("dirty=%lu writeback=%lu unstable=%lu "
> +	TP_printk("dirty=%lu writeback=%lu "
>  		  "bg_thresh=%lu thresh=%lu limit=%lu "
>  		  "dirtied=%lu written=%lu",
>  		  __entry->nr_dirty,
>  		  __entry->nr_writeback,
> -		  __entry->nr_unstable,
>  		  __entry->background_thresh,
>  		  __entry->dirty_thresh,
>  		  __entry->dirty_limit,
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index a3b97f103966..1db4b285c407 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4330,7 +4330,6 @@ void mem_cgroup_wb_stats(struct bdi_writeback *wb, unsigned long *pfilepages,
>  
>  	*pdirty = memcg_exact_page_state(memcg, NR_FILE_DIRTY);
>  
> -	/* this should eventually include NR_UNSTABLE_NFS */
>  	*pwriteback = memcg_exact_page_state(memcg, NR_WRITEBACK);
>  	*pfilepages = memcg_exact_page_state(memcg, NR_INACTIVE_FILE) +
>  			memcg_exact_page_state(memcg, NR_ACTIVE_FILE);
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index f02a71797781..d11b097c8002 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -504,7 +504,6 @@ bool node_dirty_ok(struct pglist_data *pgdat)
>  	unsigned long nr_pages = 0;
>  
>  	nr_pages += node_page_state(pgdat, NR_FILE_DIRTY);
> -	nr_pages += node_page_state(pgdat, NR_UNSTABLE_NFS);
>  	nr_pages += node_page_state(pgdat, NR_WRITEBACK);
>  
>  	return nr_pages <= limit;
> @@ -758,7 +757,7 @@ static void mdtc_calc_avail(struct dirty_throttle_control *mdtc,
>   * bounded by the bdi->min_ratio and/or bdi->max_ratio parameters, if set.
>   *
>   * Return: @wb's dirty limit in pages. The term "dirty" in the context of
> - * dirty balancing includes all PG_dirty, PG_writeback and NFS unstable pages.
> + * dirty balancing includes all PG_dirty and PG_writeback pages.
>   */
>  static unsigned long __wb_calc_thresh(struct dirty_throttle_control *dtc)
>  {
> @@ -1566,7 +1565,7 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
>  	struct dirty_throttle_control * const mdtc = mdtc_valid(&mdtc_stor) ?
>  						     &mdtc_stor : NULL;
>  	struct dirty_throttle_control *sdtc;
> -	unsigned long nr_reclaimable;	/* = file_dirty + unstable_nfs */
> +	unsigned long nr_reclaimable;	/* = file_dirty */
>  	long period;
>  	long pause;
>  	long max_pause;
> @@ -1586,14 +1585,7 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
>  		unsigned long m_thresh = 0;
>  		unsigned long m_bg_thresh = 0;
>  
> -		/*
> -		 * Unstable writes are a feature of certain networked
> -		 * filesystems (i.e. NFS) in which data may have been
> -		 * written to the server's write cache, but has not yet
> -		 * been flushed to permanent storage.
> -		 */
> -		nr_reclaimable = global_node_page_state(NR_FILE_DIRTY) +
> -					global_node_page_state(NR_UNSTABLE_NFS);
> +		nr_reclaimable = global_node_page_state(NR_FILE_DIRTY);
>  		gdtc->avail = global_dirtyable_memory();
>  		gdtc->dirty = nr_reclaimable + global_node_page_state(NR_WRITEBACK);
>  
> @@ -1963,8 +1955,7 @@ bool wb_over_bg_thresh(struct bdi_writeback *wb)
>  	 * as we're trying to decide whether to put more under writeback.
>  	 */
>  	gdtc->avail = global_dirtyable_memory();
> -	gdtc->dirty = global_node_page_state(NR_FILE_DIRTY) +
> -		      global_node_page_state(NR_UNSTABLE_NFS);
> +	gdtc->dirty = global_node_page_state(NR_FILE_DIRTY);
>  	domain_dirty_limits(gdtc);
>  
>  	if (gdtc->dirty > gdtc->bg_thresh)
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 13cc653122b7..cc406ee17ad9 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5319,7 +5319,7 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
>  
>  	printk("active_anon:%lu inactive_anon:%lu isolated_anon:%lu\n"
>  		" active_file:%lu inactive_file:%lu isolated_file:%lu\n"
> -		" unevictable:%lu dirty:%lu writeback:%lu unstable:%lu\n"
> +		" unevictable:%lu dirty:%lu writeback:%lu\n"
>  		" slab_reclaimable:%lu slab_unreclaimable:%lu\n"
>  		" mapped:%lu shmem:%lu pagetables:%lu bounce:%lu\n"
>  		" free:%lu free_pcp:%lu free_cma:%lu\n",
> @@ -5332,7 +5332,6 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
>  		global_node_page_state(NR_UNEVICTABLE),
>  		global_node_page_state(NR_FILE_DIRTY),
>  		global_node_page_state(NR_WRITEBACK),
> -		global_node_page_state(NR_UNSTABLE_NFS),
>  		global_node_page_state(NR_SLAB_RECLAIMABLE),
>  		global_node_page_state(NR_SLAB_UNRECLAIMABLE),
>  		global_node_page_state(NR_FILE_MAPPED),
> @@ -5365,7 +5364,6 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
>  			" anon_thp: %lukB"
>  #endif
>  			" writeback_tmp:%lukB"
> -			" unstable:%lukB"
>  			" all_unreclaimable? %s"
>  			"\n",
>  			pgdat->node_id,
> @@ -5387,7 +5385,6 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
>  			K(node_page_state(pgdat, NR_ANON_THPS) * HPAGE_PMD_NR),
>  #endif
>  			K(node_page_state(pgdat, NR_WRITEBACK_TEMP)),
> -			K(node_page_state(pgdat, NR_UNSTABLE_NFS)),
>  			pgdat->kswapd_failures >= MAX_RECLAIM_RETRIES ?
>  				"yes" : "no");
>  	}
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index 96d21a792b57..6c719f184843 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -1108,7 +1108,7 @@ int fragmentation_index(struct zone *zone, unsigned int order)
>  					TEXT_FOR_HIGHMEM(xx) xx "_movable",
>  
>  const char * const vmstat_text[] = {
> -	/* enum zone_stat_item countes */
> +	/* enum zone_stat_item counters */
>  	"nr_free_pages",
>  	"nr_zone_inactive_anon",
>  	"nr_zone_active_anon",
> @@ -1162,7 +1162,6 @@ const char * const vmstat_text[] = {
>  	"nr_file_hugepages",
>  	"nr_file_pmdmapped",
>  	"nr_anon_transparent_hugepages",
> -	"nr_unstable",
>  	"nr_vmscan_write",
>  	"nr_vmscan_immediate_reclaim",
>  	"nr_dirtied",
> @@ -1723,6 +1722,14 @@ static int vmstat_show(struct seq_file *m, void *arg)
>  	seq_puts(m, vmstat_text[off]);
>  	seq_put_decimal_ull(m, " ", *l);
>  	seq_putc(m, '\n');
> +
> +	if (off == NR_VMSTAT_ITEMS - 1) {
> +		/* We've come to the end - add any deprecated counters
> +		 * to avoid breaking userspace which might depend on
> +		 * them being present.
> +		 */
> +		seq_puts(m, "nr_unstable 0\n");
> +	}
>  	return 0;
>  }
>  
> -- 
> 2.26.2
> 


-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

[PATCH 2/2 V3] MM: Discard NR_UNSTABLE_NFS, use NR_WRITEBACK instead.

[NeilBrown]

[-- Attachment #1: Type: text/plain, Size: 13918 bytes --]


After an NFS page has been written it is considered "unstable" until a
COMMIT request succeeds.  If the COMMIT fails, the page will be
re-written.

These "unstable" pages are currently accounted as "reclaimable", either
in WB_RECLAIMABLE, or in NR_UNSTABLE_NFS which is included in a
'reclaimable' count.  This might have made sense when sending the COMMIT
required a separate action by the VFS/MM (e.g.  releasepage() used to
send a COMMIT).  However now that all writes generated by ->writepages()
will automatically be followed by a COMMIT (since commit 919e3bd9a875
("NFS: Ensure we commit after writeback is complete")) it makes more
sense to treat them as writeback pages.

So this patch removes NR_UNSTABLE_NFS and accounts unstable pages in
NR_WRITEBACK and WB_WRITEBACK.

A particular effect of this change is that when
wb_check_background_flush() calls wb_over_bg_threshold(), the latter
will report 'true' a lot less often as the 'unstable' pages are no
longer considered 'dirty' (as there is nothing that writeback can do
about them anyway).

Currently wb_check_background_flush() will trigger writeback to NFS even
when there are relatively few dirty pages (if there are lots of unstable
pages), this can result in small writes going to the server (10s of
Kilobytes rather than a Megabyte) which hurts throughput.
With this patch, there are fewer writes which are each larger on average.

Where the NR_UNSTABLE_NFS count was included in statistics
virtual-files, the entry is retained, but the value is hard-coded as
zero.  static trace points and warning printks which mentioned this
counter no longer report it.

Acked-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Acked-by: Michal Hocko <mhocko@suse.com> # for MM parts
Signed-off-by: NeilBrown <neilb@suse.de>
---
 Documentation/filesystems/proc.rst |  4 ++--
 drivers/base/node.c                |  2 +-
 fs/fs-writeback.c                  |  1 -
 fs/nfs/internal.h                  | 10 +++++++---
 fs/nfs/write.c                     |  4 ++--
 fs/proc/meminfo.c                  |  3 +--
 include/linux/mmzone.h             |  1 -
 include/trace/events/writeback.h   |  5 +----
 mm/memcontrol.c                    |  1 -
 mm/page-writeback.c                | 17 ++++-------------
 mm/page_alloc.c                    |  5 +----
 mm/vmstat.c                        | 11 +++++++++--
 12 files changed, 28 insertions(+), 36 deletions(-)

diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
index 38b606991065..092b7b44d158 100644
--- a/Documentation/filesystems/proc.rst
+++ b/Documentation/filesystems/proc.rst
@@ -1042,8 +1042,8 @@ PageTables
               amount of memory dedicated to the lowest level of page
               tables.
 NFS_Unstable
-              NFS pages sent to the server, but not yet committed to stable
-	      storage
+              Always zero. Previous counted pages which had been written to
+              the server, but has not been committed to stable storage.
 Bounce
               Memory used for block device "bounce buffers"
 WritebackTmp
diff --git a/drivers/base/node.c b/drivers/base/node.c
index 10d7e818e118..15f5ed6a8830 100644
--- a/drivers/base/node.c
+++ b/drivers/base/node.c
@@ -439,7 +439,7 @@ static ssize_t node_read_meminfo(struct device *dev,
 		       nid, K(i.sharedram),
 		       nid, sum_zone_node_page_state(nid, NR_KERNEL_STACK_KB),
 		       nid, K(sum_zone_node_page_state(nid, NR_PAGETABLE)),
-		       nid, K(node_page_state(pgdat, NR_UNSTABLE_NFS)),
+		       nid, 0,
 		       nid, K(sum_zone_node_page_state(nid, NR_BOUNCE)),
 		       nid, K(node_page_state(pgdat, NR_WRITEBACK_TEMP)),
 		       nid, K(sreclaimable +
diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 76ac9c7d32ec..c5bdf46e3b4b 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -1070,7 +1070,6 @@ static void bdi_split_work_to_wbs(struct backing_dev_info *bdi,
 static unsigned long get_nr_dirty_pages(void)
 {
 	return global_node_page_state(NR_FILE_DIRTY) +
-		global_node_page_state(NR_UNSTABLE_NFS) +
 		get_nr_dirty_inodes();
 }
 
diff --git a/fs/nfs/internal.h b/fs/nfs/internal.h
index 1f32a9fbfdaf..6673a77884d9 100644
--- a/fs/nfs/internal.h
+++ b/fs/nfs/internal.h
@@ -668,7 +668,8 @@ void nfs_super_set_maxbytes(struct super_block *sb, __u64 maxfilesize)
 }
 
 /*
- * Record the page as unstable and mark its inode as dirty.
+ * Record the page as unstable (an extra writeback period) and mark its
+ * inode as dirty.
  */
 static inline
 void nfs_mark_page_unstable(struct page *page, struct nfs_commit_info *cinfo)
@@ -676,8 +677,11 @@ void nfs_mark_page_unstable(struct page *page, struct nfs_commit_info *cinfo)
 	if (!cinfo->dreq) {
 		struct inode *inode = page_file_mapping(page)->host;
 
-		inc_node_page_state(page, NR_UNSTABLE_NFS);
-		inc_wb_stat(&inode_to_bdi(inode)->wb, WB_RECLAIMABLE);
+		/* This page is really still in write-back - just that the
+		 * writeback is happening on the server now.
+		 */
+		inc_node_page_state(page, NR_WRITEBACK);
+		inc_wb_stat(&inode_to_bdi(inode)->wb, WB_WRITEBACK);
 		__mark_inode_dirty(inode, I_DIRTY_DATASYNC);
 	}
 }
diff --git a/fs/nfs/write.c b/fs/nfs/write.c
index df4b87c30ac9..d9ea824accb7 100644
--- a/fs/nfs/write.c
+++ b/fs/nfs/write.c
@@ -946,9 +946,9 @@ nfs_mark_request_commit(struct nfs_page *req, struct pnfs_layout_segment *lseg,
 static void
 nfs_clear_page_commit(struct page *page)
 {
-	dec_node_page_state(page, NR_UNSTABLE_NFS);
+	dec_node_page_state(page, NR_WRITEBACK);
 	dec_wb_stat(&inode_to_bdi(page_file_mapping(page)->host)->wb,
-		    WB_RECLAIMABLE);
+		    WB_WRITEBACK);
 }
 
 /* Called holding the request lock on @req */
diff --git a/fs/proc/meminfo.c b/fs/proc/meminfo.c
index 8c1f1bb1a5ce..9bd94b5a9658 100644
--- a/fs/proc/meminfo.c
+++ b/fs/proc/meminfo.c
@@ -106,8 +106,7 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
 	show_val_kb(m, "PageTables:     ",
 		    global_zone_page_state(NR_PAGETABLE));
 
-	show_val_kb(m, "NFS_Unstable:   ",
-		    global_node_page_state(NR_UNSTABLE_NFS));
+	show_val_kb(m, "NFS_Unstable:   ", 0);
 	show_val_kb(m, "Bounce:         ",
 		    global_zone_page_state(NR_BOUNCE));
 	show_val_kb(m, "WritebackTmp:   ",
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 1b9de7d220fb..a89f47515eb1 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -193,7 +193,6 @@ enum node_stat_item {
 	NR_FILE_THPS,
 	NR_FILE_PMDMAPPED,
 	NR_ANON_THPS,
-	NR_UNSTABLE_NFS,	/* NFS unstable pages */
 	NR_VMSCAN_WRITE,
 	NR_VMSCAN_IMMEDIATE,	/* Prioritise for reclaim when writeback ends */
 	NR_DIRTIED,		/* page dirtyings since bootup */
diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
index d94def25e4dc..45b5fbdb1f62 100644
--- a/include/trace/events/writeback.h
+++ b/include/trace/events/writeback.h
@@ -542,7 +542,6 @@ TRACE_EVENT(global_dirty_state,
 	TP_STRUCT__entry(
 		__field(unsigned long,	nr_dirty)
 		__field(unsigned long,	nr_writeback)
-		__field(unsigned long,	nr_unstable)
 		__field(unsigned long,	background_thresh)
 		__field(unsigned long,	dirty_thresh)
 		__field(unsigned long,	dirty_limit)
@@ -553,7 +552,6 @@ TRACE_EVENT(global_dirty_state,
 	TP_fast_assign(
 		__entry->nr_dirty	= global_node_page_state(NR_FILE_DIRTY);
 		__entry->nr_writeback	= global_node_page_state(NR_WRITEBACK);
-		__entry->nr_unstable	= global_node_page_state(NR_UNSTABLE_NFS);
 		__entry->nr_dirtied	= global_node_page_state(NR_DIRTIED);
 		__entry->nr_written	= global_node_page_state(NR_WRITTEN);
 		__entry->background_thresh = background_thresh;
@@ -561,12 +559,11 @@ TRACE_EVENT(global_dirty_state,
 		__entry->dirty_limit	= global_wb_domain.dirty_limit;
 	),
 
-	TP_printk("dirty=%lu writeback=%lu unstable=%lu "
+	TP_printk("dirty=%lu writeback=%lu "
 		  "bg_thresh=%lu thresh=%lu limit=%lu "
 		  "dirtied=%lu written=%lu",
 		  __entry->nr_dirty,
 		  __entry->nr_writeback,
-		  __entry->nr_unstable,
 		  __entry->background_thresh,
 		  __entry->dirty_thresh,
 		  __entry->dirty_limit,
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 5beea03dd58a..2db5bbcfc17a 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4330,7 +4330,6 @@ void mem_cgroup_wb_stats(struct bdi_writeback *wb, unsigned long *pfilepages,
 
 	*pdirty = memcg_exact_page_state(memcg, NR_FILE_DIRTY);
 
-	/* this should eventually include NR_UNSTABLE_NFS */
 	*pwriteback = memcg_exact_page_state(memcg, NR_WRITEBACK);
 	*pfilepages = memcg_exact_page_state(memcg, NR_INACTIVE_FILE) +
 			memcg_exact_page_state(memcg, NR_ACTIVE_FILE);
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 9692c553526b..b3b08de01d12 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -504,7 +504,6 @@ bool node_dirty_ok(struct pglist_data *pgdat)
 	unsigned long nr_pages = 0;
 
 	nr_pages += node_page_state(pgdat, NR_FILE_DIRTY);
-	nr_pages += node_page_state(pgdat, NR_UNSTABLE_NFS);
 	nr_pages += node_page_state(pgdat, NR_WRITEBACK);
 
 	return nr_pages <= limit;
@@ -758,7 +757,7 @@ static void mdtc_calc_avail(struct dirty_throttle_control *mdtc,
  * bounded by the bdi->min_ratio and/or bdi->max_ratio parameters, if set.
  *
  * Return: @wb's dirty limit in pages. The term "dirty" in the context of
- * dirty balancing includes all PG_dirty, PG_writeback and NFS unstable pages.
+ * dirty balancing includes all PG_dirty and PG_writeback pages.
  */
 static unsigned long __wb_calc_thresh(struct dirty_throttle_control *dtc)
 {
@@ -1566,7 +1565,7 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
 	struct dirty_throttle_control * const mdtc = mdtc_valid(&mdtc_stor) ?
 						     &mdtc_stor : NULL;
 	struct dirty_throttle_control *sdtc;
-	unsigned long nr_reclaimable;	/* = file_dirty + unstable_nfs */
+	unsigned long nr_reclaimable;	/* = file_dirty */
 	long period;
 	long pause;
 	long max_pause;
@@ -1586,14 +1585,7 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
 		unsigned long m_thresh = 0;
 		unsigned long m_bg_thresh = 0;
 
-		/*
-		 * Unstable writes are a feature of certain networked
-		 * filesystems (i.e. NFS) in which data may have been
-		 * written to the server's write cache, but has not yet
-		 * been flushed to permanent storage.
-		 */
-		nr_reclaimable = global_node_page_state(NR_FILE_DIRTY) +
-					global_node_page_state(NR_UNSTABLE_NFS);
+		nr_reclaimable = global_node_page_state(NR_FILE_DIRTY);
 		gdtc->avail = global_dirtyable_memory();
 		gdtc->dirty = nr_reclaimable + global_node_page_state(NR_WRITEBACK);
 
@@ -1948,8 +1940,7 @@ bool wb_over_bg_thresh(struct bdi_writeback *wb)
 	 * as we're trying to decide whether to put more under writeback.
 	 */
 	gdtc->avail = global_dirtyable_memory();
-	gdtc->dirty = global_node_page_state(NR_FILE_DIRTY) +
-		      global_node_page_state(NR_UNSTABLE_NFS);
+	gdtc->dirty = global_node_page_state(NR_FILE_DIRTY);
 	domain_dirty_limits(gdtc);
 
 	if (gdtc->dirty > gdtc->bg_thresh)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 69827d4fa052..238b5518f3c5 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5310,7 +5310,7 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
 
 	printk("active_anon:%lu inactive_anon:%lu isolated_anon:%lu\n"
 		" active_file:%lu inactive_file:%lu isolated_file:%lu\n"
-		" unevictable:%lu dirty:%lu writeback:%lu unstable:%lu\n"
+		" unevictable:%lu dirty:%lu writeback:%lu\n"
 		" slab_reclaimable:%lu slab_unreclaimable:%lu\n"
 		" mapped:%lu shmem:%lu pagetables:%lu bounce:%lu\n"
 		" free:%lu free_pcp:%lu free_cma:%lu\n",
@@ -5323,7 +5323,6 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
 		global_node_page_state(NR_UNEVICTABLE),
 		global_node_page_state(NR_FILE_DIRTY),
 		global_node_page_state(NR_WRITEBACK),
-		global_node_page_state(NR_UNSTABLE_NFS),
 		global_node_page_state(NR_SLAB_RECLAIMABLE),
 		global_node_page_state(NR_SLAB_UNRECLAIMABLE),
 		global_node_page_state(NR_FILE_MAPPED),
@@ -5356,7 +5355,6 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
 			" anon_thp: %lukB"
 #endif
 			" writeback_tmp:%lukB"
-			" unstable:%lukB"
 			" all_unreclaimable? %s"
 			"\n",
 			pgdat->node_id,
@@ -5378,7 +5376,6 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
 			K(node_page_state(pgdat, NR_ANON_THPS) * HPAGE_PMD_NR),
 #endif
 			K(node_page_state(pgdat, NR_WRITEBACK_TEMP)),
-			K(node_page_state(pgdat, NR_UNSTABLE_NFS)),
 			pgdat->kswapd_failures >= MAX_RECLAIM_RETRIES ?
 				"yes" : "no");
 	}
diff --git a/mm/vmstat.c b/mm/vmstat.c
index 96d21a792b57..6c719f184843 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1108,7 +1108,7 @@ int fragmentation_index(struct zone *zone, unsigned int order)
 					TEXT_FOR_HIGHMEM(xx) xx "_movable",
 
 const char * const vmstat_text[] = {
-	/* enum zone_stat_item countes */
+	/* enum zone_stat_item counters */
 	"nr_free_pages",
 	"nr_zone_inactive_anon",
 	"nr_zone_active_anon",
@@ -1162,7 +1162,6 @@ const char * const vmstat_text[] = {
 	"nr_file_hugepages",
 	"nr_file_pmdmapped",
 	"nr_anon_transparent_hugepages",
-	"nr_unstable",
 	"nr_vmscan_write",
 	"nr_vmscan_immediate_reclaim",
 	"nr_dirtied",
@@ -1723,6 +1722,14 @@ static int vmstat_show(struct seq_file *m, void *arg)
 	seq_puts(m, vmstat_text[off]);
 	seq_put_decimal_ull(m, " ", *l);
 	seq_putc(m, '\n');
+
+	if (off == NR_VMSTAT_ITEMS - 1) {
+		/* We've come to the end - add any deprecated counters
+		 * to avoid breaking userspace which might depend on
+		 * them being present.
+		 */
+		seq_puts(m, "nr_unstable 0\n");
+	}
 	return 0;
 }
 
-- 
2.26.0


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Re: [PATCH 2/2 V3] MM: Discard NR_UNSTABLE_NFS, use NR_WRITEBACK instead.

[Christoph Hellwig]

On Thu, Apr 16, 2020 at 10:31:27AM +1000, NeilBrown wrote:
>  	return global_node_page_state(NR_FILE_DIRTY) +
> -		global_node_page_state(NR_UNSTABLE_NFS) +
>  		get_nr_dirty_inodes();

Nit: this could a single line now:

  	return global_node_page_state(NR_FILE_DIRTY) + get_nr_dirty_inodes();

> +		/* This page is really still in write-back - just that the
> +		 * writeback is happening on the server now.
> +		 */

This needs to switch to the canonical kernel comment style.

> +	if (off == NR_VMSTAT_ITEMS - 1) {
> +		/* We've come to the end - add any deprecated counters
> +		 * to avoid breaking userspace which might depend on
> +		 * them being present.
> +		 */

Same here.

Otherwise looks good:

Reviewed-by: Christoph Hellwig <hch@lst.de>

Re: [PATCH 2/2 V3] MM: Discard NR_UNSTABLE_NFS, use NR_WRITEBACK instead.

[Jan Kara]

On Thu 16-04-20 10:31:27, NeilBrown wrote:
> 
> After an NFS page has been written it is considered "unstable" until a
> COMMIT request succeeds.  If the COMMIT fails, the page will be
> re-written.
> 
> These "unstable" pages are currently accounted as "reclaimable", either
> in WB_RECLAIMABLE, or in NR_UNSTABLE_NFS which is included in a
> 'reclaimable' count.  This might have made sense when sending the COMMIT
> required a separate action by the VFS/MM (e.g.  releasepage() used to
> send a COMMIT).  However now that all writes generated by ->writepages()
> will automatically be followed by a COMMIT (since commit 919e3bd9a875
> ("NFS: Ensure we commit after writeback is complete")) it makes more
> sense to treat them as writeback pages.
> 
> So this patch removes NR_UNSTABLE_NFS and accounts unstable pages in
> NR_WRITEBACK and WB_WRITEBACK.
> 
> A particular effect of this change is that when
> wb_check_background_flush() calls wb_over_bg_threshold(), the latter
> will report 'true' a lot less often as the 'unstable' pages are no
> longer considered 'dirty' (as there is nothing that writeback can do
> about them anyway).
> 
> Currently wb_check_background_flush() will trigger writeback to NFS even
> when there are relatively few dirty pages (if there are lots of unstable
> pages), this can result in small writes going to the server (10s of
> Kilobytes rather than a Megabyte) which hurts throughput.
> With this patch, there are fewer writes which are each larger on average.
> 
> Where the NR_UNSTABLE_NFS count was included in statistics
> virtual-files, the entry is retained, but the value is hard-coded as
> zero.  static trace points and warning printks which mentioned this
> counter no longer report it.
> 
> Acked-by: Trond Myklebust <trond.myklebust@hammerspace.com>
> Acked-by: Michal Hocko <mhocko@suse.com> # for MM parts
> Signed-off-by: NeilBrown <neilb@suse.de>

This looks good to me. You can add:

Reviewed-by: Jan Kara <jack@suse.cz>

BTW I prefer the comment style Christoph wants but I don't insist...

								Honza

> ---
>  Documentation/filesystems/proc.rst |  4 ++--
>  drivers/base/node.c                |  2 +-
>  fs/fs-writeback.c                  |  1 -
>  fs/nfs/internal.h                  | 10 +++++++---
>  fs/nfs/write.c                     |  4 ++--
>  fs/proc/meminfo.c                  |  3 +--
>  include/linux/mmzone.h             |  1 -
>  include/trace/events/writeback.h   |  5 +----
>  mm/memcontrol.c                    |  1 -
>  mm/page-writeback.c                | 17 ++++-------------
>  mm/page_alloc.c                    |  5 +----
>  mm/vmstat.c                        | 11 +++++++++--
>  12 files changed, 28 insertions(+), 36 deletions(-)
> 
> diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
> index 38b606991065..092b7b44d158 100644
> --- a/Documentation/filesystems/proc.rst
> +++ b/Documentation/filesystems/proc.rst
> @@ -1042,8 +1042,8 @@ PageTables
>                amount of memory dedicated to the lowest level of page
>                tables.
>  NFS_Unstable
> -              NFS pages sent to the server, but not yet committed to stable
> -	      storage
> +              Always zero. Previous counted pages which had been written to
> +              the server, but has not been committed to stable storage.
>  Bounce
>                Memory used for block device "bounce buffers"
>  WritebackTmp
> diff --git a/drivers/base/node.c b/drivers/base/node.c
> index 10d7e818e118..15f5ed6a8830 100644
> --- a/drivers/base/node.c
> +++ b/drivers/base/node.c
> @@ -439,7 +439,7 @@ static ssize_t node_read_meminfo(struct device *dev,
>  		       nid, K(i.sharedram),
>  		       nid, sum_zone_node_page_state(nid, NR_KERNEL_STACK_KB),
>  		       nid, K(sum_zone_node_page_state(nid, NR_PAGETABLE)),
> -		       nid, K(node_page_state(pgdat, NR_UNSTABLE_NFS)),
> +		       nid, 0,
>  		       nid, K(sum_zone_node_page_state(nid, NR_BOUNCE)),
>  		       nid, K(node_page_state(pgdat, NR_WRITEBACK_TEMP)),
>  		       nid, K(sreclaimable +
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 76ac9c7d32ec..c5bdf46e3b4b 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -1070,7 +1070,6 @@ static void bdi_split_work_to_wbs(struct backing_dev_info *bdi,
>  static unsigned long get_nr_dirty_pages(void)
>  {
>  	return global_node_page_state(NR_FILE_DIRTY) +
> -		global_node_page_state(NR_UNSTABLE_NFS) +
>  		get_nr_dirty_inodes();
>  }
>  
> diff --git a/fs/nfs/internal.h b/fs/nfs/internal.h
> index 1f32a9fbfdaf..6673a77884d9 100644
> --- a/fs/nfs/internal.h
> +++ b/fs/nfs/internal.h
> @@ -668,7 +668,8 @@ void nfs_super_set_maxbytes(struct super_block *sb, __u64 maxfilesize)
>  }
>  
>  /*
> - * Record the page as unstable and mark its inode as dirty.
> + * Record the page as unstable (an extra writeback period) and mark its
> + * inode as dirty.
>   */
>  static inline
>  void nfs_mark_page_unstable(struct page *page, struct nfs_commit_info *cinfo)
> @@ -676,8 +677,11 @@ void nfs_mark_page_unstable(struct page *page, struct nfs_commit_info *cinfo)
>  	if (!cinfo->dreq) {
>  		struct inode *inode = page_file_mapping(page)->host;
>  
> -		inc_node_page_state(page, NR_UNSTABLE_NFS);
> -		inc_wb_stat(&inode_to_bdi(inode)->wb, WB_RECLAIMABLE);
> +		/* This page is really still in write-back - just that the
> +		 * writeback is happening on the server now.
> +		 */
> +		inc_node_page_state(page, NR_WRITEBACK);
> +		inc_wb_stat(&inode_to_bdi(inode)->wb, WB_WRITEBACK);
>  		__mark_inode_dirty(inode, I_DIRTY_DATASYNC);
>  	}
>  }
> diff --git a/fs/nfs/write.c b/fs/nfs/write.c
> index df4b87c30ac9..d9ea824accb7 100644
> --- a/fs/nfs/write.c
> +++ b/fs/nfs/write.c
> @@ -946,9 +946,9 @@ nfs_mark_request_commit(struct nfs_page *req, struct pnfs_layout_segment *lseg,
>  static void
>  nfs_clear_page_commit(struct page *page)
>  {
> -	dec_node_page_state(page, NR_UNSTABLE_NFS);
> +	dec_node_page_state(page, NR_WRITEBACK);
>  	dec_wb_stat(&inode_to_bdi(page_file_mapping(page)->host)->wb,
> -		    WB_RECLAIMABLE);
> +		    WB_WRITEBACK);
>  }
>  
>  /* Called holding the request lock on @req */
> diff --git a/fs/proc/meminfo.c b/fs/proc/meminfo.c
> index 8c1f1bb1a5ce..9bd94b5a9658 100644
> --- a/fs/proc/meminfo.c
> +++ b/fs/proc/meminfo.c
> @@ -106,8 +106,7 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
>  	show_val_kb(m, "PageTables:     ",
>  		    global_zone_page_state(NR_PAGETABLE));
>  
> -	show_val_kb(m, "NFS_Unstable:   ",
> -		    global_node_page_state(NR_UNSTABLE_NFS));
> +	show_val_kb(m, "NFS_Unstable:   ", 0);
>  	show_val_kb(m, "Bounce:         ",
>  		    global_zone_page_state(NR_BOUNCE));
>  	show_val_kb(m, "WritebackTmp:   ",
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 1b9de7d220fb..a89f47515eb1 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -193,7 +193,6 @@ enum node_stat_item {
>  	NR_FILE_THPS,
>  	NR_FILE_PMDMAPPED,
>  	NR_ANON_THPS,
> -	NR_UNSTABLE_NFS,	/* NFS unstable pages */
>  	NR_VMSCAN_WRITE,
>  	NR_VMSCAN_IMMEDIATE,	/* Prioritise for reclaim when writeback ends */
>  	NR_DIRTIED,		/* page dirtyings since bootup */
> diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
> index d94def25e4dc..45b5fbdb1f62 100644
> --- a/include/trace/events/writeback.h
> +++ b/include/trace/events/writeback.h
> @@ -542,7 +542,6 @@ TRACE_EVENT(global_dirty_state,
>  	TP_STRUCT__entry(
>  		__field(unsigned long,	nr_dirty)
>  		__field(unsigned long,	nr_writeback)
> -		__field(unsigned long,	nr_unstable)
>  		__field(unsigned long,	background_thresh)
>  		__field(unsigned long,	dirty_thresh)
>  		__field(unsigned long,	dirty_limit)
> @@ -553,7 +552,6 @@ TRACE_EVENT(global_dirty_state,
>  	TP_fast_assign(
>  		__entry->nr_dirty	= global_node_page_state(NR_FILE_DIRTY);
>  		__entry->nr_writeback	= global_node_page_state(NR_WRITEBACK);
> -		__entry->nr_unstable	= global_node_page_state(NR_UNSTABLE_NFS);
>  		__entry->nr_dirtied	= global_node_page_state(NR_DIRTIED);
>  		__entry->nr_written	= global_node_page_state(NR_WRITTEN);
>  		__entry->background_thresh = background_thresh;
> @@ -561,12 +559,11 @@ TRACE_EVENT(global_dirty_state,
>  		__entry->dirty_limit	= global_wb_domain.dirty_limit;
>  	),
>  
> -	TP_printk("dirty=%lu writeback=%lu unstable=%lu "
> +	TP_printk("dirty=%lu writeback=%lu "
>  		  "bg_thresh=%lu thresh=%lu limit=%lu "
>  		  "dirtied=%lu written=%lu",
>  		  __entry->nr_dirty,
>  		  __entry->nr_writeback,
> -		  __entry->nr_unstable,
>  		  __entry->background_thresh,
>  		  __entry->dirty_thresh,
>  		  __entry->dirty_limit,
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 5beea03dd58a..2db5bbcfc17a 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4330,7 +4330,6 @@ void mem_cgroup_wb_stats(struct bdi_writeback *wb, unsigned long *pfilepages,
>  
>  	*pdirty = memcg_exact_page_state(memcg, NR_FILE_DIRTY);
>  
> -	/* this should eventually include NR_UNSTABLE_NFS */
>  	*pwriteback = memcg_exact_page_state(memcg, NR_WRITEBACK);
>  	*pfilepages = memcg_exact_page_state(memcg, NR_INACTIVE_FILE) +
>  			memcg_exact_page_state(memcg, NR_ACTIVE_FILE);
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 9692c553526b..b3b08de01d12 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -504,7 +504,6 @@ bool node_dirty_ok(struct pglist_data *pgdat)
>  	unsigned long nr_pages = 0;
>  
>  	nr_pages += node_page_state(pgdat, NR_FILE_DIRTY);
> -	nr_pages += node_page_state(pgdat, NR_UNSTABLE_NFS);
>  	nr_pages += node_page_state(pgdat, NR_WRITEBACK);
>  
>  	return nr_pages <= limit;
> @@ -758,7 +757,7 @@ static void mdtc_calc_avail(struct dirty_throttle_control *mdtc,
>   * bounded by the bdi->min_ratio and/or bdi->max_ratio parameters, if set.
>   *
>   * Return: @wb's dirty limit in pages. The term "dirty" in the context of
> - * dirty balancing includes all PG_dirty, PG_writeback and NFS unstable pages.
> + * dirty balancing includes all PG_dirty and PG_writeback pages.
>   */
>  static unsigned long __wb_calc_thresh(struct dirty_throttle_control *dtc)
>  {
> @@ -1566,7 +1565,7 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
>  	struct dirty_throttle_control * const mdtc = mdtc_valid(&mdtc_stor) ?
>  						     &mdtc_stor : NULL;
>  	struct dirty_throttle_control *sdtc;
> -	unsigned long nr_reclaimable;	/* = file_dirty + unstable_nfs */
> +	unsigned long nr_reclaimable;	/* = file_dirty */
>  	long period;
>  	long pause;
>  	long max_pause;
> @@ -1586,14 +1585,7 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
>  		unsigned long m_thresh = 0;
>  		unsigned long m_bg_thresh = 0;
>  
> -		/*
> -		 * Unstable writes are a feature of certain networked
> -		 * filesystems (i.e. NFS) in which data may have been
> -		 * written to the server's write cache, but has not yet
> -		 * been flushed to permanent storage.
> -		 */
> -		nr_reclaimable = global_node_page_state(NR_FILE_DIRTY) +
> -					global_node_page_state(NR_UNSTABLE_NFS);
> +		nr_reclaimable = global_node_page_state(NR_FILE_DIRTY);
>  		gdtc->avail = global_dirtyable_memory();
>  		gdtc->dirty = nr_reclaimable + global_node_page_state(NR_WRITEBACK);
>  
> @@ -1948,8 +1940,7 @@ bool wb_over_bg_thresh(struct bdi_writeback *wb)
>  	 * as we're trying to decide whether to put more under writeback.
>  	 */
>  	gdtc->avail = global_dirtyable_memory();
> -	gdtc->dirty = global_node_page_state(NR_FILE_DIRTY) +
> -		      global_node_page_state(NR_UNSTABLE_NFS);
> +	gdtc->dirty = global_node_page_state(NR_FILE_DIRTY);
>  	domain_dirty_limits(gdtc);
>  
>  	if (gdtc->dirty > gdtc->bg_thresh)
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 69827d4fa052..238b5518f3c5 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5310,7 +5310,7 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
>  
>  	printk("active_anon:%lu inactive_anon:%lu isolated_anon:%lu\n"
>  		" active_file:%lu inactive_file:%lu isolated_file:%lu\n"
> -		" unevictable:%lu dirty:%lu writeback:%lu unstable:%lu\n"
> +		" unevictable:%lu dirty:%lu writeback:%lu\n"
>  		" slab_reclaimable:%lu slab_unreclaimable:%lu\n"
>  		" mapped:%lu shmem:%lu pagetables:%lu bounce:%lu\n"
>  		" free:%lu free_pcp:%lu free_cma:%lu\n",
> @@ -5323,7 +5323,6 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
>  		global_node_page_state(NR_UNEVICTABLE),
>  		global_node_page_state(NR_FILE_DIRTY),
>  		global_node_page_state(NR_WRITEBACK),
> -		global_node_page_state(NR_UNSTABLE_NFS),
>  		global_node_page_state(NR_SLAB_RECLAIMABLE),
>  		global_node_page_state(NR_SLAB_UNRECLAIMABLE),
>  		global_node_page_state(NR_FILE_MAPPED),
> @@ -5356,7 +5355,6 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
>  			" anon_thp: %lukB"
>  #endif
>  			" writeback_tmp:%lukB"
> -			" unstable:%lukB"
>  			" all_unreclaimable? %s"
>  			"\n",
>  			pgdat->node_id,
> @@ -5378,7 +5376,6 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
>  			K(node_page_state(pgdat, NR_ANON_THPS) * HPAGE_PMD_NR),
>  #endif
>  			K(node_page_state(pgdat, NR_WRITEBACK_TEMP)),
> -			K(node_page_state(pgdat, NR_UNSTABLE_NFS)),
>  			pgdat->kswapd_failures >= MAX_RECLAIM_RETRIES ?
>  				"yes" : "no");
>  	}
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index 96d21a792b57..6c719f184843 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -1108,7 +1108,7 @@ int fragmentation_index(struct zone *zone, unsigned int order)
>  					TEXT_FOR_HIGHMEM(xx) xx "_movable",
>  
>  const char * const vmstat_text[] = {
> -	/* enum zone_stat_item countes */
> +	/* enum zone_stat_item counters */
>  	"nr_free_pages",
>  	"nr_zone_inactive_anon",
>  	"nr_zone_active_anon",
> @@ -1162,7 +1162,6 @@ const char * const vmstat_text[] = {
>  	"nr_file_hugepages",
>  	"nr_file_pmdmapped",
>  	"nr_anon_transparent_hugepages",
> -	"nr_unstable",
>  	"nr_vmscan_write",
>  	"nr_vmscan_immediate_reclaim",
>  	"nr_dirtied",
> @@ -1723,6 +1722,14 @@ static int vmstat_show(struct seq_file *m, void *arg)
>  	seq_puts(m, vmstat_text[off]);
>  	seq_put_decimal_ull(m, " ", *l);
>  	seq_putc(m, '\n');
> +
> +	if (off == NR_VMSTAT_ITEMS - 1) {
> +		/* We've come to the end - add any deprecated counters
> +		 * to avoid breaking userspace which might depend on
> +		 * them being present.
> +		 */
> +		seq_puts(m, "nr_unstable 0\n");
> +	}
>  	return 0;
>  }
>  
> -- 
> 2.26.0
> 


-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR