Re: [PATCH, RFC] writeback: avoid redirtying when ->write_inode failed to clear I_DIRTY

[Wu Fengguang <fengguang.wu@intel.com>]

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Christoph,

On Sat, Aug 27, 2011 at 02:14:09PM +0800, Christoph Hellwig wrote:
> Right now ->write_inode has no way to safely return a EAGAIN without explicitly
> redirtying the inode, as we would lose the dirty state otherwise.  Most
> filesystems get this wrong, but XFS makes heavy use of it to avoid blocking
> the flusher thread when ->write_inode hits contentended inode locks.  A
> contended ilock is something XFS can hit very easibly when extending files, as
> the data I/O completion handler takes the lock to update the size, and the
> ->write_inode call can race with it fairly easily if writing enough data
> in one go so that the completion for the first write come in just before
> we call ->write_inode.
> 
> Change the handling of this case to use requeue_io for a quick retry instead
> of redirty_tail, which keeps moving out the dirtied_when data and thus keeps
> delaying the writeout more and more with every failed attempt to get the lock.

Yeah redirty_tail() does have the problem of possibly delay inodes for
too long time. However, you know requeue_io() always has the danger of
triggering busy wb_writeback() loops in corner cases.

For example, nfs_write_inode()/nfs_commit_unstable_pages() often
redirty the inode without doing anything (hence no any progress, a
prerequisite for busy loops) depending on the in flight writes, which
unfortunately further depends on _external_ network/server states..
That means some stalled network/sever state could lead to busy loops
in NFS clients.

The alternative solution may be to firstly apply the attached patch,
and change this one to:

  -			redirty_tail(inode, wb);
  +			requeue_io_wait(inode, wb);

Thanks,
Fengguang

> Signed-off-by: Christoph Hellwig <hch@lst.de>
> 
> Index: linux-2.6/fs/fs-writeback.c
> ===================================================================
> --- linux-2.6.orig/fs/fs-writeback.c	2011-08-26 14:47:42.137050059 +0200
> +++ linux-2.6/fs/fs-writeback.c	2011-08-26 15:06:47.003493601 +0200
> @@ -464,8 +464,18 @@ writeback_single_inode(struct inode *ino
>  			 * operations, such as delayed allocation during
>  			 * submission or metadata updates after data IO
>  			 * completion.
> +			 *
> +			 * For the latter case it is very important to give
> +			 * the inode another turn on b_more_io instead of
> +			 * redirtying it.  Constantly moving dirtied_when
> +			 * forward will prevent us from ever writing out
> +			 * the metadata dirtied in the I/O completion handler.
> +			 *
> +			 * For files on XFS that constantly get appended to
> +			 * calling redirty_tail means they will never get
> +			 * their updated i_size written out.
>  			 */
> -			redirty_tail(inode, wb);
> +			requeue_io(inode, wb);
>  		} else {
>  			/*
>  			 * The inode is clean.  At this point we either have

[-- Attachment #2: writeback-more_io_wait.patch --]
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Subject: writeback: introduce queue b_more_io_wait
Date: Sun Jul 31 18:44:44 CST 2011

The problem is, redirty_tail() may update i_dirtied_when and result in
30s max delay. If redirty_tail() is called often enough, some inode may
even be delayed for ever.

So introduce the b_more_io_wait queue to park inodes that for some
reason cannot be synced immediately. The inodes will be sent to b_io at
the next b_io refill time, however won't be busy retried as b_more_io.

The new data flow after converting all redirty_tail() calls to
requeue_io_wait():

b_dirty --> b_io --> b_more_io/b_more_io_wait --+
             ^                                  |
	     |                                  |
	     +----------------------------------+

Cc: Jan Kara <jack@suse.cz>
Cc: Dave Chinner <david@fromorbit.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
 fs/fs-writeback.c           |   10 ++++++++++
 include/linux/backing-dev.h |    8 +++++---
 mm/backing-dev.c            |   10 ++++++++--
 3 files changed, 23 insertions(+), 5 deletions(-)

--- linux-next.orig/fs/fs-writeback.c	2011-08-27 15:28:27.000000000 +0800
+++ linux-next/fs/fs-writeback.c	2011-08-27 15:45:10.000000000 +0800
@@ -220,6 +220,15 @@ static void requeue_io(struct inode *ino
 	list_move(&inode->i_wb_list, &wb->b_more_io);
 }
 
+/*
+ * The inode should be retried in an opportunistic way.
+ */
+static void requeue_io_wait(struct inode *inode, struct bdi_writeback *wb)
+{
+	assert_spin_locked(&wb->list_lock);
+	list_move(&inode->i_wb_list, &wb->b_more_io_wait);
+}
+
 static void inode_sync_complete(struct inode *inode)
 {
 	/*
@@ -307,6 +316,7 @@ static void queue_io(struct bdi_writebac
 	int moved;
 	assert_spin_locked(&wb->list_lock);
 	list_splice_init(&wb->b_more_io, &wb->b_io);
+	list_splice_init(&wb->b_more_io_wait, &wb->b_io);
 	moved = move_expired_inodes(&wb->b_dirty, &wb->b_io, older_than_this);
 	trace_writeback_queue_io(wb, older_than_this, moved);
 }
--- linux-next.orig/include/linux/backing-dev.h	2011-08-26 19:27:20.000000000 +0800
+++ linux-next/include/linux/backing-dev.h	2011-08-27 15:45:10.000000000 +0800
@@ -59,6 +59,7 @@ struct bdi_writeback {
 	struct list_head b_dirty;	/* dirty inodes */
 	struct list_head b_io;		/* parked for writeback */
 	struct list_head b_more_io;	/* parked for more writeback */
+	struct list_head b_more_io_wait;/* opportunistic retry io */
 	spinlock_t list_lock;		/* protects the b_* lists */
 };
 
@@ -129,9 +130,10 @@ extern struct list_head bdi_pending_list
 
 static inline int wb_has_dirty_io(struct bdi_writeback *wb)
 {
-	return !list_empty(&wb->b_dirty) ||
-	       !list_empty(&wb->b_io) ||
-	       !list_empty(&wb->b_more_io);
+	return !list_empty(&wb->b_dirty)	||
+	       !list_empty(&wb->b_io)		||
+	       !list_empty(&wb->b_more_io)	||
+	       !list_empty(&wb->b_more_io_wait);
 }
 
 static inline void __add_bdi_stat(struct backing_dev_info *bdi,
--- linux-next.orig/mm/backing-dev.c	2011-08-26 19:27:20.000000000 +0800
+++ linux-next/mm/backing-dev.c	2011-08-27 15:45:10.000000000 +0800
@@ -74,10 +74,10 @@ static int bdi_debug_stats_show(struct s
 	unsigned long background_thresh;
 	unsigned long dirty_thresh;
 	unsigned long bdi_thresh;
-	unsigned long nr_dirty, nr_io, nr_more_io;
+	unsigned long nr_dirty, nr_io, nr_more_io, nr_more_io_wait;
 	struct inode *inode;
 
-	nr_dirty = nr_io = nr_more_io = 0;
+	nr_dirty = nr_io = nr_more_io = nr_more_io_wait = 0;
 	spin_lock(&wb->list_lock);
 	list_for_each_entry(inode, &wb->b_dirty, i_wb_list)
 		nr_dirty++;
@@ -85,6 +85,8 @@ static int bdi_debug_stats_show(struct s
 		nr_io++;
 	list_for_each_entry(inode, &wb->b_more_io, i_wb_list)
 		nr_more_io++;
+	list_for_each_entry(inode, &wb->b_more_io_wait, i_wb_list)
+		nr_more_io_wait++;
 	spin_unlock(&wb->list_lock);
 
 	global_dirty_limits(&background_thresh, &dirty_thresh);
@@ -103,6 +105,7 @@ static int bdi_debug_stats_show(struct s
 		   "b_dirty:            %10lu\n"
 		   "b_io:               %10lu\n"
 		   "b_more_io:          %10lu\n"
+		   "b_more_io_wait:     %10lu\n"
 		   "bdi_list:           %10u\n"
 		   "state:              %10lx\n",
 		   (unsigned long) K(bdi_stat(bdi, BDI_WRITEBACK)),
@@ -116,6 +119,7 @@ static int bdi_debug_stats_show(struct s
 		   nr_dirty,
 		   nr_io,
 		   nr_more_io,
+		   nr_more_io_wait,
 		   !list_empty(&bdi->bdi_list), bdi->state);
 #undef K
 
@@ -635,6 +639,7 @@ static void bdi_wb_init(struct bdi_write
 	INIT_LIST_HEAD(&wb->b_dirty);
 	INIT_LIST_HEAD(&wb->b_io);
 	INIT_LIST_HEAD(&wb->b_more_io);
+	INIT_LIST_HEAD(&wb->b_more_io_wait);
 	spin_lock_init(&wb->list_lock);
 	setup_timer(&wb->wakeup_timer, wakeup_timer_fn, (unsigned long)bdi);
 }
@@ -701,6 +706,7 @@ void bdi_destroy(struct backing_dev_info
 		list_splice(&bdi->wb.b_dirty, &dst->b_dirty);
 		list_splice(&bdi->wb.b_io, &dst->b_io);
 		list_splice(&bdi->wb.b_more_io, &dst->b_more_io);
+		list_splice(&bdi->wb.b_more_io_wait, &dst->b_more_io_wait);
 		spin_unlock(&bdi->wb.list_lock);
 		spin_unlock(&dst->list_lock);
 	}