Re: [RFC PATCH 1/2] mm, vmscan: account the number of isolated pages per zone

[Michal Hocko <mhocko@kernel.org>]

On Wed 25-01-17 20:09:31, Tetsuo Handa wrote:
> Michal Hocko wrote:
> > On Wed 25-01-17 11:19:57, Christoph Hellwig wrote:
> > > On Wed, Jan 25, 2017 at 11:15:17AM +0100, Michal Hocko wrote:
> > > > I think we are missing a check for fatal_signal_pending in
> > > > iomap_file_buffered_write. This means that an oom victim can consume the
> > > > full memory reserves. What do you think about the following? I haven't
> > > > tested this but it mimics generic_perform_write so I guess it should
> > > > work.
> > > 
> > > Hi Michal,
> > > 
> > > this looks reasonable to me.  But we have a few more such loops,
> > > maybe it makes sense to move the check into iomap_apply?
> > 
> > I wasn't sure about the expected semantic of iomap_apply but now that
> > I've actually checked all the callers I believe all of them should be
> > able to handle EINTR just fine. Well iomap_file_dirty, iomap_zero_range,
> > iomap_fiemap and iomap_page_mkwriteseem do not follow the standard
> > pattern to return the number of written pages or an error but it rather
> > propagates the error out. From my limited understanding of those code
> > paths that should just be ok. I was not all that sure about iomap_dio_rw
> > that is just too convoluted for me. If that one is OK as well then
> > the following patch should be indeed better.
> 
> Is "length" in
> 
>    written = actor(inode, pos, length, data, &iomap);
> 
> call guaranteed to be small enough? If not guaranteed,
> don't we need to check SIGKILL inside "actor" functions?

You are right! Checking for signals inside iomap_apply doesn't really
solve anything because basically all users do iov_iter_count(). Blee. So
we have loops around iomap_apply which itself loops inside the actor.
iomap_write_begin seems to be used by most of them which is also where we
get the pagecache page so I guess this should be the "right" place to
put the check in. Things like dax_iomap_actor will need an explicit check.
This is quite unfortunate but I do not see any better solution.
What do you think Christoph?
---
>From 362da5cac527146a341300c2ca441245c16043e8 Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@suse.com>
Date: Wed, 25 Jan 2017 11:06:37 +0100
Subject: [PATCH] fs: break out of iomap_file_buffered_write on fatal signals

Tetsuo has noticed that an OOM stress test which performs large write
requests can cause the full memory reserves depletion. He has tracked
this down to the following path
	__alloc_pages_nodemask+0x436/0x4d0
	alloc_pages_current+0x97/0x1b0
	__page_cache_alloc+0x15d/0x1a0          mm/filemap.c:728
	pagecache_get_page+0x5a/0x2b0           mm/filemap.c:1331
	grab_cache_page_write_begin+0x23/0x40   mm/filemap.c:2773
	iomap_write_begin+0x50/0xd0             fs/iomap.c:118
	iomap_write_actor+0xb5/0x1a0            fs/iomap.c:190
	? iomap_write_end+0x80/0x80             fs/iomap.c:150
	iomap_apply+0xb3/0x130                  fs/iomap.c:79
	iomap_file_buffered_write+0x68/0xa0     fs/iomap.c:243
	? iomap_write_end+0x80/0x80
	xfs_file_buffered_aio_write+0x132/0x390 [xfs]
	? remove_wait_queue+0x59/0x60
	xfs_file_write_iter+0x90/0x130 [xfs]
	__vfs_write+0xe5/0x140
	vfs_write+0xc7/0x1f0
	? syscall_trace_enter+0x1d0/0x380
	SyS_write+0x58/0xc0
	do_syscall_64+0x6c/0x200
	entry_SYSCALL64_slow_path+0x25/0x25

the oom victim has access to all memory reserves to make a forward
progress to exit easier. But iomap_file_buffered_write and other callers
of iomap_apply loop to complete the full request. We need to check for
fatal signals and back off with a short write instead. As the
iomap_apply delegates all the work down to the actor we have to hook
into those. All callers that work with the page cache are calling
iomap_write_begin so we will check for signals there. dax_iomap_actor
has to handle the situation explicitly because it copies data to the
userspace directly. Other callers like iomap_page_mkwrite work on a
single page or iomap_fiemap_actor do not allocate memory based on the
given len.

Fixes: 68a9f5e7007c ("xfs: implement iomap based buffered write path")
Cc: stable # 4.8+
Reported-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Michal Hocko <mhocko@suse.com>
---
 fs/dax.c   | 5 +++++
 fs/iomap.c | 3 +++
 2 files changed, 8 insertions(+)

diff --git a/fs/dax.c b/fs/dax.c
index 413a91db9351..0e263dacf9cf 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -1033,6 +1033,11 @@ dax_iomap_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
 		struct blk_dax_ctl dax = { 0 };
 		ssize_t map_len;
 
+		if (fatal_signal_pending(current)) {
+			ret = -EINTR;
+			break;
+		}
+
 		dax.sector = dax_iomap_sector(iomap, pos);
 		dax.size = (length + offset + PAGE_SIZE - 1) & PAGE_MASK;
 		map_len = dax_map_atomic(iomap->bdev, &dax);
diff --git a/fs/iomap.c b/fs/iomap.c
index e57b90b5ff37..691eada58b06 100644
--- a/fs/iomap.c
+++ b/fs/iomap.c
@@ -114,6 +114,9 @@ iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, unsigned flags,
 
 	BUG_ON(pos + len > iomap->offset + iomap->length);
 
+	if (fatal_signal_pending(current))
+		return -EINTR;
+
 	page = grab_cache_page_write_begin(inode->i_mapping, index, flags);
 	if (!page)
 		return -ENOMEM;
-- 
2.11.0


-- 
Michal Hocko
SUSE Labs