Message-ID: <20150701133715.GA6287@dhcp22.suse.cz>
Date:	Wed, 1 Jul 2015 15:37:15 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Nikolay Borisov <kernel@...p.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Dave Chinner <david@...morbit.com>, Theodore Ts'o <tytso@....edu>,
	Marian Marinov <mm@...com>, Hugh Dickins <hughd@...gle.com>,
	linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>,
	linux-ext4@...r.kernel.org
Subject: Re: [PATCH] mm, vmscan: Do not wait for page writeback for GFP_NOFS
 allocations

[CCing Hugh as well]

On Wed 01-07-15 08:17:31, Michal Hocko wrote:
> On Tue 30-06-15 17:17:17, Michal Hocko wrote:
> [...]
> > Hi,
> > the issue has been reported http://marc.info/?l=linux-kernel&m=143522730927480.
> > This obviously requires a patch ot make ext4_ext_grow_indepth call
> > sb_getblk with the GFP_NOFS mask but that one makes sense on its own
> > and Ted has mentioned he will push it. I haven't marked the patch for
> > stable yet. This is the first time the issue has been reported and
> > ext4 writeout code has changed considerably in 3.11 and I am not sure
> > the issue was present before. e62e384e9da8 which has introduced the
> > wait_on_page_writeback has been merged in 3.6 which is quite some time
> > ago. If we go with stable I would suggest marking it for 3.11+ and it
> > should obviously go with the ext4_ext_grow_indepth fix.
> 
> After Dave's additional explanation
> (http://marc.info/?l=linux-ext4&m=143570521212215) it is clear that the
> lack of __GFP_FS check was wrong from the very beginning. XFS is doing
> the similar thing from before the e62e384e9da8 was merged. I guess we
> were just lucky not to hit this problem sooner.
> 
> That being said I think the patch should be marked for stable and the
> changelog updated:
> 
> As per David Chinner the xfs is doing similar thing since 2.6.15 already
> so ext4 is not the only affected filesystem. Moreover he notes:
> : For example: IO completion might require unwritten extent conversion
> : which executes filesystem transactions and GFP_NOFS allocations. The
> : writeback flag on the pages can not be cleared until unwritten
> : extent conversion completes. Hence memory reclaim cannot wait on
> : page writeback to complete in GFP_NOFS context because it is not
> : safe to do so, memcg reclaim or otherwise.
> 
> Cc: stable # 3.6+
> Fixes: e62e384e9da8 ("memcg: prevent OOM with too many dirty pages")
> 
> Andrew let me know whether I should repost the patch with the updated
> changelog or you can take it from here.

Hmm, I have double checked the original commit and it turned out my
memory was failing me. e62e384e9da8 had may_enter_fs check. This has
been changed later in the same merge window by c3b94f44fcb0 ("memcg:
further prevent OOM with too many dirty pages") and the code has been
refactored later some more. So the changelog needs some more rewording:
---
>From ca18f213a2c2c94d792e6cca5e391745dcb3484b Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@...e.cz>
Date: Tue, 30 Jun 2015 16:34:50 +0200
Subject: [PATCH] mm, vmscan: Do not wait for page writeback for GFP_NOFS
 allocations

Nikolay has reported a hang when a memcg reclaim got stuck with the
following backtrace:
PID: 18308  TASK: ffff883d7c9b0a30  CPU: 1   COMMAND: "rsync"
 #0 [ffff88177374ac60] __schedule at ffffffff815ab152
 #1 [ffff88177374acb0] schedule at ffffffff815ab76e
 #2 [ffff88177374acd0] schedule_timeout at ffffffff815ae5e5
 #3 [ffff88177374ad70] io_schedule_timeout at ffffffff815aad6a
 #4 [ffff88177374ada0] bit_wait_io at ffffffff815abfc6
 #5 [ffff88177374adb0] __wait_on_bit at ffffffff815abda5
 #6 [ffff88177374ae00] wait_on_page_bit at ffffffff8111fd4f
 #7 [ffff88177374ae50] shrink_page_list at ffffffff81135445
 #8 [ffff88177374af50] shrink_inactive_list at ffffffff81135845
 #9 [ffff88177374b060] shrink_lruvec at ffffffff81135ead
 #10 [ffff88177374b150] shrink_zone at ffffffff811360c3
 #11 [ffff88177374b220] shrink_zones at ffffffff81136eff
 #12 [ffff88177374b2a0] do_try_to_free_pages at ffffffff8113712f
 #13 [ffff88177374b300] try_to_free_mem_cgroup_pages at ffffffff811372be
 #14 [ffff88177374b380] try_charge at ffffffff81189423
 #15 [ffff88177374b430] mem_cgroup_try_charge at ffffffff8118c6f5
 #16 [ffff88177374b470] __add_to_page_cache_locked at ffffffff8112137d
 #17 [ffff88177374b4e0] add_to_page_cache_lru at ffffffff81121618
 #18 [ffff88177374b510] pagecache_get_page at ffffffff8112170b
 #19 [ffff88177374b560] grow_dev_page at ffffffff811c8297
 #20 [ffff88177374b5c0] __getblk_slow at ffffffff811c91d6
 #21 [ffff88177374b600] __getblk_gfp at ffffffff811c92c1
 #22 [ffff88177374b630] ext4_ext_grow_indepth at ffffffff8124565c
 #23 [ffff88177374b690] ext4_ext_create_new_leaf at ffffffff81246ca8
 #24 [ffff88177374b6e0] ext4_ext_insert_extent at ffffffff81246f09
 #25 [ffff88177374b750] ext4_ext_map_blocks at ffffffff8124a848
 #26 [ffff88177374b870] ext4_map_blocks at ffffffff8121a5b7
 #27 [ffff88177374b910] mpage_map_one_extent at ffffffff8121b1fa
 #28 [ffff88177374b950] mpage_map_and_submit_extent at ffffffff8121f07b
 #29 [ffff88177374b9b0] ext4_writepages at ffffffff8121f6d5
 #30 [ffff88177374bb20] do_writepages at ffffffff8112c490
 #31 [ffff88177374bb30] __filemap_fdatawrite_range at ffffffff81120199
 #32 [ffff88177374bb80] filemap_flush at ffffffff8112041c
 #33 [ffff88177374bb90] ext4_alloc_da_blocks at ffffffff81219da1
 #34 [ffff88177374bbb0] ext4_rename at ffffffff81229b91
 #35 [ffff88177374bcd0] ext4_rename2 at ffffffff81229e32
 #36 [ffff88177374bce0] vfs_rename at ffffffff811a08a5
 #37 [ffff88177374bd60] SYSC_renameat2 at ffffffff811a3ffc
 #38 [ffff88177374bf60] sys_renameat2 at ffffffff811a408e
 #39 [ffff88177374bf70] sys_rename at ffffffff8119e51e
 #40 [ffff88177374bf80] system_call_fastpath at ffffffff815afa89

Dave Chinner has properly pointed out that this is a deadlock in the
reclaim code because ext4 doesn't submit pages which are marked by
PG_writeback right away. The heuristic was introduced by e62e384e9da8
("memcg: prevent OOM with too many dirty pages") and it was applied
only when may_enter_fs was specified. The code has been changed by
c3b94f44fcb0 ("memcg: further prevent OOM with too many dirty pages")
which has removed the __GFP_FS restriction with a reasoning that we
do not get into the fs code. But this is not sufficient apparently
because the fs doesn't necessarily submit pages marked PG_writeback
for IO right away.

ext4_bio_write_page calls io_submit_add_bh but that doesn't necessarily
submit the bio. Instead it tries to map more pages into the bio and
mpage_map_one_extent might trigger memcg charge which might end up
waiting on a page which is marked PG_writeback but hasn't been submitted
yet so we would end up waiting for something that never finishes.

Fix this issue by limiting the wait to reclaim triggered by __GFP_FS
allocations to make sure we are not called from filesystem paths which
might be doing exactly this kind of IO optimizations. The page fault
path, which is the only path that triggers memcg oom killer since 3.12,
shouldn't require GFP_NOFS and so we shouldn't reintroduce the premature
OOM killer issue which was originally addressed by the heuristic.

As per David Chinner the xfs is doing similar thing since 2.6.15 already
so ext4 is not the only affected filesystem. Moreover he notes:
: For example: IO completion might require unwritten extent conversion
: which executes filesystem transactions and GFP_NOFS allocations. The
: writeback flag on the pages can not be cleared until unwritten
: extent conversion completes. Hence memory reclaim cannot wait on
: page writeback to complete in GFP_NOFS context because it is not
: safe to do so, memcg reclaim or otherwise.

Cc: stable # 3.6+
Fixes: c3b94f44fcb0 ("memcg: further prevent OOM with too many dirty pages")
Reported-by: Nikolay Borisov <kernel@...p.com>
Signed-off-by: Michal Hocko <mhocko@...e.cz>
---
 mm/vmscan.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 37e90db1520b..6c44d424968e 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -995,7 +995,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 				goto keep_locked;
 
 			/* Case 3 above */
-			} else {
+			} else if (sc->gfp_mask & __GFP_FS) {
 				wait_on_page_writeback(page);
 			}
 		}
-- 
2.1.4

-- 
Michal Hocko
SUSE Labs
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