Re: [PATCH 1/6] mm/vmscan: Wake up flushers for legacy cgroups too

Michal Hocko
2018-03-20T15:00:16Z

On Thu 15-03-18 19:45:48, Andrey Ryabinin wrote:
> Commit 726d061fbd36 ("mm: vmscan: kick flushers when we encounter
> dirty pages on the LRU") added flusher invocation to
> shrink_inactive_list() when many dirty pages on the LRU are encountered.
>
> However, shrink_inactive_list() doesn't wake up flushers for legacy
> cgroup reclaim, so the next commit bbef938429f5 ("mm: vmscan: remove
> old flusher wakeup from direct reclaim path") removed the only source
> of flusher's wake up in legacy mem cgroup reclaim path.
>
> This leads to premature OOM if there is too many dirty pages in cgroup:
> # mkdir /sys/fs/cgroup/memory/test
> # echo $$ > /sys/fs/cgroup/memory/test/tasks
> # echo 50M > /sys/fs/cgroup/memory/test/memory.limit_in_bytes
> # dd if=/dev/zero of=tmp_file bs=1M count=100
> Killed
>
> dd invoked oom-killer: gfp_mask=0x14000c0(GFP_KERNEL), nodemask=(null), order=0, oom_score_adj=0
>
> Call Trace:
> dump_stack+0x46/0x65
> dump_header+0x6b/0x2ac
> oom_kill_process+0x21c/0x4a0
> out_of_memory+0x2a5/0x4b0
> mem_cgroup_out_of_memory+0x3b/0x60
> mem_cgroup_oom_synchronize+0x2ed/0x330
> pagefault_out_of_memory+0x24/0x54
> __do_page_fault+0x521/0x540
> page_fault+0x45/0x50
>
> Task in /test killed as a result of limit of /test
> memory: usage 51200kB, limit 51200kB, failcnt 73
> memory+swap: usage 51200kB, limit 9007199254740988kB, failcnt 0
> kmem: usage 296kB, limit 9007199254740988kB, failcnt 0
> Memory cgroup stats for /test: cache:49632KB rss:1056KB rss_huge:0KB shmem:0KB
> mapped_file:0KB dirty:49500KB writeback:0KB swap:0KB inactive_anon:0KB
> active_anon:1168KB inactive_file:24760KB active_file:24960KB unevictable:0KB
> Memory cgroup out of memory: Kill process 3861 (bash) score 88 or sacrifice child
> Killed process 3876 (dd) total-vm:8484kB, anon-rss:1052kB, file-rss:1720kB, shmem-rss:0kB
> oom_reaper: reaped process 3876 (dd), now anon-rss:0kB, file-rss:0kB, shmem-rss:0kB
>
> Wake up flushers in legacy cgroup reclaim too.
>
> Fixes: bbef938429f5 ("mm: vmscan: remove old flusher wakeup from direct reclaim path")
> Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
> Cc: <stable@vger.kernel.org>

Yes, this makes sense. We are stalling on writeback pages for the legacy
memcg but we do not have any way to throttle dirty pages before the
writeback kicks in

Acked-by: Michal Hocko <mhocko@suse.cz>

> ---
> mm/vmscan.c | 31 ++++++++++++++++---------------
> 1 file changed, 16 insertions(+), 15 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 8fcd9f8d7390..4390a8d5be41 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1771,6 +1771,20 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
> if (stat.nr_writeback && stat.nr_writeback == nr_taken)
> set_bit(PGDAT_WRITEBACK, &pgdat->flags);
>
> + /*
> + * If dirty pages are scanned that are not queued for IO, it
> + * implies that flushers are not doing their job. This can
> + * happen when memory pressure pushes dirty pages to the end of
> + * the LRU before the dirty limits are breached and the dirty
> + * data has expired. It can also happen when the proportion of
> + * dirty pages grows not through writes but through memory
> + * pressure reclaiming all the clean cache. And in some cases,
> + * the flushers simply cannot keep up with the allocation
> + * rate. Nudge the flusher threads in case they are asleep.
> + */
> + if (stat.nr_unqueued_dirty == nr_taken)
> + wakeup_flusher_threads(WB_REASON_VMSCAN);
> +
> /*
> * Legacy memcg will stall in page writeback so avoid forcibly
> * stalling here.
> @@ -1783,22 +1797,9 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
> if (stat.nr_dirty && stat.nr_dirty == stat.nr_congested)
> set_bit(PGDAT_CONGESTED, &pgdat->flags);
>
> - /*
> - * If dirty pages are scanned that are not queued for IO, it
> - * implies that flushers are not doing their job. This can
> - * happen when memory pressure pushes dirty pages to the end of
> - * the LRU before the dirty limits are breached and the dirty
> - * data has expired. It can also happen when the proportion of
> - * dirty pages grows not through writes but through memory
> - * pressure reclaiming all the clean cache. And in some cases,
> - * the flushers simply cannot keep up with the allocation
> - * rate. Nudge the flusher threads in case they are asleep, but
> - * also allow kswapd to start writing pages during reclaim.
> - */
> - if (stat.nr_unqueued_dirty == nr_taken) {
> - wakeup_flusher_threads(WB_REASON_VMSCAN);
> + /* Allow kswapd to start writing pages during reclaim. */
> + if (stat.nr_unqueued_dirty == nr_taken)
> set_bit(PGDAT_DIRTY, &pgdat->flags);
> - }
>
> /*
> * If kswapd scans pages marked marked for immediate
> --
> 2.16.1

--
Michal Hocko
SUSE Labs