Message-ID: <CAAPL-u9=Bo9=NONYpu-AFQMzM7hwFCRNDxaCbb4tu8iHUerzPQ@mail.gmail.com>
Date: Tue, 8 Oct 2024 23:21:17 -0700
From: Wei Xu <weixugc@...gle.com>
To: jingxiang zeng <jingxiangzeng.cas@...il.com>
Cc: linux-mm@...ck.org, akpm@...ux-foundation.org, kasong@...cent.com, 
	linuszeng@...cent.com, linux-kernel@...r.kernel.org, tjmercier@...gle.com, 
	yuzhao@...gle.com, chrisl@...nel.org
Subject: Re: [RESEND][PATCH v4] mm/vmscan: wake up flushers conditionally to
 avoid cgroup OOM

On Tue, Oct 8, 2024 at 9:52 PM jingxiang zeng
<jingxiangzeng.cas@...il.com> wrote:
>
> On Tue, 8 Oct 2024 at 11:26, Wei Xu <weixugc@...gle.com> wrote:
> >
> > On Mon, Oct 7, 2024 at 6:57 PM Jingxiang Zeng
> > <jingxiangzeng.cas@...il.com> wrote:
> > >
> > > From: Jingxiang Zeng <linuszeng@...cent.com>
> > >
> > > Commit 14aa8b2d5c2e ("mm/mglru: don't sync disk for each aging cycle")
> > > removed the opportunity to wake up flushers during the MGLRU page
> > > reclamation process can lead to an increased likelihood of triggering OOM
> > > when encountering many dirty pages during reclamation on MGLRU.
> > >
> > > This leads to premature OOM if there are too many dirty pages in cgroup:
> > > Killed
> > >
> > > dd invoked oom-killer: gfp_mask=0x101cca(GFP_HIGHUSER_MOVABLE|__GFP_WRITE),
> > > order=0, oom_score_adj=0
> > >
> > > Call Trace:
> > >   <TASK>
> > >   dump_stack_lvl+0x5f/0x80
> > >   dump_stack+0x14/0x20
> > >   dump_header+0x46/0x1b0
> > >   oom_kill_process+0x104/0x220
> > >   out_of_memory+0x112/0x5a0
> > >   mem_cgroup_out_of_memory+0x13b/0x150
> > >   try_charge_memcg+0x44f/0x5c0
> > >   charge_memcg+0x34/0x50
> > >   __mem_cgroup_charge+0x31/0x90
> > >   filemap_add_folio+0x4b/0xf0
> > >   __filemap_get_folio+0x1a4/0x5b0
> > >   ? srso_return_thunk+0x5/0x5f
> > >   ? __block_commit_write+0x82/0xb0
> > >   ext4_da_write_begin+0xe5/0x270
> > >   generic_perform_write+0x134/0x2b0
> > >   ext4_buffered_write_iter+0x57/0xd0
> > >   ext4_file_write_iter+0x76/0x7d0
> > >   ? selinux_file_permission+0x119/0x150
> > >   ? srso_return_thunk+0x5/0x5f
> > >   ? srso_return_thunk+0x5/0x5f
> > >   vfs_write+0x30c/0x440
> > >   ksys_write+0x65/0xe0
> > >   __x64_sys_write+0x1e/0x30
> > >   x64_sys_call+0x11c2/0x1d50
> > >   do_syscall_64+0x47/0x110
> > >   entry_SYSCALL_64_after_hwframe+0x76/0x7e
> > >
> > >  memory: usage 308224kB, limit 308224kB, failcnt 2589
> > >  swap: usage 0kB, limit 9007199254740988kB, failcnt 0
> > >
> > >   ...
> > >   file_dirty 303247360
> > >   file_writeback 0
> > >   ...
> > >
> > > oom-kill:constraint=CONSTRAINT_MEMCG,nodemask=(null),cpuset=test,
> > > mems_allowed=0,oom_memcg=/test,task_memcg=/test,task=dd,pid=4404,uid=0
> > > Memory cgroup out of memory: Killed process 4404 (dd) total-vm:10512kB,
> > > anon-rss:1152kB, file-rss:1824kB, shmem-rss:0kB, UID:0 pgtables:76kB
> > > oom_score_adj:0
> > >
> > > The flusher wake up was removed to decrease SSD wearing, but if we are
> > > seeing all dirty folios at the tail of an LRU, not waking up the flusher
> > > could lead to thrashing easily.  So wake it up when a mem cgroups is about
> > > to OOM due to dirty caches.
> > >
> > > ---
> > > Changes from v3:
> > > - Avoid taking lock and reduce overhead on folio isolation by
> > >   checking the right flags and rework wake up condition, fixing the
> > >   performance regression reported by Chris Li.
> > >   [Chris Li, Kairui Song]
> > > - Move the wake up check to try_to_shrink_lruvec to cover kswapd
> > >   case as well, and update comments. [Kairui Song]
> > > - Link to v3: https://lore.kernel.org/all/20240924121358.30685-1-jingxiangzeng.cas@gmail.com/
> > > Changes from v2:
> > > - Acquire the lock before calling the folio_check_dirty_writeback
> > >   function. [Wei Xu, Jingxiang Zeng]
> > > - Link to v2: https://lore.kernel.org/all/20240913084506.3606292-1-jingxiangzeng.cas@gmail.com/
> > > Changes from v1:
> > > - Add code to count the number of unqueued_dirty in the sort_folio
> > >   function. [Wei Xu, Jingxiang Zeng]
> > > - Link to v1: https://lore.kernel.org/all/20240829102543.189453-1-jingxiangzeng.cas@gmail.com/
> > > ---
> > >
> > > Fixes: 14aa8b2d5c2e ("mm/mglru: don't sync disk for each aging cycle")
> > > Signed-off-by: Zeng Jingxiang <linuszeng@...cent.com>
> > > Signed-off-by: Kairui Song <kasong@...cent.com>
> > > Cc: T.J. Mercier <tjmercier@...gle.com>
> > > Cc: Wei Xu <weixugc@...gle.com>
> > > Cc: Yu Zhao <yuzhao@...gle.com>
> > > ---
> > >  mm/vmscan.c | 19 ++++++++++++++++---
> > >  1 file changed, 16 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > > index dc7a285b256b..2a5c2fe81467 100644
> > > --- a/mm/vmscan.c
> > > +++ b/mm/vmscan.c
> > > @@ -4291,6 +4291,7 @@ static bool sort_folio(struct lruvec *lruvec, struct folio *folio, struct scan_c
> > >                        int tier_idx)
> > >  {
> > >         bool success;
> > > +       bool dirty, writeback;
> > >         int gen = folio_lru_gen(folio);
> > >         int type = folio_is_file_lru(folio);
> > >         int zone = folio_zonenum(folio);
> > > @@ -4336,9 +4337,14 @@ static bool sort_folio(struct lruvec *lruvec, struct folio *folio, struct scan_c
> > >                 return true;
> > >         }
> > >
> > > +       dirty = folio_test_dirty(folio);
> > > +       writeback = folio_test_writeback(folio);
> > > +       if (type == LRU_GEN_FILE && dirty && !writeback)
> > > +               sc->nr.unqueued_dirty += delta;
> > > +
> >
> > This sounds good.  BTW, when shrink_folio_list() in evict_folios()
> > returns, we should add stat.nr_unqueued_dirty to sc->nr.unqueued_dirty
> > there as well.
>
> Thank you for your valuable feedback, I will implement it in the next version.
> >
> > >         /* waiting for writeback */
> > > -       if (folio_test_locked(folio) || folio_test_writeback(folio) ||
> > > -           (type == LRU_GEN_FILE && folio_test_dirty(folio))) {
> > > +       if (folio_test_locked(folio) || writeback ||
> > > +           (type == LRU_GEN_FILE && dirty)) {
> > >                 gen = folio_inc_gen(lruvec, folio, true);
> > >                 list_move(&folio->lru, &lrugen->folios[gen][type][zone]);
> > >                 return true;
> > > @@ -4454,7 +4460,7 @@ static int scan_folios(struct lruvec *lruvec, struct scan_control *sc,
> > >         trace_mm_vmscan_lru_isolate(sc->reclaim_idx, sc->order, MAX_LRU_BATCH,
> > >                                 scanned, skipped, isolated,
> > >                                 type ? LRU_INACTIVE_FILE : LRU_INACTIVE_ANON);
> > > -
> > > +       sc->nr.taken += scanned;
> >
> > I think we should only include file pages (in sort_folio) and isolated
> > pages into sc->nr.taken, instead of all scanned pages. For example, if
>
> Unqueued dirty pages are not isolated, but promoted to the newer generation
> in the sort_folio function. So I tend to wake up the flusher thread when the
> number of scanned pages is equal to the number of unqueued dirty pages.
>
> > there are only unevictable and unqueued dirty pages, we would still
> > like to wake up the flusher threads, but because nr.taken counts
> > unevictable pages as well, the wakeup condition in
> > try_to_shrink_lruvec() won't be met.
>
> The situation you mentioned will not happen because the number of
> scanned pages does not include unevicatble pages.
> However, there is another situation that needs attention. When the
> scanned pages contain anonymous pages and unqueued dirty pages,
> the flusher cannot be woken up. I will fix this situation in the next version.
>

Pages in unevictable LRUs are not scanned, but unevictable pages that
are not yet moved to unevictable LRUs can be included in scanned
pages. See the !folio_evictable(folio) check in sort_folio().

> >
> > >         /*
> > >          * There might not be eligible folios due to reclaim_idx. Check the
> > >          * remaining to prevent livelock if it's not making progress.
> > > @@ -4796,6 +4802,13 @@ static bool try_to_shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
> > >                 cond_resched();
> > >         }
> > >
> > > +       /*
> > > +        * If too many file cache in the coldest generation can't be evicted
> > > +        * due to being dirty, wake up the flusher.
> > > +        */
> > > +       if (sc->nr.unqueued_dirty && sc->nr.unqueued_dirty == sc->nr.taken)
> > > +               wakeup_flusher_threads(WB_REASON_VMSCAN);
> > > +
> >
> > try_to_shrink_lruvec() can be called from shrink_node() for global
> > reclaim as well. We need to reset sc->nr before calling
> > lru_gen_shrink_node() there. MGLRU didn't need that because it didn't
> > use sc->nr until this change.
>
> Thank you for your valuable feedback, I will implement it in the next version.
> >
> > >         /* whether this lruvec should be rotated */
> > >         return nr_to_scan < 0;
> > >  }
> > > --
> > > 2.43.5
> > >

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