Re: [v3 PATCH] mm: list_lru: set shrinker map bit when child nr_items is not zero

[Shakeel Butt <shakeelb@google.com>]

On Tue, Dec 1, 2020 at 1:25 PM Yang Shi <shy828301@gmail.com> wrote:
>
> When investigating a slab cache bloat problem, significant amount of
> negative dentry cache was seen, but confusingly they neither got shrunk
> by reclaimer (the host has very tight memory) nor be shrunk by dropping
> cache.  The vmcore shows there are over 14M negative dentry objects on lru,
> but tracing result shows they were even not scanned at all.  The further
> investigation shows the memcg's vfs shrinker_map bit is not set.  So the
> reclaimer or dropping cache just skip calling vfs shrinker.  So we have
> to reboot the hosts to get the memory back.
>
> I didn't manage to come up with a reproducer in test environment, and the
> problem can't be reproduced after rebooting.  But it seems there is race
> between shrinker map bit clear and reparenting by code inspection.  The
> hypothesis is elaborated as below.
>
> The memcg hierarchy on our production environment looks like:
>                 root
>                /    \
>           system   user
>
> The main workloads are running under user slice's children, and it creates
> and removes memcg frequently.  So reparenting happens very often under user
> slice, but no task is under user slice directly.
>
> So with the frequent reparenting and tight memory pressure, the below
> hypothetical race condition may happen:
>
>        CPU A                            CPU B
> reparent
>     dst->nr_items == 0
>                                  shrinker:
>                                      total_objects == 0
>     add src->nr_items to dst
>     set_bit
>                                      retrun SHRINK_EMPTY

return

>                                      clear_bit
> child memcg offline
>     replace child's kmemcg_id to

with

>     parent's (in memcg_offline_kmem())
>                                   list_lru_del() between shrinker runs
>                                      see parent's kmemcg_id
>                                      dec dst->nr_items
> reparent again
>     dst->nr_items may go negative
>     due to concurrent list_lru_del()
>
>                                  The second run of shrinker:
>                                      read nr_items without any
>                                      synchronization, so it may
>                                      see intermediate negative
>                                      nr_items then total_objects
>                                      may return 0 conincidently

coincidently

>
>                                      keep the bit cleared
>     dst->nr_items != 0
>     skip set_bit
>     add scr->nr_item to dst
>
> After this point dst->nr_item may never go zero, so reparenting will not
> set shrinker_map bit anymore.  And since there is no task under user
> slice directly, so no new object will be added to its lru to set the
> shrinker map bit either.  That bit is kept cleared forever.
>
> How does list_lru_del() race with reparenting?  It is because
> reparenting replaces childen's kmemcg_id to parent's without protecting

children's

> from nlru->lock, so list_lru_del() may see parent's kmemcg_id but
> actually deleting items from child's lru, but dec'ing parent's nr_items,
> so the parent's nr_items may go negative as commit
> 2788cf0c401c268b4819c5407493a8769b7007aa ("memcg: reparent list_lrus and
> free kmemcg_id on css offline") says.
>
> Since it is impossible that dst->nr_items goes negative and
> src->nr_items goes zero at the same time, so it seems we could set the
> shrinker map bit iff src->nr_items != 0.  We could synchronize
> list_lru_count_one() and reparenting with nlru->lock, but it seems
> checking src->nr_items in reparenting is the simplest and avoids lock
> contention.
>
> Fixes: fae91d6d8be5 ("mm/list_lru.c: set bit in memcg shrinker bitmap on first list_lru item appearance")
> Suggested-by: Roman Gushchin <guro@fb.com>
> Reviewed-by: Roman Gushchin <guro@fb.com>
> Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
> Cc: Kirill Tkhai <ktkhai@virtuozzo.com>
> Cc: Shakeel Butt <shakeelb@google.com>
> Cc: <stable@vger.kernel.org> v4.19+
> Signed-off-by: Yang Shi <shy828301@gmail.com>

Reviewed-by: Shakeel Butt <shakeelb@google.com>