On Tue, Dec 1, 2020 at 1:25 PM Yang Shi <shy828301@xxxxxxxxx> 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@xxxxxx>
> Reviewed-by: Roman Gushchin <guro@xxxxxx>
> Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx>
> Cc: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
> Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx> v4.19+
> Signed-off-by: Yang Shi <shy828301@xxxxxxxxx>
Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
