Re: [PATCH] mm: memcontrol: fix occasional OOMs due to proportional memory.low reclaim

[Shakeel Butt <shakeelb@google.com>]

On Tue, Aug 17, 2021 at 11:03 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> We've noticed occasional OOM killing when memory.low settings are in
> effect for cgroups. This is unexpected and undesirable as memory.low
> is supposed to express non-OOMing memory priorities between cgroups.
>
> The reason for this is proportional memory.low reclaim. When cgroups
> are below their memory.low threshold, reclaim passes them over in the
> first round, and then retries if it couldn't find pages anywhere else.
> But when cgroups are slighly above their memory.low setting, page scan

*slightly

> force is scaled down and diminished in proportion to the overage, to
> the point where it can cause reclaim to fail as well - only in that
> case we currently don't retry, and instead trigger OOM.
>
> To fix this, hook proportional reclaim into the same retry logic we
> have in place for when cgroups are skipped entirely. This way if
> reclaim fails and some cgroups were scanned with dimished pressure,

*diminished

> we'll try another full-force cycle before giving up and OOMing.
>
> Reported-by: Leon Yang <lnyng@fb.com>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

Should this be considered for stable?

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

[...]
>
>  static inline void mem_cgroup_calculate_protection(struct mem_cgroup *root,
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 4620df62f0ff..701106e1829c 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -100,9 +100,12 @@ struct scan_control {
>         unsigned int may_swap:1;
>
>         /*
> -        * Cgroups are not reclaimed below their configured memory.low,
> -        * unless we threaten to OOM. If any cgroups are skipped due to
> -        * memory.low and nothing was reclaimed, go back for memory.low.
> +        * Cgroup memory below memory.low is protected as long as we
> +        * don't threaten to OOM. If any cgroup is reclaimed at
> +        * reduced force or passed over entirely due to its memory.low
> +        * setting (memcg_low_skipped), and nothing is reclaimed as a
> +        * result, then go back back for one more cycle that reclaims

*back