Re: [PATCH v2 1/2] mm, memcg: Avoid stale protection values when cgroup is above protection

[Chris Down <chris@chrisdown.name>]

Yafang Shao writes:
>A cgroup can have both memory protection and a memory limit to isolate
>it from its siblings in both directions - for example, to prevent it
>from being shrunk below 2G under high pressure from outside, but also
>from growing beyond 4G under low pressure.
>
>Commit 9783aa9917f8 ("mm, memcg: proportional memory.{low,min} reclaim")
>implemented proportional scan pressure so that multiple siblings in
>excess of their protection settings don't get reclaimed equally but
>instead in accordance to their unprotected portion.
>
>During limit reclaim, this proportionality shouldn't apply of course:
>there is no competition, all pressure is from within the cgroup and
>should be applied as such. Reclaim should operate at full efficiency.
>
>However, mem_cgroup_protected() never expected anybody to look at the
>effective protection values when it indicated that the cgroup is above
>its protection. As a result, a query during limit reclaim may return
>stale protection values that were calculated by a previous reclaim cycle
>in which the cgroup did have siblings.
>
>When this happens, reclaim is unnecessarily hesitant and potentially
>slow to meet the desired limit. In theory this could lead to premature
>OOM kills, although it's not obvious this has occurred in practice.
>
>[hannes@cmpxchg.org: changelog]
>[mhocko@kernel.org: rework code comment]
>[chris@chrisdown.name: retitle]
>Fixes: 9783aa9917f8 ("mm, memcg: proportional memory.{low,min} reclaim")
>Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
>Acked-by: Roman Gushchin <guro@fb.com>
>Cc: Michal Hocko <mhocko@kernel.org>
>Cc: Johannes Weiner <hannes@cmpxchg.org>
>Cc: Chris Down <chris@chrisdown.name>

Thanks!

Acked-by: Chris Down <chris@chrisdown.name>