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

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Apr 28, 2020 at 07:26:47PM +0100, Chris Down wrote:
> From: Yafang Shao <laoar.shao@gmail.com>
> 
> 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.
> 
> Fixes: 9783aa9917f8 ("mm, memcg: proportional memory.{low,min} reclaim")
> Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> Signed-off-by: Chris Down <chris@chrisdown.name>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Roman Gushchin <guro@fb.com>
> 
> [hannes@cmpxchg.org: rework code comment]
> [hannes@cmpxchg.org: changelog]
> [chris@chrisdown.name: fix store tear]
> [chris@chrisdown.name: retitle]

Acked-by: Johannes Weiner <hannes@cmpxchg.org>