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

[Yafang Shao <laoar.shao@gmail.com>]

On Wed, Apr 29, 2020 at 2:26 AM Chris Down <chris@chrisdown.name> 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]
> ---
>  mm/memcontrol.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 0be00826b832..b0374be44e9e 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -6392,8 +6392,19 @@ enum mem_cgroup_protection mem_cgroup_protected(struct mem_cgroup *root,
>
>         if (!root)
>                 root = root_mem_cgroup;
> -       if (memcg == root)
> +       if (memcg == root) {
> +               /*
> +                * The cgroup is the reclaim root in this reclaim
> +                * cycle, and therefore not protected. But it may have
> +                * stale effective protection values from previous
> +                * cycles in which it was not the reclaim root - for
> +                * example, global reclaim followed by limit reclaim.
> +                * Reset these values for mem_cgroup_protection().
> +                */
> +               WRITE_ONCE(memcg->memory.emin, 0);
> +               WRITE_ONCE(memcg->memory.elow, 0);


Hi Chris,

Would you pls. add some comments above these newly added WRITE_ONCE() ?
E.g.
What does them mean to fix ?
Why do we must add WRITE_ONCE() and READ_ONCE here and there all over
the memcg protection ?
Otherwise, it may be harder to understand by the others.


>                 return MEMCG_PROT_NONE;
> +       }
>
>         usage = page_counter_read(&memcg->memory);
>         if (!usage)
> --
> 2.26.2
>


-- 
Thanks
Yafang