linux-kernel - Re: [PATCH v2 2/3] mm: memcontrol: flush percpu slab vmstats on kmem offlining

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Date:   Mon, 19 Aug 2019 15:27:44 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Roman Gushchin <guro@...com>
Cc:     <linux-mm@...ck.org>, Michal Hocko <mhocko@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        <linux-kernel@...r.kernel.org>, <kernel-team@...com>,
        Vladimir Davydov <vdavydov.dev@...il.com>
Subject: Re: [PATCH v2 2/3] mm: memcontrol: flush percpu slab vmstats on
 kmem offlining

On Mon, 19 Aug 2019 13:23:37 -0700 Roman Gushchin <guro@...com> wrote:

> I've noticed that the "slab" value in memory.stat is sometimes 0,
> even if some children memory cgroups have a non-zero "slab" value.
> The following investigation showed that this is the result
> of the kmem_cache reparenting in combination with the per-cpu
> batching of slab vmstats.
> 
> At the offlining some vmstat value may leave in the percpu cache,
> not being propagated upwards by the cgroup hierarchy. It means
> that stats on ancestor levels are lower than actual. Later when
> slab pages are released, the precise number of pages is substracted
> on the parent level, making the value negative. We don't show negative
> values, 0 is printed instead.
> 
> To fix this issue, let's flush percpu slab memcg and lruvec stats
> on memcg offlining. This guarantees that numbers on all ancestor
> levels are accurate and match the actual number of outstanding
> slab pages.
> 
> Fixes: fb2f2b0adb98 ("mm: memcg/slab: reparent memcg kmem_caches on cgroup removal")
> Signed-off-by: Roman Gushchin <guro@...com>
> Cc: Johannes Weiner <hannes@...xchg.org>
> Cc: Michal Hocko <mhocko@...nel.org>
> Cc: Vladimir Davydov <vdavydov.dev@...il.com>

[1/3] and [3/3] have cc:stable.  [2/3] does not.  However [3/3] does
not correctly apply without [2/3] having being applied.