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Date:   Thu, 2 Jul 2020 19:10:11 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Vlastimil Babka <vbabka@suse.cz>
Cc:     Roman Gushchin <guro@fb.com>,
        Naresh Kamboju <naresh.kamboju@linaro.org>,
        Shakeel Butt <shakeelb@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-mm <linux-mm@kvack.org>,
        open list <linux-kernel@vger.kernel.org>,
        lkft-triage@lists.linaro.org, Chris Down <chris@chrisdown.name>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: BUG: Bad page state in process - page dumped because: page still
 charged to cgroup
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On Thu 02-07-20 18:35:31, Vlastimil Babka wrote:
> On 7/2/20 6:22 PM, Michal Hocko wrote:
> > On Wed 01-07-20 11:45:52, Roman Gushchin wrote:
> > [...]
> >> >From c97afecd32c0db5e024be9ba72f43d22974f5bcd Mon Sep 17 00:00:00 2001
> >> From: Roman Gushchin <guro@fb.com>
> >> Date: Wed, 1 Jul 2020 11:05:32 -0700
> >> Subject: [PATCH] mm: kmem: make memcg_kmem_enabled() irreversible
> >> 
> >> Historically the kernel memory accounting was an opt-in feature, which
> >> could be enabled for individual cgroups. But now it's not true, and
> >> it's on by default both on cgroup v1 and cgroup v2.  And as long as a
> >> user has at least one non-root memory cgroup, the kernel memory
> >> accounting is on. So in most setups it's either always on (if memory
> >> cgroups are in use and kmem accounting is not disabled), either always
> >> off (otherwise).
> >> 
> >> memcg_kmem_enabled() is used in many places to guard the kernel memory
> >> accounting code. If memcg_kmem_enabled() can reverse from returning
> >> true to returning false (as now), we can't rely on it on release paths
> >> and have to check if it was on before.
> >> 
> >> If we'll make memcg_kmem_enabled() irreversible (always returning true
> >> after returning it for the first time), it'll make the general logic
> >> more simple and robust. It also will allow to guard some checks which
> >> otherwise would stay unguarded.
> >> 
> >> Signed-off-by: Roman Gushchin <guro@fb.com>
> 
> Fixes: ? or let Andrew squash it to some patch of your series (it's in mmotm I
> think)?

I would rather make it its own patch because this is really subtle.
-- 
Michal Hocko
SUSE Labs