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From:   Shakeel Butt <shakeelb@google.com>
Date:   Wed, 22 Apr 2020 11:02:22 -0700
Message-ID: <CALvZod5XC+v_ThY=QyiXu9uWgkuc6x63jwUjW9mdWueoPh3LBw@mail.gmail.com>
Subject: Re: [PATCH 02/18] mm: memcontrol: fix theoretical race in charge moving
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Joonsoo Kim <js1304@gmail.com>,
        Alex Shi <alex.shi@linux.alibaba.com>,
        Hugh Dickins <hughd@google.com>,
        Michal Hocko <mhocko@suse.com>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Roman Gushchin <guro@fb.com>, Linux MM <linux-mm@kvack.org>,
        Cgroups <cgroups@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Kernel Team <kernel-team@fb.com>
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On Mon, Apr 20, 2020 at 3:11 PM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> The move_lock is a per-memcg lock, but the VM accounting code that
> needs to acquire it comes from the page and follows page->mem_cgroup
> under RCU protection. That means that the page becomes unlocked not
> when we drop the move_lock, but when we update page->mem_cgroup. And
> that assignment doesn't imply any memory ordering. If that pointer
> write gets reordered against the reads of the page state -
> page_mapped, PageDirty etc. the state may change while we rely on it
> being stable and we can end up corrupting the counters.
>
> Place an SMP memory barrier to make sure we're done with all page
> state by the time the new page->mem_cgroup becomes visible.
>
> Also replace the open-coded move_lock with a lock_page_memcg() to make
> it more obvious what we're serializing against.
>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

Reviewed-by: Shakeel Butt <shakeelb@google.com>