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Date:   Wed, 22 Apr 2020 15:36:16 +0900
From:   Joonsoo Kim <js1304@gmail.com>
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Alex Shi <alex.shi@linux.alibaba.com>,
        Shakeel Butt <shakeelb@google.com>,
        Hugh Dickins <hughd@google.com>,
        Michal Hocko <mhocko@suse.com>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Roman Gushchin <guro@fb.com>, linux-mm@kvack.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@fb.com
Subject: Re: [PATCH 02/18] mm: memcontrol: fix theoretical race in charge
 moving
Message-ID: <20200422063615.GB6780@js1304-desktop>
References: <20200420221126.341272-1-hannes@cmpxchg.org>
 <20200420221126.341272-3-hannes@cmpxchg.org>
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On Mon, Apr 20, 2020 at 06:11:10PM -0400, Johannes Weiner 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: Joonsoo Kim <iamjoonsoo.kim@lge.com>

Thanks.