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Shutemov" , Roman Gushchin , 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200420221126.341272-3-hannes@cmpxchg.org> User-Agent: Mutt/1.5.24 (2015-08-30) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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 Reviewed-by: Joonsoo Kim Thanks.