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Mon, 20 Jan 2020 13:10:57 -0800 (PST) Received: from [2620:15c:17:3:3a5:23a7:5e32:4598] ([2620:15c:17:3:3a5:23a7:5e32:4598]) by smtp.gmail.com with ESMTPSA id j14sm38145864pgs.57.2020.01.20.13.10.57 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 20 Jan 2020 13:10:57 -0800 (PST) Date: Mon, 20 Jan 2020 13:10:56 -0800 (PST) From: David Rientjes X-X-Sender: rientjes@chino.kir.corp.google.com To: Michal Hocko cc: Andrew Morton , Wei Yang , hannes@cmpxchg.org, vdavydov.dev@gmail.com, ktkhai@virtuozzo.com, kirill.shutemov@linux.intel.com, yang.shi@linux.alibaba.com, cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, alexander.duyck@gmail.com, stable@vger.kernel.org Subject: Re: [Patch v4] mm: thp: remove the defer list related code since this will not happen In-Reply-To: <20200120072237.GA18451@dhcp22.suse.cz> Message-ID: References: <20200117233836.3434-1-richardw.yang@linux.intel.com> <20200118145421.0ab96d5d9bea21a3339d52fe@linux-foundation.org> <20200120072237.GA18451@dhcp22.suse.cz> User-Agent: Alpine 2.21 (DEB 202 2017-01-01) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII 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, 20 Jan 2020, Michal Hocko wrote: > > When migrating memcg charges of thp memory, there are two possibilities: > > > > (1) The underlying compound page is mapped by a pmd and thus does is not > > on a deferred split queue (it's mapped), or > > > > (2) The compound page is not mapped by a pmd and is awaiting split on a > > deferred split queue. > > > > The current charge migration implementation does *not* migrate charges for > > thp memory on the deferred split queue, it only migrates charges for pages > > that are mapped by a pmd. > > > > Thus, to migrate charges, the underlying compound page cannot be on a > > deferred split queue; no list manipulation needs to be done in > > mem_cgroup_move_account(). > > > > With the current code, the underlying compound page is moved to the > > deferred split queue of the memcg its memory is not charged to, so > > susbequent reclaim will consider these pages for the wrong memcg. Remove > > the deferred split queue handling in mem_cgroup_move_account() entirely. > > I believe this still doesn't describe the underlying problem to the full > extent. What happens with the page on the deferred list when it > shouldn't be there in fact? Unless I am missing something deferred_split_scan > will simply split that huge page. Which is a bit unfortunate but nothing > really critical. This should be mentioned in the changelog. > Are you referring to a compound page on the deferred split queue before a task is moved? I'm not sure this is within the scope of Wei's patch.. this is simply preventing a page from being moved to the deferred split queue of a memcg that it is not charged to. Is there a concern about why this code can be removed or a suggestion on something else it should be doing instead?