From mboxrd@z Thu Jan 1 00:00:00 1970 From: Wei Yang Subject: Re: [Patch v4] mm: thp: remove the defer list related code since this will not happen Date: Mon, 20 Jan 2020 16:17:10 +0800 Message-ID: <20200120081710.GA18028@richard> References: <20200117233836.3434-1-richardw.yang@linux.intel.com> <20200118145421.0ab96d5d9bea21a3339d52fe@linux-foundation.org> <20200120072237.GA18451@dhcp22.suse.cz> Reply-To: Wei Yang Mime-Version: 1.0 Return-path: Content-Disposition: inline In-Reply-To: <20200120072237.GA18451@dhcp22.suse.cz> Sender: stable-owner@vger.kernel.org List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Michal Hocko Cc: David Rientjes , 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 On Mon, Jan 20, 2020 at 08:22:37AM +0100, Michal Hocko wrote: >On Sat 18-01-20 15:36:06, David Rientjes wrote: >> On Sat, 18 Jan 2020, Andrew Morton wrote: >> >> > On Sat, 18 Jan 2020 07:38:36 +0800 Wei Yang wrote: >> > >> > > If compound is true, this means it is a PMD mapped THP. Which implies >> > > the page is not linked to any defer list. So the first code chunk will >> > > not be executed. >> > > >> > > Also with this reason, it would not be proper to add this page to a >> > > defer list. So the second code chunk is not correct. >> > > >> > > Based on this, we should remove the defer list related code. >> > > >> > > Fixes: 87eaceb3faa5 ("mm: thp: make deferred split shrinker memcg aware") >> > > >> > > Signed-off-by: Wei Yang >> > > Suggested-by: Kirill A. Shutemov >> > > Cc: [5.4+] >> > >> > This patch is identical to "mm: thp: grab the lock before manipulating >> > defer list", which is rather confusing. Please let people know when >> > this sort of thing is done. >> > >> > The earlier changelog mentioned a possible race condition. This >> > changelog does not. In fact this changelog fails to provide any >> > description of any userspace-visible runtime effects of the bug. >> > Please send along such a description for inclusion, as always. >> > >> >> The locking concern that Wei was originally looking at is no longer an >> issue because we determined that the code in question could simply be >> removed. >> >> I think the following can be added to the changelog: >> >> ----->o----- >> >> 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. > Per my understanding, if we do the split when it is not necessary, we probably have a lower performance due to tlb miss. For others, I don't see the impact. >With that clarified, feel free to add > >Acked-by: Michal Hocko > >-- >Michal Hocko >SUSE Labs -- Wei Yang Help you, Help me