From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 6FCCDC33CA1 for ; Mon, 20 Jan 2020 08:17:07 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 3F756207E0 for ; Mon, 20 Jan 2020 08:17:07 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 3F756207E0 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.intel.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id B595E6B05CC; Mon, 20 Jan 2020 03:17:06 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id B09AE6B05CD; Mon, 20 Jan 2020 03:17:06 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 9F89B6B05CE; Mon, 20 Jan 2020 03:17:06 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0056.hostedemail.com [216.40.44.56]) by kanga.kvack.org (Postfix) with ESMTP id 899CC6B05CC for ; Mon, 20 Jan 2020 03:17:06 -0500 (EST) Received: from smtpin15.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay03.hostedemail.com (Postfix) with SMTP id 3DD938248047 for ; Mon, 20 Jan 2020 08:17:06 +0000 (UTC) X-FDA: 76397307252.15.can65_20cac5b317e4e X-HE-Tag: can65_20cac5b317e4e X-Filterd-Recvd-Size: 5195 Received: from mga18.intel.com (mga18.intel.com [134.134.136.126]) by imf09.hostedemail.com (Postfix) with ESMTP for ; Mon, 20 Jan 2020 08:17:05 +0000 (UTC) X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from fmsmga002.fm.intel.com ([10.253.24.26]) by orsmga106.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 20 Jan 2020 00:17:02 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.70,341,1574150400"; d="scan'208";a="258624482" Received: from richard.sh.intel.com (HELO localhost) ([10.239.159.54]) by fmsmga002.fm.intel.com with ESMTP; 20 Jan 2020 00:17:00 -0800 Date: Mon, 20 Jan 2020 16:17:10 +0800 From: Wei Yang 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 Subject: Re: [Patch v4] mm: thp: remove the defer list related code since this will not happen Message-ID: <20200120081710.GA18028@richard> Reply-To: Wei Yang References: <20200117233836.3434-1-richardw.yang@linux.intel.com> <20200118145421.0ab96d5d9bea21a3339d52fe@linux-foundation.org> <20200120072237.GA18451@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200120072237.GA18451@dhcp22.suse.cz> User-Agent: Mutt/1.9.4 (2018-02-28) 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, 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