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=-2.5 required=3.0 tests=MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 EC08CCA9EC3 for ; Thu, 31 Oct 2019 09:16:05 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id B349A20862 for ; Thu, 31 Oct 2019 09:16:05 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org B349A20862 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 48AC26B0007; Thu, 31 Oct 2019 05:16:05 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 4392D6B0008; Thu, 31 Oct 2019 05:16:05 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 34FCC6B000A; Thu, 31 Oct 2019 05:16:05 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0244.hostedemail.com [216.40.44.244]) by kanga.kvack.org (Postfix) with ESMTP id 158F26B0007 for ; Thu, 31 Oct 2019 05:16:05 -0400 (EDT) Received: from smtpin26.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with SMTP id 993933D15 for ; Thu, 31 Oct 2019 09:16:04 +0000 (UTC) X-FDA: 76103523048.26.voice50_350078e1c054d X-HE-Tag: voice50_350078e1c054d X-Filterd-Recvd-Size: 3420 Received: from mx1.suse.de (mx2.suse.de [195.135.220.15]) by imf31.hostedemail.com (Postfix) with ESMTP for ; Thu, 31 Oct 2019 09:16:04 +0000 (UTC) X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 4B6CFB12D; Thu, 31 Oct 2019 09:16:02 +0000 (UTC) Date: Thu, 31 Oct 2019 10:16:01 +0100 From: Michal Hocko To: Johannes Weiner Cc: Minchan Kim , zhong jiang , akpm@linux-foundation.org, ktkhai@virtuozzo.com, linux-mm@kvack.org Subject: Re: [PATCH] mm: fix unevictable page reclaim when calling madvise_pageout Message-ID: <20191031091601.GE13102@dhcp22.suse.cz> References: <1572275317-63910-1-git-send-email-zhongjiang@huawei.com> <20191029081102.GB31513@dhcp22.suse.cz> <5DB806D1.8020503@huawei.com> <20191029094039.GH31513@dhcp22.suse.cz> <5DB81838.6020208@huawei.com> <20191030165239.GA167773@google.com> <20191030174533.GL31513@dhcp22.suse.cz> <20191030193307.GA48128@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191030193307.GA48128@cmpxchg.org> User-Agent: Mutt/1.10.1 (2018-07-13) 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 Wed 30-10-19 15:33:07, Johannes Weiner wrote: > On Wed, Oct 30, 2019 at 06:45:33PM +0100, Michal Hocko wrote: > > On Wed 30-10-19 09:52:39, Minchan Kim wrote: [...] > > > madvise_pageout could work with a shared page and one of the vmas among processes > > > could do mlock so it could pass Unevictable LRU pages into shrink_page_list. > > > It's pointless to try reclaim unevictable pages from the beginning so I want to fix > > > madvise_pageout via introducing only_evictable flag into the API so that > > > madvise_pageout uses it as "true". > > > > > > If we want to remove the PageUnevictable VM_BUG_ON_PAGE in shrink_page_list, > > > I want to see more strong reason why it happens and why caller couldn't > > > filter them out from the beginning. > > > > Why is this preferable over removing the VM_BUG_ON condition? In other > > words why should we keep PageUnevictable check there? > > The mlock LRU shuffling is a bit tricky and can race with page reclaim > or others isolating the page from the LRU list. If another isolator > wins, it has to move the page during putback on behalf of mlock. > > See the implementation and comments in __pagevec_lru_add_fn(). > > That's why page reclaim can see !page_evictable(), but it must not see > pages that have the PageUnevictable lru bit already set. Because that > would mean the isolation/putback machinery messed up somewhere and the > page LRU state is corrupt. > > As that machinery is non-trivial, it's useful to have that sanity > check in page reclaim. Thanks for the clarification! This sounds reasonable (as much as the mlock juggling does) to me. This is probably worth a comment right above the bug_on. I have to confess that I am still not clear on all the details here, though. E.g. migrate_misplaced_transhuge_page sets the flag without lru_lock and relies only on page lock IIUC and the bug on is done right after the lock is released. Maybe I am just confused or maybe the race window is too small to matter but isn't this race possible at least theoretically? -- Michal Hocko SUSE Labs