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=-5.5 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,UNPARSEABLE_RELAY,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 8FED2C433E0 for ; Tue, 11 Aug 2020 08:23:05 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 73B68206C3 for ; Tue, 11 Aug 2020 08:23:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728368AbgHKIXE (ORCPT ); Tue, 11 Aug 2020 04:23:04 -0400 Received: from out30-57.freemail.mail.aliyun.com ([115.124.30.57]:50623 "EHLO out30-57.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726397AbgHKIXE (ORCPT ); Tue, 11 Aug 2020 04:23:04 -0400 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R101e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e07425;MF=alex.shi@linux.alibaba.com;NM=1;PH=DS;RN=19;SR=0;TI=SMTPD_---0U5S3WRW_1597134176; Received: from IT-FVFX43SYHV2H.local(mailfrom:alex.shi@linux.alibaba.com fp:SMTPD_---0U5S3WRW_1597134176) by smtp.aliyun-inc.com(127.0.0.1); Tue, 11 Aug 2020 16:22:57 +0800 Subject: Re: [PATCH v17 14/21] mm/compaction: do page isolation first in compaction To: Alexander Duyck Cc: Andrew Morton , Mel Gorman , Tejun Heo , Hugh Dickins , Konstantin Khlebnikov , Daniel Jordan , Yang Shi , Matthew Wilcox , Johannes Weiner , kbuild test robot , linux-mm , LKML , cgroups@vger.kernel.org, Shakeel Butt , Joonsoo Kim , Wei Yang , "Kirill A. Shutemov" , Rong Chen References: <1595681998-19193-1-git-send-email-alex.shi@linux.alibaba.com> <1595681998-19193-15-git-send-email-alex.shi@linux.alibaba.com> <241ca157-104f-4f0d-7d5b-de394443788d@linux.alibaba.com> <8dbd004e-8eba-f1ec-a5eb-5dc551978936@linux.alibaba.com> From: Alex Shi Message-ID: Date: Tue, 11 Aug 2020 16:22:27 +0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:68.0) Gecko/20100101 Thunderbird/68.7.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 在 2020/8/10 下午10:41, Alexander Duyck 写道: > On Mon, Aug 10, 2020 at 6:10 AM Alex Shi wrote: >> >> >> >> 在 2020/8/7 下午10:51, Alexander Duyck 写道: >>> I wonder if this entire section shouldn't be restructured. This is the >>> only spot I can see where we are resetting the LRU flag instead of >>> pulling the page from the LRU list with the lock held. Looking over >>> the code it seems like something like that should be possible. I am >>> not sure the LRU lock is really protecting us in either the >>> PageCompound check nor the skip bits. It seems like holding a >>> reference on the page should prevent it from switching between >>> compound or not, and the skip bits are per pageblock with the LRU bits >>> being per node/memcg which I would think implies that we could have >>> multiple LRU locks that could apply to a single skip bit. >> >> Hi Alexander, >> >> I don't find problem yet on compound or skip bit usage. Would you clarify the >> issue do you concerned? >> >> Thanks! > > The point I was getting at is that the LRU lock is being used to > protect these and with your changes I don't think that makes sense > anymore. > > The skip bits are per-pageblock bits. With your change the LRU lock is > now per memcg first and then per node. As such I do not believe it > really provides any sort of exclusive access to the skip bits. I still > have to look into this more, but it seems like you need a lock per > either section or zone that can be used to protect those bits and deal > with this sooner rather than waiting until you have found an LRU page. > The one part that is confusing though is that the definition of the > skip bits seems to call out that they are a hint since they are not > protected by a lock, but that is exactly what has been happening here. > The skip bits are safe here, since even it race with other skip action, It will still skip out. The skip action is try not to compaction too much, not a exclusive action needs avoid race. > The point I was getting at with the PageCompound check is that instead > of needing the LRU lock you should be able to look at PageCompound as > soon as you call get_page_unless_zero() and preempt the need to set > the LRU bit again. Instead of trying to rely on the LRU lock to > guarantee that the page hasn't been merged you could just rely on the > fact that you are holding a reference to it so it isn't going to > switch between being compound or order 0 since it cannot be freed. It > spoils the idea I originally had of combining the logic for > get_page_unless_zero and TestClearPageLRU into a single function, but > the advantage is you aren't clearing the LRU flag unless you are > actually going to pull the page from the LRU list. Sorry, I still can not follow you here. Compound code part is unchanged and follow the original logical. So would you like to pose a new code to see if its works? Thanks Alex > > My main worry is that this is the one spot where we appear to be > clearing the LRU bit without ever actually pulling the page off of the > LRU list, and I am thinking we would be better served by addressing > the skip and PageCompound checks earlier rather than adding code to > set the bit again if either of those cases are encountered. This way > we don't pseudo-pin pages in the LRU if they are compound or supposed > to be skipped. > > Thanks. > > - Alex >