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=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,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 E3308C433FF for ; Wed, 31 Jul 2019 07:21:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B5C5C218A4 for ; Wed, 31 Jul 2019 07:21:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1564557664; bh=BRyD0XI4GwRe0VUf5a7t1pGnEl0FtkVXZqDPETjZ4Go=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=sAc9R62jIcubnJYxorT/k4d8aRspUXCvDEnXt4IZdfHqhiwwP9kR+YpW0fA22I7N4 ZGVBNg3yTh0eq1yejSAXH6Gc/udS/j65GgbACds5VzTlHB0tpUHUKg2WL33kN3Npk1 NOT4p5Fx0aBMbdmqSSfSwX/70hoA0wIfMTzoI3DE= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727488AbfGaHVD (ORCPT ); Wed, 31 Jul 2019 03:21:03 -0400 Received: from mx2.suse.de ([195.135.220.15]:38478 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1725913AbfGaHVD (ORCPT ); Wed, 31 Jul 2019 03:21:03 -0400 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 E663DAE52; Wed, 31 Jul 2019 07:21:01 +0000 (UTC) Date: Wed, 31 Jul 2019 09:21:01 +0200 From: Michal Hocko To: Minchan Kim Cc: Andrew Morton , LKML , linux-mm , Miguel de Dios , Wei Wang , Johannes Weiner , Mel Gorman , Nicholas Piggin Subject: Re: [PATCH] mm: release the spinlock on zap_pte_range Message-ID: <20190731072101.GX9330@dhcp22.suse.cz> References: <20190729071037.241581-1-minchan@kernel.org> <20190729074523.GC9330@dhcp22.suse.cz> <20190729082052.GA258885@google.com> <20190729083515.GD9330@dhcp22.suse.cz> <20190730121110.GA184615@google.com> <20190730123237.GR9330@dhcp22.suse.cz> <20190730123935.GB184615@google.com> <20190730125751.GS9330@dhcp22.suse.cz> <20190731054447.GB155569@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190731054447.GB155569@google.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 31-07-19 14:44:47, Minchan Kim wrote: > On Tue, Jul 30, 2019 at 02:57:51PM +0200, Michal Hocko wrote: > > [Cc Nick - the email thread starts http://lkml.kernel.org/r/20190729071037.241581-1-minchan@kernel.org > > A very brief summary is that mark_page_accessed seems to be quite > > expensive and the question is whether we still need it and why > > SetPageReferenced cannot be used instead. More below.] > > > > On Tue 30-07-19 21:39:35, Minchan Kim wrote: [...] > > > commit bf3f3bc5e73 > > > Author: Nick Piggin > > > Date: Tue Jan 6 14:38:55 2009 -0800 > > > > > > mm: don't mark_page_accessed in fault path > > > > > > Doing a mark_page_accessed at fault-time, then doing SetPageReferenced at > > > unmap-time if the pte is young has a number of problems. > > > > > > mark_page_accessed is supposed to be roughly the equivalent of a young pte > > > for unmapped references. Unfortunately it doesn't come with any context: > > > after being called, reclaim doesn't know who or why the page was touched. > > > > > > So calling mark_page_accessed not only adds extra lru or PG_referenced > > > manipulations for pages that are already going to have pte_young ptes anyway, > > > but it also adds these references which are difficult to work with from the > > > context of vma specific references (eg. MADV_SEQUENTIAL pte_young may not > > > wish to contribute to the page being referenced). > > > > > > Then, simply doing SetPageReferenced when zapping a pte and finding it is > > > young, is not a really good solution either. SetPageReferenced does not > > > correctly promote the page to the active list for example. So after removing > > > mark_page_accessed from the fault path, several mmap()+touch+munmap() would > > > have a very different result from several read(2) calls for example, which > > > is not really desirable. > > > > Well, I have to say that this is rather vague to me. Nick, could you be > > more specific about which workloads do benefit from this change? Let's > > say that the zapped pte is the only referenced one and then reclaim > > finds the page on inactive list. We would go and reclaim it. But does > > that matter so much? Hot pages would be referenced from multiple ptes > > very likely, no? > > As Nick mentioned in the description, without mark_page_accessed in > zapping part, repeated mmap + touch + munmap never acticated the page > while several read(2) calls easily promote it. And is this really a problem? If we refault the same page then the refaults detection should catch it no? In other words is the above still a problem these days? -- Michal Hocko SUSE Labs