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[209.85.220.65]) by mx.google.com with SMTPS id q1sor68019256pfb.34.2019.08.06.03.55.15 for (Google Transport Security); Tue, 06 Aug 2019 03:55:16 -0700 (PDT) Received-SPF: pass (google.com: domain of minchan.kim@gmail.com designates 209.85.220.65 as permitted sender) client-ip=209.85.220.65; Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=blv8rluI; spf=pass (google.com: domain of minchan.kim@gmail.com designates 209.85.220.65 as permitted sender) smtp.mailfrom=minchan.kim@gmail.com; dmarc=fail (p=NONE sp=NONE dis=NONE) header.from=kernel.org DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=sender:date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=oa5z6zUvCHcd7r910UMaS4iQDg5URkCU1Ylsu0mgnns=; b=blv8rluI18O6mziiEb2dE4tZgdd+rRx2neLdiacDFpSj6cKi/JVgKIEpfl3XbLdHZ3 z2Z4xTX0KMsT6r+sQv4cwLvWDRXLPdJ7HxEFdqDAS69w2UjEgYqX1O54hmoPijaPpOlg 4fPbhHpn0CBaDJeekIvtg6hyw+Fh7fyBune0pzfEetyOLl1L+SOwPdqGNQv6/CUArNrn Ahtddt682EsIVhH5tqFu7VRgj+wmbX61q9Xj6PqIbMeCkJ/7w+UXuorVaVkKZ7n3e/eB 3re38XoitkuxZjIsHBlY2sfzc9MyeITx+MK0hNdTz/4P3hwVyHQxTtogGSezk/+NYOuJ T6IA== X-Google-Smtp-Source: APXvYqyQN7R6BSpr072n52qskPQBVMmzd3IQWnQI+qk4MMW2i6gA9H2POAcCLk9dZyoMIglmWEsqWg== X-Received: by 2002:a62:5c3:: with SMTP id 186mr3056758pff.144.1565088915615; Tue, 06 Aug 2019 03:55:15 -0700 (PDT) Received: from google.com ([2401:fa00:d:0:98f1:8b3d:1f37:3e8]) by smtp.gmail.com with ESMTPSA id z24sm15294361pga.2.2019.08.06.03.55.11 (version=TLS1_3 cipher=AEAD-AES256-GCM-SHA384 bits=256/256); Tue, 06 Aug 2019 03:55:14 -0700 (PDT) Date: Tue, 6 Aug 2019 19:55:09 +0900 From: Minchan Kim To: Michal Hocko 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: <20190806105509.GA94582@google.com> 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> <20190731072101.GX9330@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190731072101.GX9330@dhcp22.suse.cz> 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, Jul 31, 2019 at 09:21:01AM +0200, Michal Hocko wrote: > 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? I admit we have been not fair for them because read(2) syscall pages are easily promoted regardless of zap timing unlike mmap-based pages. However, if we remove the mark_page_accessed in the zap_pte_range, it would make them more unfair in that read(2)-accessed pages are easily promoted while mmap-based page should go through refault to be promoted. I also want to remove the costly overhead from the hot path but couldn't come up with nice solution.