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 7F79FC31E40 for ; Tue, 6 Aug 2019 11:57:10 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5645620C01 for ; Tue, 6 Aug 2019 11:57:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1565092630; bh=jmdggg38TboDRj5TTtGvby9FdDxzrB8Veg6nuDixYJ4=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=YR5BV/y26XVbKUX9KAIJU351YOPxpRJ+sr6wPb75RX96rcQ2Zvy66TocMyH88Wpr/ 8ApSf1gTwIjCbGnnSY5ft1wHSSkqH2BIoqqkgOHsEW+Yij6GXSN8PKlpWw/HamrHxI mbZSSKUZ9SlAtyTmAFrZqpGfbOnBYLtjGYQ2nM6I= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730992AbfHFL5J (ORCPT ); Tue, 6 Aug 2019 07:57:09 -0400 Received: from mx2.suse.de ([195.135.220.15]:33706 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726243AbfHFL5I (ORCPT ); Tue, 6 Aug 2019 07:57:08 -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 CF36EAD95; Tue, 6 Aug 2019 11:57:05 +0000 (UTC) Date: Tue, 6 Aug 2019 13:57:03 +0200 From: Michal Hocko To: Joel Fernandes Cc: linux-kernel@vger.kernel.org, Robin Murphy , Alexey Dobriyan , Andrew Morton , Borislav Petkov , Brendan Gregg , Catalin Marinas , Christian Hansen , dancol@google.com, fmayer@google.com, "H. Peter Anvin" , Ingo Molnar , Jonathan Corbet , Kees Cook , kernel-team@android.com, linux-api@vger.kernel.org, linux-doc@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, Mike Rapoport , minchan@kernel.org, namhyung@google.com, paulmck@linux.ibm.com, Roman Gushchin , Stephen Rothwell , surenb@google.com, Thomas Gleixner , tkjos@google.com, Vladimir Davydov , Vlastimil Babka , Will Deacon Subject: Re: [PATCH v4 3/5] [RFC] arm64: Add support for idle bit in swap PTE Message-ID: <20190806115703.GY11812@dhcp22.suse.cz> References: <20190805170451.26009-1-joel@joelfernandes.org> <20190805170451.26009-3-joel@joelfernandes.org> <20190806084203.GJ11812@dhcp22.suse.cz> <20190806103627.GA218260@google.com> <20190806104755.GR11812@dhcp22.suse.cz> <20190806111446.GA117316@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190806111446.GA117316@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 Tue 06-08-19 07:14:46, Joel Fernandes wrote: > On Tue, Aug 06, 2019 at 12:47:55PM +0200, Michal Hocko wrote: > > On Tue 06-08-19 06:36:27, Joel Fernandes wrote: > > > On Tue, Aug 06, 2019 at 10:42:03AM +0200, Michal Hocko wrote: > > > > On Mon 05-08-19 13:04:49, Joel Fernandes (Google) wrote: > > > > > This bit will be used by idle page tracking code to correctly identify > > > > > if a page that was swapped out was idle before it got swapped out. > > > > > Without this PTE bit, we lose information about if a page is idle or not > > > > > since the page frame gets unmapped. > > > > > > > > And why do we need that? Why cannot we simply assume all swapped out > > > > pages to be idle? They were certainly idle enough to be reclaimed, > > > > right? Or what does idle actualy mean here? > > > > > > Yes, but other than swapping, in Android a page can be forced to be swapped > > > out as well using the new hints that Minchan is adding? > > > > Yes and that is effectivelly making them idle, no? > > That depends on how you think of it. I would much prefer to have it documented so that I do not have to guess ;) > If you are thinking of a monitoring > process like a heap profiler, then from the heap profiler's (that only cares > about the process it is monitoring) perspective it will look extremely odd if > pages that are recently accessed by the process appear to be idle which would > falsely look like those processes are leaking memory. The reality being, > Android forced those pages into swap because of other reasons. I would like > for the swapping mechanism, whether forced swapping or memory reclaim, not to > interfere with the idle detection. Hmm, but how are you going to handle situation when the page is unmapped and refaulted again (e.g. a normal reclaim of a pagecache)? You are losing that information same was as in the swapout case, no? Or am I missing something? > This is just an effort to make the idle tracking a little bit better. We > would like to not lose the 'accessed' information of the pages. > > Initially, I had proposed what you are suggesting as well however the above > reasons made me to do it like this. Also Minchan and Konstantin suggested > this, so there are more people interested in the swap idle bit. Minchan, can > you provide more thoughts here? (He is on 2-week vacation from today so > hopefully replies before he vanishes ;-)). We can move on with the rest of the series in the mean time but I would like to see a proper justification for the swap entries and why they should be handled special. > Also assuming all swap pages as idle has other "semantic" issues. It is quite > odd if a swapped page is automatically marked as idle without userspace > telling it to. Consider the following set of events: 1. Userspace marks only > a certain memory region as idle. 2. Userspace reads back the bits > corresponding to a bigger region. Part of this bigger region is swapped. > Userspace expects all of the pages it did not mark, to have idle bit set to > '0' because it never marked them as idle. However if it is now surprised by > what it read back (not all '0' read back). Since a page is swapped, it will > be now marked "automatically" as idle as per your proposal, even if userspace > never marked it explicity before. This would be quite confusing/ambiguous. OK, I see. I guess the primary question I have is how do you distinguish Idle page which got unmapped and faulted in again from swapped out page and refaulted - including the time the pte is not present. -- Michal Hocko SUSE Labs