From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-f69.google.com (mail-ed1-f69.google.com [209.85.208.69]) by kanga.kvack.org (Postfix) with ESMTP id 14A816B02CD for ; Thu, 15 Nov 2018 07:20:00 -0500 (EST) Received: by mail-ed1-f69.google.com with SMTP id l45so9385580edb.1 for ; Thu, 15 Nov 2018 04:20:00 -0800 (PST) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id x52si1056683edx.285.2018.11.15.04.19.58 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 15 Nov 2018 04:19:58 -0800 (PST) Date: Thu, 15 Nov 2018 13:19:50 +0100 From: Michal Hocko Subject: Re: [PATCH RFC 2/6] mm: convert PG_balloon to PG_offline Message-ID: <20181115121950.GQ23831@dhcp22.suse.cz> References: <20181114211704.6381-1-david@redhat.com> <20181114211704.6381-3-david@redhat.com> <20181114222321.GB1784@bombadil.infradead.org> <20181115020725.GC2353@rapoport-lnx> <5730ee16-9b18-ad3d-0fb3-e9edb55e2298@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5730ee16-9b18-ad3d-0fb3-e9edb55e2298@redhat.com> Sender: owner-linux-mm@kvack.org List-ID: To: David Hildenbrand Cc: Mike Rapoport , Matthew Wilcox , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, devel@linuxdriverproject.org, linux-fsdevel@vger.kernel.org, linux-pm@vger.kernel.org, xen-devel@lists.xenproject.org, Jonathan Corbet , Alexey Dobriyan , Mike Rapoport , Andrew Morton , Christian Hansen , Vlastimil Babka , "Kirill A. Shutemov" , Stephen Rothwell , "Michael S. Tsirkin" , Pavel Tatashin , Alexander Duyck , Naoya Horiguchi , Miles Chen , David Rientjes , Konstantin Khlebnikov [Cc Konstantin - the patch is http://lkml.kernel.org/r/20181114211704.6381-3-david@redhat.com] On Thu 15-11-18 10:21:13, David Hildenbrand wrote: > On 15.11.18 03:07, Mike Rapoport wrote: > > On Wed, Nov 14, 2018 at 11:49:15PM +0100, David Hildenbrand wrote: > >> On 14.11.18 23:23, Matthew Wilcox wrote: > >>> On Wed, Nov 14, 2018 at 10:17:00PM +0100, David Hildenbrand wrote: > >>>> Rename PG_balloon to PG_offline. This is an indicator that the page is > >>>> logically offline, the content stale and that it should not be touched > >>>> (e.g. a hypervisor would have to allocate backing storage in order for the > >>>> guest to dump an unused page). We can then e.g. exclude such pages from > >>>> dumps. > >>>> > >>>> In following patches, we will make use of this bit also in other balloon > >>>> drivers. While at it, document PGTABLE. > >>> > >>> Thank you for documenting PGTABLE. I didn't realise I also had this > >>> document to update when I added PGTABLE. > >> > >> Thank you for looking into this :) > >> > >>> > >>>> +++ b/Documentation/admin-guide/mm/pagemap.rst > >>>> @@ -78,6 +78,8 @@ number of times a page is mapped. > >>>> 23. BALLOON > >>>> 24. ZERO_PAGE > >>>> 25. IDLE > >>>> + 26. PGTABLE > >>>> + 27. OFFLINE > >>> > >>> So the offline *user* bit is new ... even though the *kernel* bit > >>> just renames the balloon bit. I'm not sure how I feel about this. > >>> I'm going to think about it some more. Could you share your decision > >>> process with us? > >> > >> BALLOON was/is documented as > >> > >> "23 - BALLOON > >> balloon compaction page > >> " > >> > >> and only includes all virtio-ballon pages after the non-lru migration > >> feature has been implemented for ballooned pages. Since then, this flag > >> does basically no longer stands for what it actually was supposed to do. > > > > Perhaps I missing something, but how the user should interpret "23" when he > > reads /proc/kpageflags? > > Looking at the history in more detail: > > commit 09316c09dde33aae14f34489d9e3d243ec0d5938 > Author: Konstantin Khlebnikov > Date: Thu Oct 9 15:29:32 2014 -0700 > > mm/balloon_compaction: add vmstat counters and kpageflags bit > > Always mark pages with PageBalloon even if balloon compaction is > disabled > and expose this mark in /proc/kpageflags as KPF_BALLOON. > > > So KPF_BALLOON was exposed when virtio-balloon pages were always marked > with PG_balloon. So the documentation is actually wrong ("balloon page" > vs. "balloon compaction page"). > > I have no idea who actually used that information. I suspect this was > just some debugging aid. > > > > >> To not break uapi I decided to not rename it but instead to add a new flag. > > > > I've got a feeling that uapi was anyway changed for the BALLON flag > > meaning. > > Yes. If we *replace* KPF_BALLOON by KPF_OFFLINE > > a) Some applications might no longer compile (I guess that's ok) > b) Some old applications will treat KPF_OFFLINE like KPF_BALLOON (which > should at least for virtio-balloon usage until now be fine - it is just > more generic) I do not think any compilation could break. If the semantic of the flag is preserved then everything should work as expected. -- Michal Hocko SUSE Labs