From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qk1-f200.google.com (mail-qk1-f200.google.com [209.85.222.200]) by kanga.kvack.org (Postfix) with ESMTP id 8F5586B0008 for ; Thu, 15 Nov 2018 04:21:29 -0500 (EST) Received: by mail-qk1-f200.google.com with SMTP id 67so43149407qkj.18 for ; Thu, 15 Nov 2018 01:21:29 -0800 (PST) Received: from mx1.redhat.com (mx1.redhat.com. [209.132.183.28]) by mx.google.com with ESMTPS id x65-v6si1214157qke.54.2018.11.15.01.21.27 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 15 Nov 2018 01:21:28 -0800 (PST) Subject: Re: [PATCH RFC 2/6] mm: convert PG_balloon to PG_offline References: <20181114211704.6381-1-david@redhat.com> <20181114211704.6381-3-david@redhat.com> <20181114222321.GB1784@bombadil.infradead.org> <20181115020725.GC2353@rapoport-lnx> From: David Hildenbrand Message-ID: <5730ee16-9b18-ad3d-0fb3-e9edb55e2298@redhat.com> Date: Thu, 15 Nov 2018 10:21:13 +0100 MIME-Version: 1.0 In-Reply-To: <20181115020725.GC2353@rapoport-lnx> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: owner-linux-mm@kvack.org List-ID: To: Mike Rapoport Cc: 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" , Michal Hocko , Pavel Tatashin , Alexander Duyck , Naoya Horiguchi , Miles Chen , David Rientjes 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) So I guess it's up to Maintainers/Matthew to decide :) -- Thanks, David / dhildenb