From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Wed, 14 Nov 2018 18:07:26 -0800 From: Mike Rapoport To: David Hildenbrand 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 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Message-Id: <20181115020725.GC2353@rapoport-lnx> Sender: owner-linux-mm@kvack.org List-ID: 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? > 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. > > > > I have no objection to renaming the balloon bit inside the kernel; I > > think that's a wise idea. I'm just not sure whether we should rename > > the user balloon bit rather than adding a new bit. > > > > Can we rename without breaking uapi? > > -- > > Thanks, > > David / dhildenb > -- Sincerely yours, Mike.