[Cc Konstantin - the patch is http://lkml.kernel.org/r/20181114211704.6381-3-david@xxxxxxxxxx] 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 <k.khlebnikov@xxxxxxxxxxx> > 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
