On Wed, Jun 24, 2020 at 06:01:02PM +0200, David Hildenbrand wrote: > On 24.06.20 17:37, Michael S. Tsirkin wrote: > > On Wed, Jun 24, 2020 at 05:28:59PM +0200, David Hildenbrand wrote: > >>> So at the high level the idea was simple, we just clear the dirty bit > >>> when page is hinted, unless we sent a new command since. Implementation > >>> was reviewed by migration maintainers. If there's a consensus the code > >>> is written so badly we can't maintain it, maybe we should remove it. > >>> Which parts are unmaintainable in your eyes - migration or virtio ones? > >> > >> QEMU implementation without a propert virtio specification. I hope that > >> we can *at least* finally document the expected behavior. Alex gave it a > >> shot, and I was hoping that Wei could jump in to clarify, help move this > >> forward ... after all he implemented (+designed?) the feature and the > >> virtio interface. > >> > >>> Or maybe it's the general thing that interface was never specced > >>> properly. > >> > >> Yes, a spec would be definitely a good starter ... > >> > >> [...] > >> > >>>> > >>>> 1. If migration fails during RAM precopy, the guest will never receive a > >>>> DONE notification. Probably easy to fix. > >>>> > >>>> 2. Unclear semantics. Alex tried to document what the actual semantics > >>>> of hinted pages are. > >>> > >>> I'll reply to that now. > >>> > >>>> Assume the following in the guest to a previously > >>>> hinted page > >>>> > >>>> /* page was hinted and is reused now */ > >>>> if (page[x] != Y) > >>>> page[x] == Y; > >>>> /* migration ends, we now run on the destination */ > >>>> BUG_ON(page[x] != Y); > >>>> /* BUG, because the content chan > >>> > >>> The assumption hinting makes is that data in page is writtent to before > >>> it's used. > >>> > >>> > >>>> A guest can observe that. And that could be a random driver that just > >>>> allocated a page. > >>>> > >>>> (I *assume* in Linux we might catch that using kasan, but I am not 100% > >>>> sure, also, the actual semantics to document are unclear - e.g., for > >>>> other guests) > >>> > >>> I think it's basically simple: hinting means it's ok to > >>> fill page with trash unless it has been modified since the command > >>> ID supplied. > >> > >> Yeah, I quite dislike the semantics, especially, as they are different > >> to well-know semantics as e.g., represent in MADV_FREE. Getting changed > >> content when reading is really weird. But it seemed to be easier to > >> implement (low hanging fruit) and nobody complained back then. Well, now > >> we are stuck with it. > >> > >> [..] > > > > The difference with MADV_FREE is > > - asynchronous (using cmd id to synchronize) > > - zero not guaranteed > > > > right? > > *looking into man page*, yes, when reading you either get the old > content or zero. > > (I remember that a re-read also makes the content stable, but looks like > you really have to write to a page) > > We should most probably do what Alex suggested and initialize pages (at > least write a single byte) when leaking them from the shrinker in the > guest while hinting is active, such that the content is stable for > anybody to allocate and reuse a page.
Drivers ignore old content from slab though, so I don't really see the point. > -- > Thanks, > > David / dhildenb
