On Mon 28-01-19 15:38:38, David Hildenbrand wrote: > On 28.01.19 14:35, Michal Hocko wrote: > > On Mon 28-01-19 14:22:52, David Hildenbrand wrote: > >> On 28.01.19 14:21, Michal Hocko wrote: > >>> On Mon 28-01-19 14:14:28, David Hildenbrand wrote: > >>>> On 28.01.19 14:07, Michal Hocko wrote: > >>>>> On Mon 28-01-19 13:16:09, David Hildenbrand wrote: > >>>>> [...] > >>>>>> My theory: > >>>>>> > >>>>>> In __unmap_and_move(), we lock the old and newpage and perform the > >>>>>> migration. In case of vitio-balloon, the new page will become > >>>>>> movable, the old page will no longer be movable. > >>>>>> > >>>>>> However, after unlocking newpage, I think there is nothing stopping > >>>>>> the newpage from getting dequeued and freed by virtio-balloon. This > >>>>>> will result in the newpage > >>>>>> 1. No longer having PageMovable() > >>>>>> 2. Getting moved to the local list before finally freeing it (using > >>>>>> page->lru) > >>>>> > >>>>> Does that mean that the virtio-balloon can change the Movable state > >>>>> while there are other users of the page? Can you point to the code that > >>>>> does it? How come this can be safe at all? Or is the PageMovable stable > >>>>> only under the page lock? > >>>>> > >>>> > >>>> PageMovable is stable under the lock. The relevant instructions are in > >>>> > >>>> mm/balloon_compaction.c and include/linux/balloon_compaction.h > >>> > >>> OK, I have just checked __ClearPageMovable and it indeed requires > >>> PageLock. Then we also have to move is_lru = __PageMovable(page) after > >>> the page lock. > >>> > >> > >> I assume that is fine as is as the page is isolated? (yes, it will be > >> modified later when moving but we are interested in the original state) > > > > OK, I've missed that the page is indeed isolated. Then the patch makes > > sense to me. > > > > Thanks Michal. I assume this has broken ever since balloon compaction > was introduced. I'll wait a little more and then resend as !RFC with a > cc-stable tag. Please make sure to CC Minchan when reposting. Btw. Acked-by: Michal Hocko <mhocko@xxxxxxxx> -- Michal Hocko SUSE Labs
