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. -- Michal Hocko SUSE Labs
