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. -- Michal Hocko SUSE Labs
