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) -- Thanks, David / dhildenb
