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 balloon_page_insert() and balloon_page_delete() modify PageMovable and are only called with both, the page locked and b_dev_info->pages_lock locked. Especially: 1. balloon_page_dequeue() which tries to find an unlocked page that is not isolated. and deletes it via balloon_page_delete(). 2. balloon_page_migrate() will effectively call balloon_page_delete() on old page and balloon_page_insert() newpage. Both pages are locked before calling balloon_page_migrate() and the b_dev_info->pages_lock is taken internally. So what's left is a very small race window when migrating to a new page, after dropping the lock of the newpage. -- Thanks, David / dhildenb
