Re: [PATCH RFC] mm: migrate: don't rely on PageMovable() of newpage after unlocking it

[Michal Hocko]

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