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

[Michal Hocko]

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