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

[Michal Hocko]

On Mon 28-01-19 15:38:38, David Hildenbrand wrote:
> On 28.01.19 14:35, Michal Hocko wrote:
> > 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.
> > 
> 
> Thanks Michal. I assume this has broken ever since balloon compaction
> was introduced. I'll wait a little more and then resend as !RFC with a
> cc-stable tag.

Please make sure to CC Minchan when reposting.

Btw.
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
-- 
Michal Hocko
SUSE Labs