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

[Rafael Aquini]

On Mon, Jan 28, 2019 at 03:38:38PM +0100, 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.
>

Yes, balloon deflation could always race against migration
This race was a problem, initially, and was dealt with, via:

commit 117aad1e9e4d97448d1df3f84b08bd65811e6d6a
Author: Rafael Aquini <aquini@xxxxxxxxxx>
Date:   Mon Sep 30 13:45:16 2013 -0700

    mm: avoid reinserting isolated balloon pages into LRU lists

 

I think this upstream patch has re-introduced it, in a more subtle way,
as we're stumbling on it now, again:

commit d6d86c0a7f8ddc5b38cf089222cb1d9540762dc2
Author: Konstantin Khlebnikov <k.khlebnikov@xxxxxxxxxxx>
Date:   Thu Oct 9 15:29:27 2014 -0700

    mm/balloon_compaction: redesign ballooned pages management
    


On this particular race against migration case, virtio ballon deflation would 
not see it before

commit b1123ea6d3b3da25af5c8a9d843bd07ab63213f4
Author: Minchan Kim <minchan@xxxxxxxxxx>
Date:   Tue Jul 26 15:23:09 2016 -0700

    mm: balloon: use general non-lru movable page feature

as the recently released balloon page would be post-processed 
without the page->lru list handling, which for migration stability
purposes must be done under the protection of page_lock.


get rid of balloon reference count.


-- Rafael