On Mon, Jan 28, 2019 at 10:01:56AM -0500, Rafael Aquini wrote: > 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. > > missing part here: I think your patch adresses this new case. Acked-by: Rafael Aquini <aquini@xxxxxxxxxx> > get rid of balloon reference count. ^^ this was a left over (sorry about my fat-fingers) > > > -- Rafael
