Messages in this thread | | | Date | Mon, 28 Jan 2019 10:01:56 -0500 | From | Rafael Aquini <> | Subject | Re: [PATCH RFC] mm: migrate: don't rely on PageMovable() of newpage after unlocking it |
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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@redhat.com> 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@samsung.com> 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@kernel.org> 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
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