On 28.01.19 21:19, Michal Hocko wrote:
On Mon 28-01-19 21:02:52, David Hildenbrand wrote:
On 28.01.19 17:04, David Hildenbrand wrote:
While debugging some crashes related to virtio-balloon deflation that happened under the old balloon migration code, I stumbled over a race that still exists today.
What we experienced:
drivers/virtio/virtio_balloon.c:release_pages_balloon():
- WARNING: CPU: 13 PID: 6586 at lib/list_debug.c:59 __list_del_entry+0xa1/0xd0
- list_del corruption. prev->next should be ffffe253961090a0, but was dead000000000100
Turns out after having added the page to a local list when dequeuing, the page would suddenly be moved to an LRU list before we would free it via the local list, corrupting both lists. So a page we own and that is !LRU was moved to an LRU list.
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, there is nothing stopping the newpage from getting dequeued and freed by virtio-balloon. This will result in the newpage
- No longer having PageMovable()
- Getting moved to the local list before finally freeing it (using page->lru)
Back in the migration thread in __unmap_and_move(), we would after unlocking the newpage suddenly no longer have PageMovable(newpage) and will therefore call putback_lru_page(newpage), modifying page->lru although that list is still in use by virtio-balloon.
To summarize, we have a race between migrating the newpage and checking for PageMovable(newpage). Instead of checking PageMovable(newpage), we can simply rely on is_lru of the original page.
Looks like this was introduced by d6d86c0a7f8d ("mm/balloon_compaction: redesign ballooned pages management"), which was backported up to 3.12. Old compaction code used PageBalloon() via -_is_movable_balloon_page() instead of PageMovable(), however with the same semantics.
Cc: Andrew Morton akpm@linux-foundation.org Cc: Mel Gorman mgorman@techsingularity.net Cc: "Kirill A. Shutemov" kirill.shutemov@linux.intel.com Cc: Michal Hocko mhocko@suse.com Cc: Naoya Horiguchi n-horiguchi@ah.jp.nec.com Cc: Jan Kara jack@suse.cz Cc: Andrea Arcangeli aarcange@redhat.com Cc: Dominik Brodowski linux@dominikbrodowski.net Cc: Matthew Wilcox willy@infradead.org Cc: Vratislav Bendel vbendel@redhat.com Cc: Rafael Aquini aquini@redhat.com Cc: Konstantin Khlebnikov k.khlebnikov@samsung.com Cc: Minchan Kim minchan@kernel.org Cc: stable@vger.kernel.org # 3.12+ Fixes: d6d86c0a7f8d ("mm/balloon_compaction: redesign ballooned pages management") Reported-by: Vratislav Bendel vbendel@redhat.com Acked-by: Michal Hocko mhocko@suse.com Acked-by: Rafael Aquini aquini@redhat.com Signed-off-by: David Hildenbrand david@redhat.com
mm/migrate.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-)
diff --git a/mm/migrate.c b/mm/migrate.c index 4512afab46ac..31e002270b05 100644 --- a/mm/migrate.c +++ b/mm/migrate.c @@ -1135,10 +1135,12 @@ static int __unmap_and_move(struct page *page, struct page *newpage, * If migration is successful, decrease refcount of the newpage * which will not free the page because new page owner increased * refcounter. As well, if it is LRU page, add the page to LRU
* list in here.
* list in here. Don't rely on PageMovable(newpage), as that could* already have changed after unlocking newpage (e.g.* virtio-balloon deflation).
if (unlikely(__PageMovable(newpage)))
if (unlikely(!is_lru)) put_page(newpage);Vratislav just pointed out that this issue should not happen on upstream as __PageMovable(newpage) will still return true even after __ClearPageMovable(newpage). Only PageMovable(newpage) would actually return false.
(not sure if I am happy about this, this is horribly confusing and complicated)
It is confusing as hell! __ClearPageMovable is a misnomer and I have to admit I have misread the implementation to actually ~PAGE_MAPPING_MOVABLE.
I am not 100% sure yet, but I guess Vratislav is right. So it was effectively fixed by
b1123ea6d3b3 ("mm: balloon: use general non-lru movable page feature"), which checks for __PageMovable(newpage) instead of __is_movable_balloon_page(newpage).
So this is not just a clean up. Sigh!
Anybody wanting to fix stable kernels either has to backport something proposed in this patch or b1123ea6d3b3.
I think we should go with a simple patch for stable so this patch sounds like a good thing. *PageMovable thingy needs a much better documentation and ideally a cleaner implementation as well. The current state is just incomprehensible.
Especially as __ClearPageMovable will not make __PageMovable fail, fun with code :)
David, could you reformulate the changelog accordingly please? My ack still holds.
You mean reformulating + resending for stable kernels only?
b1123ea6d3b3 was merged with v4.8. d6d86c0a7f8d was backported to v3.12+.
So v3.12 - v4.7 are affected and will free pages to the LRU list.
Without 195a8c43e93d ("virtio-balloon: deflate via a page list") - merged with v4.13 - this BUG is not immediately visible I guess. Pages are still added to the wrong list (LRU although they shouldn't) but the virtio-balloon local list does not exist/corrupt. I assume adding these pages to the LRU list is bad itself, right?
If a distro backported 195a8c43e93d without b1123ea6d3b3, the BUG becomes directly visible (this is how we hit it).
Thanks!