From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qk1-f198.google.com (mail-qk1-f198.google.com [209.85.222.198]) by kanga.kvack.org (Postfix) with ESMTP id 19E128E0001 for ; Mon, 28 Jan 2019 07:16:25 -0500 (EST) Received: by mail-qk1-f198.google.com with SMTP id k66so18116297qkf.1 for ; Mon, 28 Jan 2019 04:16:25 -0800 (PST) Received: from mx1.redhat.com (mx1.redhat.com. [209.132.183.28]) by mx.google.com with ESMTPS id f64si1446125qva.93.2019.01.28.04.16.23 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 28 Jan 2019 04:16:24 -0800 (PST) From: David Hildenbrand Subject: [PATCH RFC] mm: migrate: don't rely on PageMovable() of newpage after unlocking it Date: Mon, 28 Jan 2019 13:16:09 +0100 Message-Id: <20190128121609.9528-1-david@redhat.com> Sender: owner-linux-mm@kvack.org List-ID: To: linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, David Hildenbrand , Andrew Morton , Mel Gorman , "Kirill A. Shutemov" , Michal Hocko , Naoya Horiguchi , Jan Kara , Andrea Arcangeli , Dominik Brodowski , Matthew Wilcox , Vratislav Bendel , Rafael Aquini While debugging some crashes related to virtio-balloon deflation that happened under the old balloon migration code, I stumbled over a possible race I think. 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. For us, this was triggered by backporting 195a8c43e93d8 ("virtio-balloon: deflate via a page list") onto old balloon compaction code, but I think this only made the BUG become visible and the race still exists in new !LRU migraton code. 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) 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. Because if the old page was not an LRU page, the new page also shouldn't be. If there is a problem with that, we could check for __PageMovable(newpage) before unlocking the new page. As I am not yet sure if this race actually exists, especially also upstream, I am sending this as RFC. Cc: Andrew Morton Cc: Mel Gorman Cc: "Kirill A. Shutemov" Cc: Michal Hocko Cc: Naoya Horiguchi Cc: Jan Kara Cc: Andrea Arcangeli Cc: Dominik Brodowski Cc: Matthew Wilcox Cc: Vratislav Bendel Cc: Rafael Aquini Reported-by: Vratislav Bendel Signed-off-by: David Hildenbrand --- 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 (rc == MIGRATEPAGE_SUCCESS) { - if (unlikely(__PageMovable(newpage))) + if (unlikely(!is_lru)) put_page(newpage); else putback_lru_page(newpage); -- 2.17.2