On Wed 16-07-14 12:04:14, Johannes Weiner wrote: > On Wed, Jul 16, 2014 at 10:34:56AM +0200, Michal Hocko wrote: > > [Sorry I have missed this thread] > > > > On Tue 15-07-14 10:45:39, Johannes Weiner wrote: > > [...] > > > From 274b94ad83b38fe7dc1707a8eb4015b3ab1673c5 Mon Sep 17 00:00:00 2001 > > > From: Johannes Weiner <hannes@xxxxxxxxxxx> > > > Date: Thu, 10 Jul 2014 01:02:11 +0000 > > > Subject: [patch] mm: memcontrol: rewrite uncharge API fix - double migration > > > > > > Hugh reports: > > > > > > VM_BUG_ON_PAGE(!(pc->flags & PCG_MEM)) > > > mm/memcontrol.c:6680! > > > page had count 1 mapcount 0 mapping anon index 0x196 > > > flags locked uptodate reclaim swapbacked, pcflags 1, memcg not root > > > mem_cgroup_migrate < move_to_new_page < migrate_pages < compact_zone < > > > compact_zone_order < try_to_compact_pages < __alloc_pages_direct_compact < > > > __alloc_pages_nodemask < alloc_pages_vma < do_huge_pmd_anonymous_page < > > > handle_mm_fault < __do_page_fault > > > > > > mem_cgroup_migrate() assumes that a page is only migrated once and > > > then freed immediately after. > > > > > > However, putting the page back on the LRU list and dropping the > > > isolation refcount is not done atomically. This allows a PFN-based > > > migrator like compaction to isolate the page, see the expected > > > anonymous page refcount of 1, and migrate the page once more. > > > > > > Furthermore, once the charges are transferred to the new page, the old > > > page no longer has a pin on the memcg, which might get released before > > > the page itself now. pc->mem_cgroup is invalid at this point, but > > > PCG_USED suggests otherwise, provoking use-after-free. > > > > The same applies to to the new page because we are transferring only > > statistics. The old page with PCG_USED would uncharge the res_counter > > and so the new page is not backed by any and so memcg can go away. > > This sounds like a more probable scenario to me because old page should > > go away quite early after successful migration. > > No, the charges are carried by PCG_MEM and PCG_MEMSW, not PCG_USED. Dang. I am blind. Sorry about the noise... -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>