From 486cf46f3f9be5f2a966016c1a8fe01e32cde09e Mon Sep 17 00:00:00 2001 From: Hugh Dickins Date: Wed, 19 Oct 2011 12:50:35 -0700 Subject: [PATCH] mm: fix race between mremap and removing migration entry MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit I don't usually pay much attention to the stale "? " addresses in stack backtraces, but this lucky report from Pawel Sikora hints that mremap's move_ptes() has inadequate locking against page migration. 3.0 BUG_ON(!PageLocked(p)) in migration_entry_to_page(): kernel BUG at include/linux/swapops.h:105! RIP: 0010:[] [] migration_entry_wait+0x156/0x160 [] handle_pte_fault+0xae1/0xaf0 [] ? __pte_alloc+0x42/0x120 [] ? do_huge_pmd_anonymous_page+0xab/0x310 [] handle_mm_fault+0x181/0x310 [] ? vma_adjust+0x537/0x570 [] do_page_fault+0x11d/0x4e0 [] ? do_mremap+0x2d5/0x570 [] page_fault+0x1f/0x30 mremap's down_write of mmap_sem, together with i_mmap_mutex or lock, and pagetable locks, were good enough before page migration (with its requirement that every migration entry be found) came in, and enough while migration always held mmap_sem; but not enough nowadays, when there's memory hotremove and compaction. The danger is that move_ptes() lets a migration entry dodge around behind remove_migration_pte()'s back, so it's in the old location when looking at the new, then in the new location when looking at the old. Either mremap's move_ptes() must additionally take anon_vma lock(), or migration's remove_migration_pte() must stop peeking for is_swap_entry() before it takes pagetable lock. Consensus chooses the latter: we prefer to add overhead to migration than to mremapping, which gets used by JVMs and by exec stack setup. Reported-and-tested-by: Paweł Sikora Signed-off-by: Hugh Dickins Acked-by: Andrea Arcangeli Acked-by: Mel Gorman Cc: stable@vger.kernel.org Signed-off-by: Linus Torvalds --- mm/migrate.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/mm/migrate.c b/mm/migrate.c index 666e4e677414..14d0a6a632f6 100644 --- a/mm/migrate.c +++ b/mm/migrate.c @@ -120,10 +120,10 @@ static int remove_migration_pte(struct page *new, struct vm_area_struct *vma, ptep = pte_offset_map(pmd, addr); - if (!is_swap_pte(*ptep)) { - pte_unmap(ptep); - goto out; - } + /* + * Peek to check is_swap_pte() before taking ptlock? No, we + * can race mremap's move_ptes(), which skips anon_vma lock. + */ ptl = pte_lockptr(mm, pmd); }