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mm: fix race between mremap and removing migration entry 

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:[<ffffffff81127b76>]  [<ffffffff81127b76>]
                       migration_entry_wait+0x156/0x160
  [<ffffffff811016a1>] handle_pte_fault+0xae1/0xaf0
  [<ffffffff810feee2>] ? __pte_alloc+0x42/0x120
  [<ffffffff8112c26b>] ? do_huge_pmd_anonymous_page+0xab/0x310
  [<ffffffff81102a31>] handle_mm_fault+0x181/0x310
  [<ffffffff81106097>] ? vma_adjust+0x537/0x570
  [<ffffffff81424bed>] do_page_fault+0x11d/0x4e0
  [<ffffffff81109a05>] ? do_mremap+0x2d5/0x570
  [<ffffffff81421d5f>] 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 <pluto@agmk.net>
Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Andrea Arcangeli <aarcange@redhat.com>
Acked-by: Mel Gorman <mgorman@suse.de>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Hugh Dickins 2011-10-19 12:50:35 -07:00 committed by Linus Torvalds
parent e4fcd69c9e
commit 486cf46f3f
1 changed files with 4 additions and 4 deletions
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8
mm/migrate.c
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@ -120,10 +120,10 @@ static int remove_migration_pte(struct page *new, struct vm_area_struct *vma,
ptep = pte_offset_map(pmd, addr); ptep = pte_offset_map(pmd, addr);
if (!is_swap_pte(*ptep)) { /*
pte_unmap(ptep); * Peek to check is_swap_pte() before taking ptlock? No, we
goto out; * can race mremap's move_ptes(), which skips anon_vma lock.
} */
ptl = pte_lockptr(mm, pmd); ptl = pte_lockptr(mm, pmd);
} }