• Hugh Dickins's avatar
    mm: fix migration hangs on anon_vma lock · 1ce82b69
    Hugh Dickins authored
    Increased usage of page migration in mmotm reveals that the anon_vma
    locking in unmap_and_move() has been deficient since 2.6.36 (or even
    earlier).  Review at the time of f1819427
    ("mm: fix hang on anon_vma->root->lock") missed the issue here: the
    anon_vma to which we get a reference may already have been freed back to
    its slab (it is in use when we check page_mapped, but that can change),
    and so its anon_vma->root may be switched at any moment by reuse in
    anon_vma_prepare.
    
    Perhaps we could fix that with a get_anon_vma_unless_zero(), but let's
    not: just rely on page_lock_anon_vma() to do all the hard thinking for us,
    then we don't need any rcu read locking over here.
    
    In removing the rcu_unlock label: since PageAnon is a bit in
    page->mapping, it's impossible for a !page->mapping page to be anon; but
    insert VM_BUG_ON in case the implementation ever changes.
    
    [akpm@linux-foundation.org: coding-style fixes]
    Signed-off-by: default avatarHugh Dickins <hughd@google.com>
    Reviewed-by: default avatarMel Gorman <mel@csn.ul.ie>
    Reviewed-by: default avatarRik van Riel <riel@redhat.com>
    Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
    Cc: "Jun'ichi Nomura" <j-nomura@ce.jp.nec.com>
    Cc: Andi Kleen <ak@linux.intel.com>
    Cc: <stable@kernel.org> [2.6.37, 2.6.36]
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    1ce82b69
migrate.c 31.7 KB