• Hugh Dickins's avatar
    SHM_UNLOCK: fix Unevictable pages stranded after swap · 24513264
    Hugh Dickins authored
    Commit cc39c6a9 ("mm: account skipped entries to avoid looping in
    find_get_pages") correctly fixed an infinite loop; but left a problem
    that find_get_pages() on shmem would return 0 (appearing to callers to
    mean end of tree) when it meets a run of nr_pages swap entries.
    The only uses of find_get_pages() on shmem are via pagevec_lookup(),
    called from invalidate_mapping_pages(), and from shmctl SHM_UNLOCK's
    scan_mapping_unevictable_pages().  The first is already commented, and
    not worth worrying about; but the second can leave pages on the
    Unevictable list after an unusual sequence of swapping and locking.
    Fix that by using shmem_find_get_pages_and_swap() (then ignoring the
    swap) instead of pagevec_lookup().
    But I don't want to contaminate vmscan.c with shmem internals, nor
    shmem.c with LRU locking.  So move scan_mapping_unevictable_pages() into
    shmem.c, renaming it shmem_unlock_mapping(); and rename
    check_move_unevictable_page() to check_move_unevictable_pages(), looping
    down an array of pages, oftentimes under the same lock.
    Leave out the "rotate unevictable list" block: that's a leftover from
    when this was used for /proc/sys/vm/scan_unevictable_pages, whose flawed
    handling involved looking at pages at tail of LRU.
    Was there significance to the sequence first ClearPageUnevictable, then
    test page_evictable, then SetPageUnevictable here? I think not, we're
    under LRU lock, and have no barriers between those.
    Signed-off-by: default avatarHugh Dickins <hughd@google.com>
    Reviewed-by: default avatarKOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
    Cc: Minchan Kim <minchan.kim@gmail.com>
    Cc: Rik van Riel <riel@redhat.com>
    Cc: Shaohua Li <shaohua.li@intel.com>
    Cc: Eric Dumazet <eric.dumazet@gmail.com>
    Cc: Johannes Weiner <hannes@cmpxchg.org>
    Cc: Michel Lespinasse <walken@google.com>
    Cc: <stable@vger.kernel.org> [back to 3.1 but will need respins]
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
shmem.c 64.4 KB