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    SubjectRe: [PATCH 2/2] SHM_UNLOCK: fix Unevictable pages stranded after swap


    2012/1/6 Hugh Dickins <hughd@google.com>:
    > Commit cc39c6a9bbde "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.

    If I understand correctly, this is not exactly correct. Because of,
    PG_mlocked operation is not protected by LRU lock. So, I think we
    have three choice.

    1) check_move_unevictable_pages() aimed retry logic and put pages back
    into correct lru.
    2) check_move_unevictable_pages() unconditionally move the pages into
    evictable lru, and vmacan put them back into correct lru later.
    3) To protect PG_mlock operation by lru lock.


    other parts looks fine to me.



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