2012/1/6 Hugh Dickins [off-list ref]:

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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