Messages in this thread | | | Date | Mon, 09 Jan 2012 15:42:46 -0500 | From | KOSAKI Motohiro <> | Subject | Re: [PATCH 2/2] SHM_UNLOCK: fix Unevictable pages stranded after swap |
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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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