The patch titled vmscan: order evictable rescue in LRU putback has been removed from the -mm tree. Its filename was mm-order-evictable-rescue-in-lru-putback.patch This patch was dropped because it was merged into mainline or a subsystem tree The current -mm tree may be found at http://userweb.kernel.org/~akpm/mmotm/ ------------------------------------------------------ Subject: vmscan: order evictable rescue in LRU putback From: Johannes Weiner <hannes@xxxxxxxxxxx> Isolators putting a page back to the LRU do not hold the page lock, and if the page is mlocked, another thread might munlock it concurrently. Expecting this, the putback code re-checks the evictability of a page when it just moved it to the unevictable list in order to correct its decision. The problem, however, is that ordering is not garuanteed between setting PG_lru when moving the page to the list and checking PG_mlocked afterwards: #0: #1 spin_lock() if (TestClearPageMlocked()) if (PageLRU()) move to evictable list SetPageLRU() spin_unlock() if (!PageMlocked()) move to evictable list The PageMlocked() check may get reordered before SetPageLRU() in #0, resulting in #0 not moving the still mlocked page, and in #1 failing to isolate and move the page as well. The page is now stranded on the unevictable list. The race condition is very unlikely. The consequence currently is one page falling off the reclaim grid and eventually getting freed with PG_unevictable set, which triggers a warning in the page allocator. TestClearPageMlocked() in #1 already provides full memory barrier semantics. This patch adds an explicit full barrier to force ordering between SetPageLRU() and PageMlocked() so that either one of the competitors rescues the page. Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx> Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx> Cc: Hugh Dickins <hugh.dickins@xxxxxxxxxxxxx> Cc: Mel Gorman <mel@xxxxxxxxx> Cc: Lee Schermerhorn <Lee.Schermerhorn@xxxxxx> Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> Reviewed-by: Rik van Riel <riel@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- diff -puN mm/vmscan.c~mm-order-evictable-rescue-in-lru-putback mm/vmscan.c --- a/mm/vmscan.c~mm-order-evictable-rescue-in-lru-putback +++ a/mm/vmscan.c @@ -544,6 +544,16 @@ redo: */ lru = LRU_UNEVICTABLE; add_page_to_unevictable_list(page); + /* + * When racing with an mlock clearing (page is + * unlocked), make sure that if the other thread does + * not observe our setting of PG_lru and fails + * isolation, we see PG_mlocked cleared below and move + * the page back to the evictable list. + * + * The other side is TestClearPageMlocked(). + */ + smp_mb(); } /* _ Patches currently in -mm which might be from hannes@xxxxxxxxxxx are origin.patch mm-warn-once-when-a-page-is-freed-with-pg_mlocked-set.patch -- To unsubscribe from this list: send the line "unsubscribe mm-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html