Re: [PATCH 3/5] mm: Activate !PageLRU pages on mark_page_accessed ifpage is on local pagevec

From: Johannes Weiner
Date: Fri May 17 2013 - 16:39:16 EST

On Fri, May 17, 2013 at 10:48:05AM +0100, Mel Gorman wrote:
> If a page is on a pagevec then it is !PageLRU and mark_page_accessed()
> may fail to move a page to the active list as expected. Now that the LRU
> is selected at LRU drain time, mark pages PageActive if they are on the
> local pagevec so it gets moved to the correct list at LRU drain time.
> Using a debugging patch it was found that for a simple git checkout based
> workload that pages were never added to the active file list in practice
> but with this patch applied they are.
> before after
> LRU Add Active File 0 750583
> LRU Add Active Anon 2640587 2702818
> LRU Add Inactive File 8833662 8068353
> LRU Add Inactive Anon 207 200
> Note that only pages on the local pagevec are considered on purpose. A
> !PageLRU page could be in the process of being released, reclaimed, migrated
> or on a remote pagevec that is currently being drained. Marking it PageActive
> is vunerable to races where PageLRU and Active bits are checked at the
> wrong time. Page reclaim will trigger VM_BUG_ONs but depending on when the
> race hits, it could also free a PageActive page to the page allocator and
> trigger a bad_page warning. Similarly a potential race exists between a
> per-cpu drain on a pagevec list and an activation on a remote CPU.
> lru_add_drain_cpu
> __pagevec_lru_add
> lru = page_lru(page);
> mark_page_accessed
> if (PageLRU(page))
> activate_page
> else
> SetPageActive
> SetPageLRU(page);
> add_page_to_lru_list(page, lruvec, lru);
> In this case a PageActive page is added to the inactivate list and later the
> inactive/active stats will get skewed. While the PageActive checks in vmscan
> could be removed and potentially dealt with, a skew in the statistics would
> be very difficult to detect. Hence this patch deals just with the common case
> where a page being marked accessed has just been added to the local pagevec.
> Signed-off-by: Mel Gorman <mgorman@xxxxxxx>

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
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