mm: vmscan: only write dirty pages that the scanner has seen twice

Dirty pages can easily reach the end of the LRU while there are still
clean pages to reclaim around.  Don't let kswapd write them back just
because there are a lot of them.  It costs more CPU to find the clean
pages, but that's almost certainly better than to disrupt writeback from
the flushers with LRU-order single-page writes from reclaim.  And the
flushers have been woken up by that point, so we spend IO capacity on
flushing and CPU capacity on finding the clean cache.

Only start writing dirty pages if they have cycled around the LRU twice
now and STILL haven't been queued on the IO device.  It's possible that
the dirty pages are so sparsely distributed across different bdis,
inodes, memory cgroups, that the flushers take forever to get to the
ones we want reclaimed.  Once we see them twice on the LRU, we know
that's the quicker way to find them, so do LRU writeback.

Link: http://lkml.kernel.org/r/20170123181641.23938-5-hannes@cmpxchg.org
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Minchan Kim <minchan@kernel.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Mel Gorman <mgorman@suse.de>
Acked-by: Hillf Danton <hillf.zj@alibaba-inc.com>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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