mm, vmscan: clear PGDAT_WRITEBACK when zone is balanced

Hillf Danton pointed out that since commit 1d82de61 ("mm, vmscan:
make kswapd reclaim in terms of nodes") that PGDAT_WRITEBACK is no
longer cleared.

It was not noticed as triggering it requires pages under writeback to
cycle twice through the LRU and before kswapd gets stalled.
Historically, such issues tended to occur on small machines writing
heavily to slow storage such as a USB stick.

Once kswapd stalls, direct reclaim stalls may be higher but due to the
fact that memory pressure is required, it would not be very noticable.

Michal Hocko suggested removing the flag entirely but the conservative
fix is to restore the intended PGDAT_WRITEBACK behaviour and clear the
flag when a suitable zone is balanced.

Change-Id: I583b795a06520651437429e7c80b5b5756215c25
Fixes: 1d82de61 ("mm, vmscan: make kswapd reclaim in terms of nodes")
Link: http://lkml.kernel.org/r/20170203203222.gq7hk66yc36lpgtb@suse.de


Signed-off-by: Mel Gorman <mgorman@suse.de>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Hillf Danton <hillf.zj@alibaba-inc.com>
Cc: Minchan Kim <minchan.kim@gmail.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>
Git-commit: c2f83143f1c67d186520b72b6cefbf0aa07a34ee
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git


Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>