mm: vmscan: do not scale writeback pages when deciding whether to set ZONE_WRITEBACK

After the patch "mm: vmscan: Flatten kswapd priority loop" was merged
the scanning priority of kswapd changed.

The priority now rises until it is scanning enough pages to meet the
high watermark.  shrink_inactive_list sets ZONE_WRITEBACK if a number of
pages were encountered under writeback but this value is scaled based on
the priority.  As kswapd frequently scans with a higher priority now it
is relatively easy to set ZONE_WRITEBACK.  This patch removes the
scaling and treates writeback pages similar to how it treats unqueued
dirty pages and congested pages.  The user-visible effect should be that
kswapd will writeback fewer pages from reclaim context.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Cc: Rik van Riel <riel@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 1 insertions, 15 deletions

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 2385663ae5e5..2cff0d491c6d 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1443,25 +1443,11 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
          * as there is no guarantee the dirtying process is throttled in the
          * same way balance_dirty_pages() manages.
          *
-         * This scales the number of dirty pages that must be under writeback
-         * before a zone gets flagged ZONE_WRITEBACK. It is a simple backoff
-         * function that has the most effect in the range DEF_PRIORITY to
-         * DEF_PRIORITY-2 which is the priority reclaim is considered to be
-         * in trouble and reclaim is considered to be in trouble.
-         *
-         * DEF_PRIORITY   100% isolated pages must be PageWriteback to throttle
-         * DEF_PRIORITY-1  50% must be PageWriteback
-         * DEF_PRIORITY-2  25% must be PageWriteback, kswapd in trouble
-         * ...
-         * DEF_PRIORITY-6 For SWAP_CLUSTER_MAX isolated pages, throttle if any
-         *                     isolated page is PageWriteback
-         *
          * Once a zone is flagged ZONE_WRITEBACK, kswapd will count the number
          * of pages under pages flagged for immediate reclaim and stall if any
          * are encountered in the nr_immediate check below.
          */
-        if (nr_writeback && nr_writeback >=
-                        (nr_taken >> (DEF_PRIORITY - sc->priority)))
+        if (nr_writeback && nr_writeback == nr_taken)
                 zone_set_flag(zone, ZONE_WRITEBACK);
 
         /*