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>

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 2385663ae5e5ab3e3366727403dabebe0335da67..2cff0d491c6dca84391edd100e1726696c1475d5 100644 (file)
--- 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);
 
        /*