[PATCH] mm: fix draining remote pageset

[Huang Ying <ying.huang@intel.com>]

If there is no memory allocation/freeing in the remote pageset after
some time (3 seconds for now), the remote pageset will be drained to
avoid memory wastage.

But in the current implementation, vmstat updater worker may not be
re-queued when we are waiting for the timeout (pcp->expire != 0) if
there are no vmstat changes, for example, when CPU goes idle.

This is fixed via guaranteeing that the vmstat updater worker will
always be re-queued when we are waiting for the timeout.

We can reproduce the bug via allocating/freeing pages from remote
node, then go idle.  And the patch can fix it.

Fixes: 7cc36bbddde5 ("vmstat: on-demand vmstat workers V8")
Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Michal Hocko <mhocko@kernel.org>
---
 mm/vmstat.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/vmstat.c b/mm/vmstat.c
index b731d57996c5..111118741abf 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -856,8 +856,10 @@ static int refresh_cpu_vm_stats(bool do_pagesets)
 				continue;
 			}
 
-			if (__this_cpu_dec_return(pcp->expire))
+			if (__this_cpu_dec_return(pcp->expire)) {
+				changes++;
 				continue;
+			}
 
 			if (__this_cpu_read(pcp->count)) {
 				drain_zone_pages(zone, this_cpu_ptr(pcp));
-- 
2.39.2