[PATCH RFC] mm: don't miss the last page because of round-off error

[Roman Gushchin <guro@fb.com>]

I've noticed, that dying memory cgroups are  often pinned
in memory by a single pagecache page. Even under moderate
memory pressure they sometimes stayed in such state
for a long time. That looked strange.

My investigation showed that the problem is caused by
applying the LRU pressure balancing math:

  scan = div64_u64(scan * fraction[lru], denominator),

where

  denominator = fraction[anon] + fraction[file] + 1.

Because fraction[lru] is always less than denominator,
if the initial scan size is 1, the result is always 0.

This means the last page is not scanned and has
no chances to be reclaimed.

Fix this by skipping the balancing logic if the initial
scan count is 1.

In practice this change significantly improves the speed
of dying cgroups reclaim.

Signed-off-by: Roman Gushchin <guro@fb.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Rik van Riel <riel@surriel.com>
Cc: Konstantin Khlebnikov <koct9i@gmail.com>
---
 mm/vmscan.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 03822f86f288..f85c5ec01886 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2287,9 +2287,12 @@ static void get_scan_count(struct lruvec *lruvec, struct mem_cgroup *memcg,
 			/*
 			 * Scan types proportional to swappiness and
 			 * their relative recent reclaim efficiency.
+			 * Make sure we don't miss the last page
+			 * because of a round-off error.
 			 */
-			scan = div64_u64(scan * fraction[file],
-					 denominator);
+			if (scan > 1)
+				scan = div64_u64(scan * fraction[file],
+						 denominator);
 			break;
 		case SCAN_FILE:
 		case SCAN_ANON:
-- 
2.17.1