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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-ID: <20180817231834.15959-1-guro@fb.com>
Date:   Fri, 17 Aug 2018 16:18:34 -0700
From:   Roman Gushchin <guro@...com>
To:     <linux-mm@...ck.org>
CC:     <linux-kernel@...r.kernel.org>, <kernel-team@...com>,
        Roman Gushchin <guro@...com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>, Tejun Heo <tj@...nel.org>,
        Rik van Riel <riel@...riel.com>,
        Konstantin Khlebnikov <koct9i@...il.com>
Subject: [PATCH RFC] mm: don't miss the last page because of round-off error

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@...com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Johannes Weiner <hannes@...xchg.org>
Cc: Michal Hocko <mhocko@...nel.org>
Cc: Tejun Heo <tj@...nel.org>
Cc: Rik van Riel <riel@...riel.com>
Cc: Konstantin Khlebnikov <koct9i@...il.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