From 68600f623d69da428c6163275f97ca126e1a8ec5 Mon Sep 17 00:00:00 2001
From: Roman Gushchin <guro@fb.com>
Date: Fri, 26 Oct 2018 15:03:27 -0700
Subject: [PATCH] mm: don't miss the last page because of round-off error
Git-commit: 68600f623d69da428c6163275f97ca126e1a8ec5
Patch-mainline: v4.20-rc1
References: bnc#1118798

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 rounding up the result of the division.

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

[guro@fb.com: prevent double calculation of DIV64_U64_ROUND_UP() arguments]
  Link: http://lkml.kernel.org/r/20180829213311.GA13501@castle
Link: http://lkml.kernel.org/r/20180827162621.30187-3-guro@fb.com
Signed-off-by: Roman Gushchin <guro@fb.com>
Reviewed-by: 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>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Michal Hocko <mhocko@suse.com>

---
 include/linux/math64.h |    3 +++
 mm/vmscan.c            |    6 ++++--
 2 files changed, 7 insertions(+), 2 deletions(-)

--- a/include/linux/math64.h
+++ b/include/linux/math64.h
@@ -253,4 +253,7 @@ static inline u64 mul_u64_u32_div(u64 a,
 }
 #endif /* mul_u64_u32_div */
 
+#define DIV64_U64_ROUND_UP(ll, d)	\
+	({ u64 _tmp = (d); div64_u64((ll) + _tmp - 1, _tmp); })
+
 #endif /* _LINUX_MATH64_H */
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2291,9 +2291,11 @@ out:
 			/*
 			 * 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);
+			scan = DIV64_U64_ROUND_UP(scan * fraction[file],
+						  denominator);
 			break;
 		case SCAN_FILE:
 		case SCAN_ANON: