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authorRoman Gushchin <guro@fb.com>2018-10-26 15:03:27 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2018-10-26 16:25:19 -0700
commit68600f623d69da428c6163275f97ca126e1a8ec5 (patch)
tree3894896d27376c03b8242a8be5d20e70af2b28a1 /mm/vmscan.c
parent591edfb10a949d635ed770c6e85ec5286206c07e (diff)
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 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>
Diffstat (limited to 'mm/vmscan.c')
-rw-r--r--mm/vmscan.c6
1 files changed, 4 insertions, 2 deletions
diff --git a/mm/vmscan.c b/mm/vmscan.c
index c5ef7240cbcb..961401c46334 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2456,9 +2456,11 @@ 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: