Commit 68600f62 authored by Roman Gushchin's avatar Roman Gushchin Committed by Linus Torvalds

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),


  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

[ prevent double calculation of DIV64_U64_ROUND_UP() arguments]
Link: default avatarRoman Gushchin <>
Reviewed-by: default avatarAndrew Morton <>
Cc: Johannes Weiner <>
Cc: Michal Hocko <>
Cc: Tejun Heo <>
Cc: Rik van Riel <>
Cc: Konstantin Khlebnikov <>
Cc: Matthew Wilcox <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent 591edfb1
......@@ -281,4 +281,7 @@ static inline u64 mul_u64_u32_div(u64 a, u32 mul, u32 divisor)
#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 */
......@@ -2456,9 +2456,11 @@ 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],
scan = DIV64_U64_ROUND_UP(scan * fraction[file],
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