From: Roman Gushchin <guro@xxxxxx> Subject: 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@xxxxxx: 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@xxxxxx Signed-off-by: Roman Gushchin <guro@xxxxxx> Reviewed-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Cc: Johannes Weiner <hannes@xxxxxxxxxxx> Cc: Michal Hocko <mhocko@xxxxxxxxxx> Cc: Tejun Heo <tj@xxxxxxxxxx> Cc: Rik van Riel <riel@xxxxxxxxxxx> Cc: Konstantin Khlebnikov <koct9i@xxxxxxxxx> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- include/linux/math64.h | 3 +++ mm/vmscan.c | 6 ++++-- 2 files changed, 7 insertions(+), 2 deletions(-) --- a/include/linux/math64.h~mm-dont-miss-the-last-page-because-of-round-off-error +++ a/include/linux/math64.h @@ -281,4 +281,7 @@ 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~mm-dont-miss-the-last-page-because-of-round-off-error +++ a/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: _
