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SubjectRe: [PATCH v3 3/3] mm: don't miss the last page because of round-off error
On Mon, 27 Aug 2018 09:26:21 -0700 Roman Gushchin <guro@fb.com> wrote:

> 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.
>
> ...
>
> --- a/include/linux/math64.h
> +++ b/include/linux/math64.h
> @@ -281,4 +281,6 @@ 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) div64_u64((ll) + (d) - 1, (d))

This macro references arg `d' more than once. That can cause problems
if the passed expression has side-effects and is poor practice. Can
we please redo this with a temporary?

> #endif /* _LINUX_MATH64_H */
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index d649b242b989..2c67a0121c6d 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2446,9 +2446,11 @@ 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],
> - denominator);
> + scan = DIV64_U64_ROUND_UP(scan * fraction[file],
> + denominator);
> break;
> case SCAN_FILE:
> case SCAN_ANON:

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