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SubjectRE: [PATCH 03/12] percpu: introduce helper to determine if two regions overlap
Date
Hi Dennis,

> -----Original Message-----
> From: owner-linux-mm@kvack.org [mailto:owner-linux-mm@kvack.org] On
> Behalf Of Dennis Zhou
> Sent: 2019年2月28日 10:19
> To: Dennis Zhou <dennis@kernel.org>; Tejun Heo <tj@kernel.org>; Christoph
> Lameter <cl@linux.com>
> Cc: Vlad Buslov <vladbu@mellanox.com>; kernel-team@fb.com;
> linux-mm@kvack.org; linux-kernel@vger.kernel.org
> Subject: [PATCH 03/12] percpu: introduce helper to determine if two regions
> overlap
>
> While block hints were always accurate, it's possible when spanning across
> blocks that we miss updating the chunk's contig_hint. Rather than rely on
> correctness of the boundaries of hints, do a full overlap comparison.
>
> Signed-off-by: Dennis Zhou <dennis@kernel.org>
> ---
> mm/percpu.c | 31 +++++++++++++++++++++++++++----
> 1 file changed, 27 insertions(+), 4 deletions(-)
>
> diff --git a/mm/percpu.c b/mm/percpu.c
> index 69ca51d238b5..b40112b2fc59 100644
> --- a/mm/percpu.c
> +++ b/mm/percpu.c
> @@ -546,6 +546,24 @@ static inline int pcpu_cnt_pop_pages(struct
> pcpu_chunk *chunk, int bit_off,
> bitmap_weight(chunk->populated, page_start); }
>
> +/*
> + * pcpu_region_overlap - determines if two regions overlap
> + * @a: start of first region, inclusive
> + * @b: end of first region, exclusive
> + * @x: start of second region, inclusive
> + * @y: end of second region, exclusive
> + *
> + * This is used to determine if the hint region [a, b) overlaps with
> +the
> + * allocated region [x, y).
> + */
> +static inline bool pcpu_region_overlap(int a, int b, int x, int y) {
> + if ((x >= a && x < b) || (y > a && y <= b) ||
> + (x <= a && y >= b))

I think this could be simplified:
(a < y) && (x < b) could be used to do overlap check.

Regards,
Peng.

> + return true;
> + return false;
> +}
> +
> /**
> * pcpu_chunk_update - updates the chunk metadata given a free area
> * @chunk: chunk of interest
> @@ -710,8 +728,11 @@ static void pcpu_block_update_hint_alloc(struct
> pcpu_chunk *chunk, int bit_off,
> PCPU_BITMAP_BLOCK_BITS,
> s_off + bits);
>
> - if (s_off >= s_block->contig_hint_start &&
> - s_off < s_block->contig_hint_start + s_block->contig_hint) {
> + if (pcpu_region_overlap(s_block->contig_hint_start,
> + s_block->contig_hint_start +
> + s_block->contig_hint,
> + s_off,
> + s_off + bits)) {
> /* block contig hint is broken - scan to fix it */
> pcpu_block_refresh_hint(chunk, s_index);
> } else {
> @@ -764,8 +785,10 @@ static void pcpu_block_update_hint_alloc(struct
> pcpu_chunk *chunk, int bit_off,
> * contig hint is broken. Otherwise, it means a smaller space
> * was used and therefore the chunk contig hint is still correct.
> */
> - if (bit_off >= chunk->contig_bits_start &&
> - bit_off < chunk->contig_bits_start + chunk->contig_bits)
> + if (pcpu_region_overlap(chunk->contig_bits_start,
> + chunk->contig_bits_start + chunk->contig_bits,
> + bit_off,
> + bit_off + bits))
> pcpu_chunk_refresh_hint(chunk);
> }
>
> --
> 2.17.1

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