RE: [PATCH 03/12] percpu: introduce helper to determine if two regions overlap

[Peng Fan]

Hi Dennis,

> -----Original Message-----
> From: owner-linux-mm@xxxxxxxxx [mailto:owner-linux-mm@xxxxxxxxx] On
> Behalf Of Dennis Zhou
> Sent: 2019年2月28日 10:19
> To: Dennis Zhou <dennis@xxxxxxxxxx>; Tejun Heo <tj@xxxxxxxxxx>; Christoph
> Lameter <cl@xxxxxxxxx>
> Cc: Vlad Buslov <vladbu@xxxxxxxxxxxx>; kernel-team@xxxxxx;
> linux-mm@xxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> 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@xxxxxxxxxx>
> ---
>  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