RE: [PATCH 02/12] percpu: do not search past bitmap when allocating an area

[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 10:18
> 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 02/12] percpu: do not search past bitmap when allocating an
> area
> 
> pcpu_find_block_fit() guarantees that a fit is found within
> PCPU_BITMAP_BLOCK_BITS. Iteration is used to determine the first fit as it
> compares against the block's contig_hint. This can lead to incorrectly scanning
> past the end of the bitmap. The behavior was okay given the check after for
> bit_off >= end and the correctness of the hints from pcpu_find_block_fit().
> 
> This patch fixes this by bounding the end offset by the number of bits in a
> chunk.
> 
> Signed-off-by: Dennis Zhou <dennis@xxxxxxxxxx>
> ---
>  mm/percpu.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/percpu.c b/mm/percpu.c
> index 53bd79a617b1..69ca51d238b5 100644
> --- a/mm/percpu.c
> +++ b/mm/percpu.c
> @@ -988,7 +988,8 @@ static int pcpu_alloc_area(struct pcpu_chunk *chunk,
> int alloc_bits,
>  	/*
>  	 * Search to find a fit.
>  	 */
> -	end = start + alloc_bits + PCPU_BITMAP_BLOCK_BITS;
> +	end = min_t(int, start + alloc_bits + PCPU_BITMAP_BLOCK_BITS,
> +		    pcpu_chunk_map_bits(chunk));
>  	bit_off = bitmap_find_next_zero_area(chunk->alloc_map, end, start,
>  					     alloc_bits, align_mask);
>  	if (bit_off >= end)
> --

From pcpu_alloc_area itself, I think this is correct to avoid bitmap_find_next_zero_area
scan past the boundaries of alloc_map, so

Reviewed-by: Peng Fan <peng.fan@xxxxxxx>

There are a few points I did not understand well,
Per understanding pcpu_find_block_fit is to find the first bit off in a chunk which could satisfy
the bits allocation, so bits might be larger than PCPU_BITMAP_BLOCK_BITS. And if
pcpu_find_block_fit returns a good off, it means there is a area in the chunk could satisfy
the bits allocation, then the following pcpu_alloc_area will not scan past the boundaries of
alloc_map, right?

Thanks,
Peng.

> 2.17.1