Re: [PATCH] mm/page-writeback: avoid overflow when reporting BDI byte limits

[Jan Kara]

On Tue 03-11-26 19:36:26, Hungyu Lin wrote:
> bdi_get_bytes() multiplies unsigned long operands before assigning the
> result to u64. On 32-bit kernels, the intermediate product wraps and
> min_bytes and max_bytes report incorrect limits. For example, with
> vm.dirty_bytes set to 104857600 and max_ratio set to 100, max_bytes
> reports 268 instead of 104857600.
> 
> Convert the page count to bytes in u64 and use mul_u64_u32_div() to avoid
> overflow in the subsequent multiplication. This also handles large dirty
> limits on systems with larger pages. Combining the two divisions keeps
> the existing rounding behavior.
> 
> Fixes: 00df7d51263b ("mm: add bdi_get_max_bytes() function")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Hungyu Lin <dennylin0707@xxxxxxxxx>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

								Honza

> ---
>  mm/page-writeback.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 499a35473e4f..ddebec79d8d9 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -665,7 +665,8 @@ static u64 bdi_get_bytes(unsigned int ratio)
>  	u64 bytes;
>  
>  	global_dirty_limits(&background_thresh, &dirty_thresh);
> -	bytes = (dirty_thresh * PAGE_SIZE * ratio) / BDI_RATIO_SCALE / 100;
> +	bytes = mul_u64_u32_div((u64)dirty_thresh << PAGE_SHIFT, ratio,
> +				100 * BDI_RATIO_SCALE);
>  
>  	return bytes;
>  }
> -- 
> 2.43.0
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR