Re: [PATCH v2] mm/page-writeback: avoid 32-bit overflow in dirty threshold shares

[Gregory Price]

On Wed, Oct 21, 2026 at 06:06:54AM +0000, Hungyu Lin wrote:
> The ratios used by __wb_calc_thresh() are scaled to parts per million,
> but the threshold and ratio operands are unsigned long. On 32-bit
> kernels, their multiplication can wrap before the result is assigned
> to u64.
> 
> For example, with a threshold of 25600 pages and the default maximum
> ratio of 1000000, wb_max_thresh becomes 4125 pages instead of 25600.
> The shared threshold and minimum share calculations can overflow in
> the same way, producing incorrect per-writeback dirty limits.
> 
> Use a u64 threshold for the three ratio multiplications and div_u64()
> to retain the existing division order and rounding.
> 
> Fixes: ae82291e9ca4 ("mm: use part per 1000000 for bdi ratios")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Hungyu Lin <dennylin0707@xxxxxxxxx>

Reviewed-by: Gregory Price (Meta) <gourry@xxxxxxxxxx>