Re: [PATCH v2 1/2] mm/page_counter: avoid integer overflow in effective_protection()

[Barry Song]

On Fri, Aug 28, 2026 at 7:09 PM Ridong Chen <ridong.chen@xxxxxxxxx> wrote:
>
> From: Ridong Chen <chenridong@xxxxxxxxxx>
>
> effective_protection() scales a parent's protection by a ratio of page
> counts, e.g. for recursive protection:
>
>         (parent_effective - siblings_protected) * (usage - protected)
>                 / (parent_usage - siblings_protected)
>
> The multiply is done at unsigned long width before dividing. On systems
> with >= 16TB RAM the product can exceed 2^64 and wrap, giving a bogus
> protection value and silently breaking memory.min/low enforcement.
>
> Use mul_u64_u64_div_u64() to multiply in a 128-bit intermediate. Because
> usage and parent_usage are not read atomically (a child is charged
> before its parent), usage - protected can briefly exceed the divisor,
> making the quotient overflow 64 bits and trap (#DE on x86). Cap it so
> the ratio stays <= 1.
>
> Reported by the sashiko review tool [1].
>
> [1] https://sashiko.dev/#/patchset/20260826133054.88529-1-ridong.chen@xxxxxxxxx?part=1
>
> Fixes: bc50bcc6e00b ("mm: memcontrol: clean up and document effective low/min calculations")
> Fixes: 8a931f801340 ("mm: memcontrol: recursive memory.low protection")
> Cc: stable@xxxxxxxxxxxxxxx
> Assisted-by: Claude:claude-opus-4-8
> Signed-off-by: Ridong Chen <chenridong@xxxxxxxxxx>
> ---

LGTM,
Reviewed-by: Barry Song <baohua@xxxxxxxxxx>