Re: [RFC PATCH 3/3] mm/damon/core: avoid overflows in quota goal metrics and scores

[sashiko-bot@kernel.org]

> The node memory, node memcg memory, LRU [in]active memory, and hugepage
> memory quota goal metrics, as well as damos_quota_score(), use
> mult_frac() to scale a value by 10000.  mult_frac() multiplies the
> remainder by 10000 in unsigned long, which overflows on 32-bit kernels
> even when the result fits:
> 
> - For the page based metrics, this happens once the numerator exceeds
>   about 429,496 pages, or 1.6 GiB with 4 KiB pages.  For example, 2 GiB
>   used out of 3 GiB returns 1205 bp instead of 6666 bp.  The LRU
>   [in]active and hugepage metrics do not depend on NUMA.
> 
> - For quota goal scores, any current value above about 429,496 can
>   overflow.  For example, a DAMOS_QUOTA_SOME_MEM_PSI_US goal with a
>   target of one second and a current value of half a second returns 705
>   instead of 5000.  On any architecture, mult_frac() also silently wraps
> [ ... ]
> Fixes: 4835e2871321 ("mm/damon/core: introduce [in]active memory ratio damos quota goal metric")
> Fixes: f465dc71aba0 ("mm/damon: introduce DAMOS_QUOTA_HUGEPAGE auto tuning")
> Fixes: 9138e27a3bc3 ("mm/damon: add node_eligible_mem_bp goal metric")
> Fixes: 06ba5b309ed8 ("mm/damon/core: let goal specified with only target and current values")
> Signed-off-by: DaeMyung Kang <charsyam@gmail.com>

Sashiko has reviewed this patch and found no issues.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261027135932.2301646-1-charsyam@gmail.com?part=3