Re: [RFC PATCH v1.2 8/8] mm/damon/core: handle extreme memory state in get_in_active_mem_bp()

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] Silent integer overflow on 32-bit systems in mult_frac() usage across DAMON quota metrics, leading to drastically incorrect memory metrics.
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commit ba2ca9efc083502fb5555a81a42e9de1ab623b4d
Author: SJ Park <sj@kernel.org>

mm/damon/core: handle extreme memory state in get_in_active_mem_bp()

damos_get_in_active_mem_bp() uses the sum of the active and inactive
memory amount as a denominator.  In an extreme and unlikely environment,
active and inactive memory might be zero.  In this case, hence, it
results in a divide by zero problem.  Avoid it by changing the
denominator to one if it is zero, before it is being used.

Link: https://lore.kernel.org/20260721034756.147011-1-sj@kernel.org

> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index 3779a04753061..f8dddbff74a77 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -3015,7 +3015,7 @@ static unsigned int damos_get_in_active_mem_bp(bool active_ratio)
>  		global_node_page_state(NR_LRU_BASE + LRU_ACTIVE_FILE);
>  	inactive = global_node_page_state(NR_LRU_BASE + LRU_INACTIVE_ANON) +
>  		global_node_page_state(NR_LRU_BASE + LRU_INACTIVE_FILE);
> -	total = active + inactive;
> +	total = max(active + inactive, 1);
>  	if (active_ratio)
>  		return mult_frac(active, 10000, total);

[Severity: High]
This isn't a bug introduced by this patch, but does the use of mult_frac()
here cause a silent integer overflow on 32-bit systems?

The mult_frac(x, n, d) macro calculates (x / d) * n + (x % d) * n / d. On
32-bit architectures, intermediate multiplications like (x % d) * n are
evaluated as 32-bit unsigned integers if x is an unsigned long. This
silently overflows if the result exceeds ~4.29 billion.

For example, in damos_get_in_active_mem_bp(), active is counted in pages.
If active memory reaches roughly 429,496 pages (about 1.7 GB),
429,496 * 10,000 exceeds the 32-bit maximum of 4,294,967,295 and wraps
around, resulting in drastically incorrect memory metrics.

This same unsigned long overflow seems to be pre-existing in other DAMON
quota metrics in mm/damon/core.c as well:

- damos_get_node_mem_bp(): same issue with node memory.
- damos_get_node_eligible_mem_bp(): node_eligible is cast to unsigned
  long bytes, which can overflow if eligible memory exceeds ~419 KB.
- damos_set_effective_quota(): total_charged_sz is in bytes, which
  overflows when multiplied by 1,000,000 if it exceeds 4294 bytes.
- damos_quota_score(): goal->current_value tracks PSI in microseconds,
  which can easily be millions, overflowing when multiplied by 10,000.

Could these calculations be performed using 64-bit math on 32-bit systems
to prevent runaway aggressive memory reclaim or total failure to reclaim?

>  	return mult_frac(inactive, 10000, total);
>  }

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Sashiko AI review · https://sashiko.dev/#/patchset/20260831140254.72004-1-sj@kernel.org?part=8