Re: [RFC PATCH v1.3 01/11] Docs/mm/damon/design: update for DAMOS_QUOTA_NODE_ELIGIBLE_MEM_BP

[SeongJae Park <sj@kernel.org>]

On Fri, 26 Jun 2026 00:24:48 +0000 sashiko-bot@kernel.org wrote:

> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> 
> Pre-existing issues:
> - [High] Truncation of 64-bit phys_addr_t to 32-bit unsigned long in damos_get_node_eligible_mem_bp leads to incorrect metric calculation on 32-bit systems.
> --
> 
> commit 569eb38d08bfaf4718faa6440d86cad7ba9478b7
> Author: SeongJae Park <sj@kernel.org>
> 
> Docs/mm/damon/design: update for DAMOS_QUOTA_NODE_ELIGIBLE_MEM_BP
> 
> Commit 9138e27a3bc3 introduced DAMOS_QUOTA_NODE_ELIGIBLE_MEM_BP but forgot
> updating the DAMON design document for that. This commit updates the
> documentation to include the missing metric.
> 
> > diff --git a/Documentation/mm/damon/design.rst b/Documentation/mm/damon/design.rst
> > --- a/Documentation/mm/damon/design.rst
> > +++ b/Documentation/mm/damon/design.rst
> > @@ -686,9 +686,11 @@ mechanism tries to make ``current_value`` of ``target_metric`` be same to
> >    (1/10,000).
> >  - ``inactive_mem_bp``: Inactive to active + inactive (LRU) memory size ratio in
> >    bp (1/10,000).
> > +- ``node_eligible_mem_bp``: Scheme target access pattern-eligible memory ratio
> > +  of a node in bp (1/10,000).
> 
> [Severity: High]
> This isn't a bug introduced by this patch, but while reviewing this metric, I
> noticed a pre-existing issue in its implementation.
> 
> In mm/damon/core.c:damos_get_node_eligible_mem_bp(), 64-bit phys_addr_t byte
> counts are cast to 32-bit unsigned long before ratio calculation:
> 
> 	if (!(unsigned long)total_eligible)
> 		return 0;
> 
> 	return mult_frac((unsigned long)node_eligible, 10000,
> 			(unsigned long)total_eligible);
> 
> On 32-bit systems with PAE, if the amount of eligible memory exceeds 4GB,
> will this truncation cause the metric to calculate completely incorrect ratios
> (e.g., reporting > 10000 basis points) or erroneously return 0 due to modulo
> arithmetic?
> 
> This could cause the DAMOS quota tuner to adjust quotas wrongly, potentially
> resulting in extremely aggressive or completely ineffective memory actions.

Known low priority issue.  Not a blocker.


Thanks,
SJ

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