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

[SJ Park <sj@kernel.org>]

On Tue, 01 Sep 2026 13:36:50 +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] 32-bit integer overflow in DAMOS metric calculations.
> --
> 
> commit 4a4658bd5d39473e6ccb07618a88e601987f2acf
> Author: SJ Park <sj@kernel.org>
> 
> mm/damon/core: handle extreme memory state in get_in_active_mem_bp()
> 
> The function damos_get_in_active_mem_bp() uses the sum of active and inactive
> memory as a denominator. In extreme environments where both are zero, this
> results in a divide by zero error. This patch avoids the issue by changing
> the denominator to one if it is zero.
> 
> Link: https://lore.kernel.org/20260721034756.147011-1-sj@kernel.org
> 
> > diff --git a/mm/damon/core.c b/mm/damon/core.c
> > index 3060edf5e4fa6..0df785e72438f 100644
> > --- a/mm/damon/core.c
> > +++ b/mm/damon/core.c
> > @@ -3006,7 +3006,7 @@ 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);
> >  	return mult_frac(inactive, 10000, total);
> 
> [Severity: High]
> This isn't a bug introduced by this patch, but it's a pre-existing high
> severity issue in the exact function modified.
> 
> Could this mult_frac() calculation overflow on 32-bit architectures?
> 
> When active or inactive memory exceeds approximately 429,496 pages (about
> 1.7GB) on a 32-bit system, the intermediate product in mult_frac():
> (r * n / d) where r is (active % total) and n is 10000
> 
> will overflow the 32-bit unsigned long before division. This wraps around
> and causes Damon to apply completely wrong quotas, degrading memory goal
> reliability.
> 
> Does this same 32-bit overflow issue also affect other metric calculations
> in mm/damon/core.c, such as damos_get_node_mem_bp() and
> damos_get_node_memcg_used_bp() which similarly compute:
> mult_frac(numerator, 10000, i.totalram) ?

Known low priority issue.

> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20260901131850.98037-1-sj@kernel.org?part=8
> 


Thanks,
SJ