Re: [RFC PATCH v1.1 8/9] mm/damon/core: handle extreme memory state in get_node_memcg_used_bp()

[SJ Park <sj@kernel.org>]

On Sun, 02 Aug 2026 16:38:33 +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] This problem wasn't introduced by this patch, but damos_get_node_mem_bp() contains a severe logic bug and underflow when free memory exceeds total memory (i.totalram < i.freeram).
> --
> 
> --- Patch [8]: [RFC PATCH v1.1 8/9] mm/damon/core: handle extreme memory state in get_node_memcg_used_bp() ---
> commit 0fb31bc202c766c30f5d776d57b867135fba4aa6
> Author: SJ Park <sj@kernel.org>
> mm/damon/core: handle extreme memory state in get_node_memcg_used_bp()
> 
> This commit addresses division by zero and underflow issues in
> damos_get_node_memcg_used_bp() by handling cases where total memory is zero or
> used memory exceeds total memory. It returns 100% and 0% for these respective
> edge cases to prevent anomalous behavior.
> 
> Link: https://lore.kernel.org/20260329154813.47382-1-sj@kernel.org
> 
> > diff --git a/mm/damon/core.c b/mm/damon/core.c
> > index e3f3ee75a3d33..67ad1f07c29a4 100644
> > --- a/mm/damon/core.c
> > +++ b/mm/damon/core.c
> > @@ -2862,10 +2862,16 @@ static unsigned long damos_get_node_memcg_used_bp(
> >  	mem_cgroup_put(memcg);
> >  
> >  	si_meminfo_node(&i, goal->nid);
> > -	if (goal->metric = DAMOS_QUOTA_NODE_MEMCG_USED_BP)
> > +	if (!i.totalram)
> > +		return 10000;
> > +	if (goal->metric = DAMOS_QUOTA_NODE_MEMCG_USED_BP) {
> >  		numerator = used_pages;
> > -	else	/* DAMOS_QUOTA_NODE_MEMCG_FREE_BP */
> > +	} else {
> > +		/* DAMOS_QUOTA_NODE_MEMCG_FREE_BP */
> > +		if (i.totalram < used_pages)
> > +			return 0;
> 
> [Severity: High]
> This isn't a bug introduced by this patch, but does the adjacent
> damos_get_node_mem_bp() function contain a logic bug when free memory exceeds
> total memory?
> 
> If i.totalram < i.freeram, it appears that for DAMOS_QUOTA_NODE_MEM_USED_BP,
> i.totalram - i.freeram will unconditionally underflow, falsely throttling or
> activating schemes. For DAMOS_QUOTA_NODE_MEM_FREE_BP, returning 0 means it
> reports 0% free memory when the node is actually >100% free, which could cause
> schemes waiting for low free memory to trigger aggressively.
> 
> mm/damon/core.c:damos_get_node_mem_bp() {
> 	...
> 	if (goal->metric = DAMOS_QUOTA_NODE_MEM_USED_BP) {
> 		numerator = i.totalram - i.freeram;
> 	} else {
> 		/* DAMOS_QUOTA_NODE_MEM_FREE_BP */
> 		if (i.totalram < i.freeram)
> 			return 0;
> 		numerator = i.freeram;
> 	}
> 	...
> }
> 
> Should this pre-existing issue in damos_get_node_mem_bp() be addressed as well,
> perhaps by returning 0 for USED_BP and 10000 for FREE_BP when
> i.totalram < i.freeram?

The previous patch was completely broken by my mistake.  I will fix.

> 
> >  		numerator = i.totalram - used_pages;
> > +	}
> >  	return mult_frac(numerator, 10000, i.totalram);
> >  }
> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20260802162631.90304-1-sj@kernel.org?part=8
> 


Thanks,
SJ