Re: (sashiko review) [PATCH v2] mm/damon/core: validate goal nid before accessing node data

[SeongJae Park <sj@kernel.org>]

On Wed, 25 Mar 2026 17:10:50 -0700 SeongJae Park <sj@kernel.org> wrote:

> Forwarding Sashiko review for doing discussions via mails.
> 
> # review url: https://sashiko.dev/#/patchset/20260325155221.202700-1-objecting@objecting.org
> # start of sashiko.dev inline review
> commit 24e25058b8f508495d0b3db33bed16564df7fed4
> Author: Josh Law <objecting@objecting.org>
> Subject: mm/damon/core: validate goal nid before accessing node data
> 
> This patch adds bounds and node_state(N_MEMORY) checks before using
> goal->nid from userspace to prevent out-of-bounds access in NODE_DATA().
> 
> Cc: stable@vger.kernel.org
> 
> > diff --git a/mm/damon/core.c b/mm/damon/core.c
> > index db6c67e52d2b..ed45a7443abe 100644
> > --- a/mm/damon/core.c
> > +++ b/mm/damon/core.c
> > @@ -2245,6 +2245,10 @@ static __kernel_ulong_t damos_get_node_mem_bp(
> >  	struct sysinfo i;
> >  	__kernel_ulong_t numerator;
> >  
> > +	if (goal->nid < 0 || goal->nid >= MAX_NUMNODES ||
> > +	    !node_state(goal->nid, N_MEMORY))
> > +		return 0;
> > +
> 
> When goal->nid is invalid or offline, this unconditionally returns 0.
> For the DAMOS_QUOTA_NODE_MEM_FREE_BP metric, wouldn't returning 0 imply
> 0% free memory?
> 
> Could this cause the DAMON feedback loop to interpret the goal as
> severely under-achieved and drastically increase the scheme's
> aggressiveness?
> 
> Should this fallback logic check the metric type and return 10000
> (100% free) for FREE_BP metrics to fail gracefully, mirroring the
> existing fallback logic used when a memory cgroup lookup fails?

I am also thinking similar to sashiko.  Could you please make the change?

> 
> >  	si_meminfo_node(&i, goal->nid);
> >  	if (goal->metric == DAMOS_QUOTA_NODE_MEM_USED_BP)
> >  		numerator = i.totalram - i.freeram;
> >  	else	/* DAMOS_QUOTA_NODE_MEM_FREE_BP */
> >  		numerator = i.freeram;
> >  	return mult_frac(numerator, 10000, i.totalram);
> 
> While not introduced by this patch, since this function is being
> modified to handle node edge cases, is it possible for a node to have
> N_MEMORY set, but have 0 managed pages? For example, if all of its
> memory is reserved for hugetlb or unmanaged ZONE_DEVICE memory.

I'm not very sure if this is really possible.  Josh, do you know?

> 
> If so, si_meminfo_node() would set i.totalram to 0. Would the call
> to mult_frac() then cause a divide-by-zero panic when it evaluates
> x_ / d_ ?

If sashiko's theory is true, I think we should add a zero value check here?

> 
> > @@ -2261,6 +2265,10 @@ static unsigned long damos_get_node_memcg_used_bp(
> >  	unsigned long used_pages, numerator;
> >  	struct sysinfo i;
> >  
> > +	if (goal->nid < 0 || goal->nid >= MAX_NUMNODES ||
> > +	    !node_state(goal->nid, N_MEMORY))
> > +		return 0;
> > +
> 
> Similar to the function above, returning 0 unconditionally here for
> DAMOS_QUOTA_NODE_MEMCG_FREE_BP might artificially trigger maximum
> reclaim aggressiveness.

Similar to my above comment, I agree to sashiko.


Thanks,
SJ

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