Re: [PATCH v9] mm/damon: add node_eligible_mem_bp goal metric

[SeongJae Park <sj@kernel.org>]

On Sun, 26 Apr 2026 22:04:08 +0000 sashiko-bot@kernel.org wrote:

> Sashiko AI review found 4 potential issue(s):
> - [High] Missing `cond_resched()` inside the inner address-scanning loop causes CPU soft lockups.

Unlikely.

> - [High] Direct use of `mult_frac()` with 64-bit `phys_addr_t` causes a linker error on 32-bit architectures with PAE.

Good finding.  Read below for detailed answer.

> - [High] Integer overflow in `mult_frac()` on 32-bit systems without PAE leads to incorrect metric calculations.

No real issue.

> - [High] Unbounded O(N) page-by-page memory scan in quota evaluation breaks DAMON's low-overhead design.

Intended behavior.

[...]
> > +static unsigned long damos_get_node_eligible_mem_bp(struct damon_ctx *c,
> > +		struct damos *s, int nid)
> > +{
> > +	phys_addr_t total_eligible = 0;
> > +	phys_addr_t node_eligible;
> > +
> > +	if (c->ops.id != DAMON_OPS_PADDR)
> > +		return 0;
> > +
> > +	if (nid < 0 || nid >= MAX_NUMNODES || !node_online(nid))
> > +		return 0;
> > +
> > +	node_eligible = damos_calc_eligible_bytes(c, s, nid, &total_eligible);
> > +
> > +	if (!total_eligible)
> > +		return 0;
> > +
> > +	return mult_frac(node_eligible, 10000, total_eligible);
> 
> Can this cause a linker error on 32-bit architectures with PAE?
> 
> On 32-bit systems with CONFIG_PHYS_ADDR_T_64BIT enabled, phys_addr_t is a
> 64-bit integer. The mult_frac() macro performs native division using the
> type of its arguments. Since the kernel does not link 64-bit division
> routines (__udivdi3) on 32-bit platforms, this will cause a build failure.

Heh, good finding.  I suggested a fixup on my reply [1] to the patch.

[1] https://lore.kernel.org/20260426230322.106206-1-sj@kernel.org


Thanks,
SJ

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