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

[SeongJae Park <sj@kernel.org>]

[...]
> Changes since v8:
> =================
> https://lore.kernel.org/linux-mm/20260426003245.2687-1-ravis.opensrc@gmail.com/
> 
> - Removed unnecessary casts in mult_frac() call to fix potential
>   divide-by-zero on 32-bit PAE systems (Sashiko/SJ review)
> - Kept original parameter name 'ctx' in damos_set_effective_quota()
>   to avoid unnecessary diff (SJ review)
[...]
> +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);
> +}

Sashiko found [1] this can cause __udivdi3 linking issue on 32bit machine with
CONFIG_PHYS_ADDR_T_64BIT.  Seems that is correct to me.  I believe below
attaching fixup can fix it?

Ravi, if my fixup looks good to you, could you please post another version
(say, v9.1) for another Sashiko review round?

[1] https://lore.kernel.org/20260426220409.95B26C2BCAF@smtp.kernel.org


Thanks,
SJ

[...]

=== >8 ===
From 828ddfdfa3a996efc4620919a3cc77d55088c466 Mon Sep 17 00:00:00 2001
From: SeongJae Park <sj@kernel.org>
Date: Sun, 26 Apr 2026 15:54:48 -0700
Subject: [PATCH] mm/damon/core: avoid __udivdi3 linking issue

On 32bit machines having PHYS_ADDR_T_64BIT, mult_frac() with phys_addr_t
will cause __udivdi3 linking issue.  Cast values to 'unsigned long'
before calling mult_frac().  This could cause divide-by-zero if the
denominator becomes zero by casting.  Avoid it by checking the zero case
with the casted value.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/core.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 0ea747d487465..aec43314fc4a9 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -2921,10 +2921,11 @@ static unsigned long damos_get_node_eligible_mem_bp(struct damon_ctx *c,

        node_eligible = damos_calc_eligible_bytes(c, s, nid, &total_eligible);

-       if (!total_eligible)
+       if (!(unsigned long)total_eligible)
                return 0;

-       return mult_frac(node_eligible, 10000, total_eligible);
+       return mult_frac((unsigned long)node_eligible, 10000,
+                       (unsigned long)total_eligible);
 }
 #else /* CONFIG_DAMON_PADDR */
 static unsigned long damos_get_node_eligible_mem_bp(struct damon_ctx *c,
--
2.47.3