Re: [PATCH RFC v6 06/18] fs/resctrl: Let bandwidth resources default to min_bw at reset

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] Incorrect generation of default cache allocation bitmask using BIT_MASK on 32-bit architectures.
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commit f1fc8abd972980fe11863cf1116bc25deb926540
Author: Drew Fustini <fustini@kernel.org>

fs/resctrl: Let bandwidth resources default to min_bw at reset

This commit modifies bandwidth resources to default to min_bw instead of
max_bw upon reset and group creation when the hardware enforces a
sum-constrained reservation. The default_to_min flag is introduced in
struct resctrl_membw for this purpose.

> diff --git a/include/linux/resctrl.h b/include/linux/resctrl.h
> index 9529ed0d1fdf2..bcbc166412ef1 100644
> --- a/include/linux/resctrl.h
> +++ b/include/linux/resctrl.h

[ ... ]

> @@ -405,7 +412,7 @@ static inline u32 resctrl_get_default_ctrl(struct rdt_resource *r)
>  	case RESCTRL_SCHEMA_BITMAP:
>  		return BIT_MASK(r->cache.cbm_len) - 1;
[Severity: High]
This is a pre-existing issue, but does the use of BIT_MASK cause an incorrect
default mask to be generated on 32-bit architectures?

If a system has a maximum cache bitmask length of 32, which is
architecturally allowed by the Intel CAT specification, BITS_PER_LONG is 32
on 32-bit systems. The BIT_MASK macro evaluates to 
(1UL << ((nr) % BITS_PER_LONG)).

For a cbm_len of 32, 32 % 32 evaluates to 0, which makes BIT_MASK(32) equal
to 1. Subtracting 1 results in a default cache allocation mask of 0 instead
of the expected 0xFFFFFFFF. Could this functionally break the cache
allocation mechanism by starving groups of L3/L2 cache on group creation
and reset?

>  	case RESCTRL_SCHEMA_RANGE:
> -		return r->membw.max_bw;
> +		return r->membw.default_to_min ? r->membw.min_bw : r->membw.max_bw;
>  	}
>  
>  	return WARN_ON_ONCE(1);

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Sashiko AI review · https://sashiko.dev/#/patchset/20260601-ssqosid-cbqri-rqsc-v7-0-v6-0-baf00f50028a@kernel.org?part=6