Re: [RESEND RFC PATCH v2 4/5] mm/damon: introduce DAMOS_SPLIT action

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] Sysfs state inconsistency when an invalid target_order is supplied for DAMOS_SPLIT
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commit 2142d0d071963cdc56d472e03ea29ec38eeda286
Author: Lian Wang <lianux.mm@gmail.com>

mm/damon: introduce DAMOS_SPLIT action

This commit adds the DAMOS_SPLIT action for splitting large folios into
smaller mTHP-order folios. It exposes this action through the DAMON sysfs
interface and validates the target_order to be within the allowed bounds.

[ ... ]

> diff --git a/mm/damon/sysfs-schemes.c b/mm/damon/sysfs-schemes.c
> index 9067cb66632ed..198b64ca22598 100644
> --- a/mm/damon/sysfs-schemes.c
> +++ b/mm/damon/sysfs-schemes.c

[ ... ]

> @@ -3063,11 +3067,23 @@ static struct damos *damon_sysfs_mk_scheme(
>  			HPAGE_PMD_ORDER, HPAGE_PMD_ORDER);
>  		target_order = 0;
>  	}
> +	if (sysfs_scheme->action == DAMOS_SPLIT &&
> +	    (target_order == 0 ||
> +	     target_order >= HPAGE_PMD_ORDER)) {
> +		pr_warn("DAMON split: target_order %u invalid, need 2..%u. Defaulting to 2.\n",
> +			target_order,
> +			HPAGE_PMD_ORDER - 1);
> +		target_order = 2;
> +	}

[Severity: Medium]
Does this create a state inconsistency between the user-facing sysfs
interface and the actual running kernel configuration?

When an invalid target_order is supplied, target_order is overridden to a
safe default for the internal scheme here, but the sysfs_scheme->target_order
field is left unmodified. As a result, the sysfs read interface will
continue to report the incorrect value (such as 0) instead of the actual
value (2) being used by the active scheme.

>  #else
>  	if (sysfs_scheme->action == DAMOS_COLLAPSE && target_order != 0) {
>  		pr_warn("DAMON collapse: target_order not supported without THP. Use 0.\n");
>  		target_order = 0;
>  	}
> +	if (sysfs_scheme->action == DAMOS_SPLIT) {
> +		pr_warn("DAMON split: not supported without THP.\n");
> +		target_order = 2;
> +	}
>  #endif
>  	scheme->target_order = target_order;
>

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260702095227.75866-1-lianux.mm@gmail.com?part=4