Re: [PATCH 3/4] mm/damon/core: copy nid for eligible_mem_bp damos quota goal commit

[Kunwu Chan <kunwu.chan@gmail.com>]

On Tue,  1 Sep 2026 17:27:22 -0700 SJ Park <sj@kernel.org> wrote:

> damos_commit_quota_goal_union() is not updating the ->nid union field
> when the goal metric is DAMOS_QUITA_NODE_ELIGIBLE_MEM_BP.  Hence, if a
> DAMOS quota goal of the type is online committed in a way that it will
> reuse other quota goal's memory space, the new goal will work with a
> garbage nid value.  As a result, the DAMOS scheme can show unexpected
> aggressiveness.  Do the update.
> 
> The user impact is not catastrophic.  No leak or crash happens.  Doing
> the quota goal online commit that can reproduce the issue is expected to
> be not common.  This issue was not found by real users but the AI
> review.  That said, the issue can reliably be reproduced.
> 
> This issue was discovered [1] by Sashiko.
> 
> [1] https://lore.kkernel.org/20260827045035.94611-1-sj@kernel.org

Hi SJ,

The fix is consistent with the other node-based quota metrics. I also 
checked that damos_get_node_eligible_mem_bp() directly uses goal->nid 
when calculating the current value.

One minor typo: the [1] has an extra k (kkernel should be kernel).

Reviewed-by: Kunwu Chan <kunwu.chan@gmail.com>

Thanks,
Kunwu

> 
> Fixes: 9138e27a3bc3 ("mm/damon: add node_eligible_mem_bp goal metric")
> Cc: <stable@vger.kernel.org> # 7.2.x
> Signed-off-by: SJ Park <sj@kernel.org>
> ---
>  mm/damon/core.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index 20748b0a71026..53c3f97e3f126 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -1143,6 +1143,9 @@ static void damos_commit_quota_goal_union(
>  		dst->nid = src->nid;
>  		dst->memcg_id = src->memcg_id;
>  		break;
> +	case DAMOS_QUOTA_NODE_ELIGIBLE_MEM_BP:
> +		dst->nid = src->nid;
> +		break;
>  	default:
>  		break;
>  	}
> -- 
> 2.47.3
> 

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