Re: [PATCH] memcg: damon: get memcg reference before access

[SeongJae Park <sj@kernel.org>]

On Tue, 23 Dec 2025 19:45:27 -0800 Shakeel Butt <shakeel.butt@linux.dev> wrote:

> The commit b74a120bcf507 ("mm/damon/core: implement
> DAMOS_QUOTA_NODE_MEMCG_USED_BP") added accesses to memcg structure
> without getting reference to it. This is unsafe. Let's get the reference
> before accessing the memcg.

Thank you for catching and fixing this!

Nit.  On the subject, could we use 'mm/damon/core:' prefix instead of 'memcg:
damon:' for keeping the file's commit log subjects consistent?

> 
> Fixes: b74a120bcf507 ("mm/damon/core: implement DAMOS_QUOTA_NODE_MEMCG_USED_BP")

I was firsty thinking we might need to Cc: stable@.  But I realized the broken
commit has merged into 6.19-rc1.  So Cc: stable@ is not needed.

> Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>

Other than the tirivial subject prefix inconsistency, looks good to me.

Reviewed-by: SeongJae Park <sj@kernel.org>

> ---
>  mm/damon/core.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index 4ad5f290d382..89982e0229f0 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -2051,13 +2051,15 @@ static unsigned long damos_get_node_memcg_used_bp(
>  
>  	rcu_read_lock();
>  	memcg = mem_cgroup_from_id(goal->memcg_id);
> -	rcu_read_unlock();
> -	if (!memcg) {
> +	if (!memcg || !mem_cgroup_tryget(memcg)) {

For this part, I was thinking '!memcg' part seems not technically needed
because mem_cgroup_tryget() does the check.  But I think that's just trivial,
so this also looks good to me.

> +		rcu_read_unlock();
>  		if (goal->metric == DAMOS_QUOTA_NODE_MEMCG_USED_BP)
>  			return 0;
>  		else	/* DAMOS_QUOTA_NODE_MEMCG_FREE_BP */
>  			return 10000;
>  	}
> +	rcu_read_unlock();
> +
>  	mem_cgroup_flush_stats(memcg);
>  	lruvec = mem_cgroup_lruvec(memcg, NODE_DATA(goal->nid));
>  	used_pages = lruvec_page_state(lruvec, NR_ACTIVE_ANON);
> @@ -2065,6 +2067,8 @@ static unsigned long damos_get_node_memcg_used_bp(
>  	used_pages += lruvec_page_state(lruvec, NR_ACTIVE_FILE);
>  	used_pages += lruvec_page_state(lruvec, NR_INACTIVE_FILE);
>  
> +	mem_cgroup_put(memcg);
> +
>  	si_meminfo_node(&i, goal->nid);
>  	if (goal->metric == DAMOS_QUOTA_NODE_MEMCG_USED_BP)
>  		numerator = used_pages;
> -- 
> 2.47.3


Thanks,
SJ

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