> diff --git a/mm/bpf_memcontrol.c b/mm/bpf_memcontrol.c
> index 415e3c59b6a6..0ded1fdd0de8 100644
> --- a/mm/bpf_memcontrol.c
> +++ b/mm/bpf_memcontrol.c

[ ... ]

> static int bpf_memcg_ops_reg(void *kdata, struct bpf_link *link)
> {
> 	struct bpf_struct_ops_link *ops_link
> 		= container_of(link, struct bpf_struct_ops_link, link);
> 	struct memcg_bpf_ops *ops = kdata;
> 	struct mem_cgroup *memcg, *iter = NULL;
> 	int err = 0;
>
> +	if (ops_link->flags & ~BPF_F_ALLOW_OVERRIDE) {
> +		pr_err("attach only support BPF_F_ALLOW_OVERRIDE\n");
> +		return -EOPNOTSUPP;
> +	}
> +
> 	memcg = mem_cgroup_get_from_ino(ops_link->cgroup_id);
> 	if (IS_ERR_OR_NULL(memcg))
> 		return PTR_ERR(memcg);
>
> 	cgroup_lock();
> +
> +	if (READ_ONCE(memcg->bpf_ops)) {
> +		/* Check if bpf_ops of the parent is BPF_F_ALLOW_OVERRIDE. */
> +		if (memcg->bpf_ops_flags & BPF_F_ALLOW_OVERRIDE) {
> +			iter = parent_mem_cgroup(memcg);
> +
> +			if (!iter)
> +				goto busy_out;
> +			if (READ_ONCE(iter->bpf_ops) !=
> +			    READ_ONCE(memcg->bpf_ops))
> +				goto busy_out;
> +		} else {
> +busy_out:
> +			err = -EBUSY;
> +			goto unlock_out;
> +		}
> +	}
> +
> 	while ((iter = mem_cgroup_iter(memcg, iter, NULL))) {
                                      ^^^^

When the override check passes (all conditions at lines 363-377 are met),
iter is set to parent_mem_cgroup(memcg) but is never reset to NULL before
this loop. Should iter be reset to NULL here?

The mem_cgroup_iter() function expects prev (iter) to be either NULL or a
previous result from iterating within memcg's descendants. When iter points
to memcg's parent, the iteration will not correctly traverse memcg's
subtree since parent is outside that subtree.

For comparison, bpf_memcg_ops_unreg() correctly resets iter to NULL before
its loop:

> +	iter = NULL;
> +	while ((iter = mem_cgroup_iter(memcg, iter, NULL))) {

> 		if (READ_ONCE(iter->bpf_ops)) {
> -			mem_cgroup_iter_break(memcg, iter);
> -			err = -EBUSY;
> -			break;
> +			/* cannot override existing bpf_ops of sub-cgroup. */
> +			continue;
> 		}
> 		WRITE_ONCE(iter->bpf_ops, ops);
> +		iter->bpf_ops_flags = ops_link->flags;
> 	}

[ ... ]


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/21280790825