Re: [PATCH bpf-next v4 3/7] bpf: Implement cgroup storage available to non-cgroup-attached bpf progs

[Martin KaFai Lau <martin.lau@linux.dev>]

On 10/23/22 11:05 AM, Yonghong Song wrote:
> +void bpf_cgrp_storage_free(struct cgroup *cgroup)
> +{
> +	struct bpf_local_storage *local_storage;
> +	struct bpf_local_storage_elem *selem;
> +	bool free_cgroup_storage = false;
> +	struct hlist_node *n;
> +	unsigned long flags;
> +
> +	rcu_read_lock();
> +	local_storage = rcu_dereference(cgroup->bpf_cgrp_storage);
> +	if (!local_storage) {
> +		rcu_read_unlock();
> +		return;
> +	}
> +
> +	/* Neither the bpf_prog nor the bpf_map's syscall
> +	 * could be modifying the local_storage->list now.
> +	 * Thus, no elem can be added to or deleted from the
> +	 * local_storage->list by the bpf_prog or by the bpf_map's syscall.
> +	 *
> +	 * It is racing with __bpf_local_storage_map_free() alone
> +	 * when unlinking elem from the local_storage->list and
> +	 * the map's bucket->list.
> +	 */
> +	bpf_cgrp_storage_lock();
> +	raw_spin_lock_irqsave(&local_storage->lock, flags);
> +	hlist_for_each_entry_safe(selem, n, &local_storage->list, snode) {
> +		bpf_selem_unlink_map(selem);
> +		/* If local_storage list has only one element, the
> +		 * bpf_selem_unlink_storage_nolock() will return true.
> +		 * Otherwise, it will return false. The current loop iteration
> +		 * intends to remove all local storage. So the last iteration
> +		 * of the loop will set the free_cgroup_storage to true.
> +		 */
> +		free_cgroup_storage =
> +			bpf_selem_unlink_storage_nolock(local_storage, selem, false, false);
> +	}
> +	raw_spin_unlock_irqrestore(&local_storage->lock, flags);
> +	bpf_cgrp_storage_unlock();
> +	rcu_read_unlock();
> +
> +	if (free_cgroup_storage)
> +		kfree_rcu(local_storage, rcu);
> +}

[ ... ]

> +/* *gfp_flags* is a hidden argument provided by the verifier */
> +BPF_CALL_5(bpf_cgrp_storage_get, struct bpf_map *, map, struct cgroup *, cgroup,
> +	   void *, value, u64, flags, gfp_t, gfp_flags)
> +{
> +	struct bpf_local_storage_data *sdata;
> +
> +	WARN_ON_ONCE(!bpf_rcu_lock_held());
> +	if (flags & ~(BPF_LOCAL_STORAGE_GET_F_CREATE))
> +		return (unsigned long)NULL;
> +
> +	if (!cgroup)
> +		return (unsigned long)NULL;
> +
> +	if (!bpf_cgrp_storage_trylock())
> +		return (unsigned long)NULL;
> +
> +	sdata = cgroup_storage_lookup(cgroup, map, true);
> +	if (sdata)
> +		goto unlock;
> +
> +	/* only allocate new storage, when the cgroup is refcounted */
> +	if (!percpu_ref_is_dying(&cgroup->self.refcnt) &&
> +	    (flags & BPF_LOCAL_STORAGE_GET_F_CREATE))
> +		sdata = bpf_local_storage_update(cgroup, (struct bpf_local_storage_map *)map,
> +						 value, BPF_NOEXIST, gfp_flags);
> +
> +unlock:
> +	bpf_cgrp_storage_unlock();
> +	return IS_ERR_OR_NULL(sdata) ? (unsigned long)NULL : (unsigned long)sdata->data;
> +}

[ ... ]

> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> index 764bdd5fd8d1..32145d066a09 100644
> --- a/kernel/cgroup/cgroup.c
> +++ b/kernel/cgroup/cgroup.c
> @@ -5227,6 +5227,10 @@ static void css_free_rwork_fn(struct work_struct *work)
>   	struct cgroup_subsys *ss = css->ss;
>   	struct cgroup *cgrp = css->cgroup;
>   
> +#ifdef CONFIG_BPF_SYSCALL
> +	bpf_cgrp_storage_free(cgrp);
> +#endif


After revisiting comment 4bfc0bb2c60e, some of the commit message came to my mind:

" ...... it blocks a possibility to implement
   the memcg-based memory accounting for bpf objects, because a circular
   reference dependency will occur. Charged memory pages are pinning the
   corresponding memory cgroup, and if the memory cgroup is pinning
   the attached bpf program, nothing will be ever released."

Considering the bpf_map_kzalloc() is used in bpf_local_storage_map.c and it can 
charge the memcg, I wonder if the cgrp_local_storage will have similar refcnt 
loop issue here.

If here is the right place to free the cgrp_local_storage() and enough to break 
this refcnt loop, it will be useful to add some explanation and its 
consideration in the commit message.