bpf: Remember smap in bpf_local_storage

This patch remembers which smap triggers the allocation
of a 'struct bpf_local_storage' object. The local_storage is
allocated during the very first selem added to the owner.
The smap pointer is needed when using the bpf_mem_cache_free
in a later patch because it needs to free to the correct
smap's bpf_mem_alloc object.

When a selem is being removed, it needs to check if it is
the selem that triggers the creation of the local_storage.
If it is, the local_storage->smap pointer will be reset to NULL.
This NULL reset is done under the local_storage->lock in
bpf_selem_unlink_storage_nolock() when a selem is being removed.
Also note that the local_storage may not go away even
local_storage->smap is NULL because there may be other
selem still stored in the local_storage.

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20230308065936.1550103-6-martin.lau@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/linux/bpf_local_storage.h

@@ -83,6 +83,7 @@ struct bpf_local_storage_elem {
 
 struct bpf_local_storage {
 	struct bpf_local_storage_data __rcu *cache[BPF_LOCAL_STORAGE_CACHE_SIZE];
+	struct bpf_local_storage_map __rcu *smap;
 	struct hlist_head list; /* List of bpf_local_storage_elem */
 	void *owner;		/* The object that owns the above "list" of
 				 * bpf_local_storage_elem.

kernel/bpf/bpf_local_storage.c

@@ -213,6 +213,9 @@ static bool bpf_selem_unlink_storage_nolock(struct bpf_local_storage *local_stor
 			kfree_rcu(selem, rcu);
 	}
 
+	if (rcu_access_pointer(local_storage->smap) == smap)
+		RCU_INIT_POINTER(local_storage->smap, NULL);
+
 	return free_local_storage;
 }
 
@@ -368,6 +371,7 @@ int bpf_local_storage_alloc(void *owner,
 		goto uncharge;
 	}
 
+	RCU_INIT_POINTER(storage->smap, smap);
 	INIT_HLIST_HEAD(&storage->list);
 	raw_spin_lock_init(&storage->lock);
 	storage->owner = owner;