bpf: Disable migration when destroying inode storage

When destroying inode storage, it invokes bpf_local_storage_destroy() to
remove all storage elements saved in the inode storage. The destroy
procedure will call bpf_selem_free() to free the element, and
bpf_selem_free() calls bpf_obj_free_fields() to free the special fields
in map value (e.g., kptr). Since kptrs may be allocated from bpf memory
allocator, migrate_{disable|enable} pairs are necessary for the freeing
of these kptrs.

To simplify reasoning about when migrate_disable() is needed for the
freeing of these dynamically-allocated kptrs, let the caller to
guarantee migration is disabled before invoking bpf_obj_free_fields().
Therefore, the patch adds migrate_{disable|enable} pair in
bpf_inode_storage_free(). The migrate_{disable|enable} pairs in the
underlying implementation of bpf_obj_free_fields() will be removed by
the following patch.

Signed-off-by: Hou Tao <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

Author: Hou Tao

kernel/bpf/bpf_inode_storage.c

@@ -62,16 +62,17 @@ void bpf_inode_storage_free(struct inode *inode)
 	if (!bsb)
 		return;
 
+	migrate_disable();
 	rcu_read_lock();
 
 	local_storage = rcu_dereference(bsb->storage);
-	if (!local_storage) {
-		rcu_read_unlock();
-		return;
-	}
+	if (!local_storage)
+		goto out;
 
 	bpf_local_storage_destroy(local_storage);
+out:
 	rcu_read_unlock();
+	migrate_enable();
 }
 
 static void *bpf_fd_inode_storage_lookup_elem(struct bpf_map *map, void *key)