net: page_pool: stash the NAPI ID for easier access

To avoid any issues with race conditions on accessing napi
and having to think about the lifetime of NAPI objects
in netlink GET - stash the napi_id to which page pool
was linked at creation time.
Reviewed-by: Eric Dumazet <edumazet@google.com>
Acked-by: Jesper Dangaard Brouer <hawk@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

include/net/page_pool/types.h

@@ -193,6 +193,7 @@ struct page_pool {
 	/* User-facing fields, protected by page_pools_lock */
 	struct {
 		struct hlist_node list;
+		u32 napi_id;
 		u32 id;
 	} user;
 };

net/core/page_pool_user.c

@@ -37,9 +37,11 @@ int page_pool_list(struct page_pool *pool)
 	if (err < 0)
 		goto err_unlock;
 
-	if (pool->slow.netdev)
+	if (pool->slow.netdev) {
 		hlist_add_head(&pool->user.list,
 			       &pool->slow.netdev->page_pools);
+		pool->user.napi_id = pool->p.napi ? pool->p.napi->napi_id : 0;
+	}
 
 	mutex_unlock(&page_pools_lock);
 	return 0;