[PATCH net-next] net: replace mc_dev with a flag in netdev_hw_addr_list

[Yuyang Huang <yuyanghuang@google.com>]

Commit 646cb8868bd2 ("net: add a generation counter for dev->mc
changes") added an mc_dev back pointer to struct netdev_hw_addr_list,
so the list helpers can find the netns of a dev->mc list. Only dev->mc
sets it, but the pointer makes every list 8 bytes larger: 32 to 40
bytes on 64-bit. struct net_device holds four of these lists, and some
drivers keep arrays of them.

Use an is_dev_mc flag in the hole after _count instead, and get the
device with container_of(). The struct is 32 bytes again.

Link: https://lore.kernel.org/netdev/8dcd9564-e9a3-4b00-93b8-3cb023d68047@redhat.com/
Signed-off-by: Yuyang Huang <yuyanghuang@google.com>
---
 include/linux/netdevice.h |  6 +++---
 net/core/dev_addr_lists.c | 12 ++++++++----
 2 files changed, 11 insertions(+), 7 deletions(-)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index c0ae442c4344..e00687e8e750 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -257,11 +257,11 @@ struct netdev_hw_addr_list {
 	 */
 	int			_count;
 
+	/* Set for dev->mc only */
+	bool			is_dev_mc;
+
 	/* Auxiliary tree for faster lookup on addition and deletion */
 	struct rb_root		tree;
-
-	/* Set for dev->mc only, the device the list belongs to */
-	struct net_device	*mc_dev;
 };
 
 #define netdev_hw_addr_list_count(l) ((l)->_count)
diff --git a/net/core/dev_addr_lists.c b/net/core/dev_addr_lists.c
index d827cc21867b..b6f25e487984 100644
--- a/net/core/dev_addr_lists.c
+++ b/net/core/dev_addr_lists.c
@@ -25,8 +25,12 @@
  */
 static void __hw_addr_changed(struct netdev_hw_addr_list *list)
 {
-	if (list->mc_dev)
-		atomic_inc(&dev_net(list->mc_dev)->dev_mc_genid);
+	struct net_device *dev;
+
+	if (!list->is_dev_mc)
+		return;
+	dev = container_of(list, struct net_device, mc);
+	atomic_inc(&dev_net(dev)->dev_mc_genid);
 }
 
 static void __hw_addr_count_add(struct netdev_hw_addr_list *list, int value)
@@ -538,7 +542,7 @@ void __hw_addr_init(struct netdev_hw_addr_list *list)
 	INIT_LIST_HEAD(&list->list);
 	list->_count = 0;
 	list->tree = RB_ROOT;
-	list->mc_dev = NULL;
+	list->is_dev_mc = false;
 }
 EXPORT_SYMBOL(__hw_addr_init);
 
@@ -1215,7 +1219,7 @@ EXPORT_SYMBOL(dev_mc_flush);
 void dev_mc_init(struct net_device *dev)
 {
 	__hw_addr_init(&dev->mc);
-	dev->mc.mc_dev = dev;
+	dev->mc.is_dev_mc = true;
 }
 EXPORT_SYMBOL(dev_mc_init);
 
-- 
2.56.0.542.g3ef89b7020-goog