[PATCH] net: limit a number of namespaces which can be cleaned up concurrently

[Andrei Vagin <avagin-GEFAQzZX7r8dnm+yROfE0A@public.gmane.org>]

From: Andrey Vagin <avagin-GEFAQzZX7r8dnm+yROfE0A@public.gmane.org>

The operation of destroying netns is heavy and it is executed under
net_mutex. If many namespaces are destroyed concurrently, net_mutex can
be locked for a long time. It is impossible to create a new netns during
this period of time.

In our days when userns allows to create network namespaces to
unprivilaged users, it may be a real problem.

On my laptop (fedora 24, i5-5200U, 12GB) 1000 namespaces requires about
300MB of RAM and are being destroyed for 8 seconds.

In this patch, a number of namespaces which can be cleaned up
concurrently is limited by 32. net_mutex is released after handling each
portion of net namespaces and then it is locked again to handle the next
one. It allows other users to lock it without waiting for a long
time.

I am not sure whether we need to add a sysctl to costomize this limit.
Let me know if you think it's required.

Cc: "David S. Miller" <davem-fT/PcQaiUtIeIZ0/mPfg9Q@public.gmane.org>
Cc: "Eric W. Biederman" <ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
Signed-off-by: Andrei Vagin <avagin-GEFAQzZX7r8dnm+yROfE0A@public.gmane.org>
---
 net/core/net_namespace.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/net/core/net_namespace.c b/net/core/net_namespace.c
index 989434f..33dd3b7 100644
--- a/net/core/net_namespace.c
+++ b/net/core/net_namespace.c
@@ -406,10 +406,20 @@ static void cleanup_net(struct work_struct *work)
 	struct net *net, *tmp;
 	struct list_head net_kill_list;
 	LIST_HEAD(net_exit_list);
+	int i = 0;
 
 	/* Atomically snapshot the list of namespaces to cleanup */
 	spin_lock_irq(&cleanup_list_lock);
-	list_replace_init(&cleanup_list, &net_kill_list);
+	list_for_each_entry_safe(net, tmp, &cleanup_list, cleanup_list)
+		if (++i == 32)
+			break;
+	if (i == 32) {
+		list_cut_position(&net_kill_list,
+				  &cleanup_list, &net->cleanup_list);
+		queue_work(netns_wq, work);
+	} else {
+		list_replace_init(&cleanup_list, &net_kill_list);
+	}
 	spin_unlock_irq(&cleanup_list_lock);
 
 	mutex_lock(&net_mutex);
-- 
2.7.4