[PATCH net-next v2 0/3] net: make moving a device to another netns cheaper

[PATCH net-next v2 0/3] net: make moving a device to another netns cheaper

[Qiliang Yuan]

Moving a device to another netns waits in rcu_barrier() with RTNL held.
Container and sandbox managers move one end of a veth pair for every
new network namespace, so this barrier limits how fast such namespaces
can be created on the whole system, to about 75 per second in a 16
vCPU guest no matter how many threads create them.

The barrier was added for NETDEV_UNREGISTER_FINAL, which is long gone,
but it still keeps cached IPv4 routes through the moved device from
being accepted in the old netns. Routes through nexthop objects are not
covered by it: their cached dst entries are released by an RCU callback
queued from another RCU callback, so a socket in the old netns can still
send through a device that now belongs to another netns whenever the
route generation numbers of the two netns happen to match.

Patches 1 and 2 draw the IPv4 rt_genid and the IPv6 fib6_sernum bumps
from sequences shared by all netns, so a cached route no longer matches
another netns. This fixes that leak for nexthop objects and leaves
nothing that relies on the barrier. Patch 3 removes it. Patches 1 and 2
fix bugs that exist today and can go to net on their own if preferred.

Creating 400 netns, each with a veth pair whose host end is moved back
to the initial netns, a tap device, addresses and routes, median of 5
runs:

                   before      after
  1 thread        68 /s      385 /s
  4 threads       74 /s      368 /s
  16 threads      76 /s      263 /s

Signed-off-by: Qiliang Yuan <odys.yuan@gmail.com>
---
V1 -> V2:
- 2/3: keep the initial fib6_sernum of each netns and never hand it out
  from the shared sequence, instead of drawing a value at netns creation
  (Eric Dumazet); 1/3: do the same for rt_genid
- 2/3: note that fib6_sernum is no longer monotonic per netns (Eric
  Dumazet); 1/3: likewise for rt_genid
- Drop Kuniyuki's Reviewed-by from 1/3 and 2/3, which changed
- Rebase onto net-next and remeasure

v1: https://lore.kernel.org/r/20261016-bug-net-netns-move-rcu-barrier-v1-0-4157d798658f@gmail.com

---
Qiliang Yuan (3):
      ipv4: draw rt_genid from a sequence shared by all netns
      ipv6: take fib6_sernum from a sequence shared by all netns
      net: remove rcu_barrier() from __dev_change_net_namespace()

 include/net/net_namespace.h |  5 +----
 net/core/dev.c              |  1 -
 net/ipv4/route.c            | 20 ++++++++++++++++++++
 net/ipv6/ip6_fib.c          | 15 ++++++++++++---
 4 files changed, 33 insertions(+), 8 deletions(-)
---
base-commit: 98c2ad4f5d805dbc1ea436a334ee59311806a48c
change-id: 20261016-bug-net-netns-move-rcu-barrier-d998c6395830

Best regards,
-- 
Qiliang Yuan <odys.yuan@gmail.com>

[PATCH net-next v2 1/3] ipv4: draw rt_genid from a sequence shared by all netns

[Qiliang Yuan]

An IPv4 route records the rt_genid of the netns it was created in, and
rt_is_expired() compares it with the rt_genid of the netns its device
belongs to when the route is checked. Every netns starts its rt_genid
at 0 and increments it on each cache flush, so different netns often
hold the same value.

When a device moves to another netns, routes cached through it remain
usable until dst_dev_put() points them to blackhole_netdev. For a route
using a nexthop object this happens in nexthop_free_rcu(), which
free_fib_info_rcu() queues from inside an RCU callback, so the
rcu_barrier() in __dev_change_net_namespace() does not wait for it. If
the rt_genid of the new netns matches the cached one in that window,
ipv4_dst_check() accepts the route and a socket in the old netns keeps
sending through a device that now belongs to another netns.

Take every rt_genid bump from one sequence shared by all netns, which
never hands out 0, the value every netns starts with. A netns bumps its
rt_genid on NETDEV_UP before its devices can carry traffic, so a cached
route never matches the rt_genid of the netns its device moves to.
Concurrent bumps of one netns can leave the older of two values, so
rt_genid is no longer monotonic per netns. It is only compared for
equality, so this costs at most an extra route lookup.

A connected UDP socket in netns A caches a route through a nexthop
object on a veth. The veth is moved to netns B and brought up with one
RTM_NEWLINK, then the socket sends 200000 datagrams. B's rt_genid is
bumped 0 to 20 times before the move, 63 trials:

                                before     after
  trials sending after move          2         0
  datagrams sent after move       2616         0

Fixes: 4c7e8084fd46 ("ipv4: Plumb support for nexthop object in a fib_info")
Signed-off-by: Qiliang Yuan <odys.yuan@gmail.com>
---
 include/net/net_namespace.h |  5 +----
 net/ipv4/route.c            | 20 ++++++++++++++++++++
 2 files changed, 21 insertions(+), 4 deletions(-)

diff --git a/include/net/net_namespace.h b/include/net/net_namespace.h
index 46b4c67e2966f..9c78f1d90fb62 100644
--- a/include/net/net_namespace.h
+++ b/include/net/net_namespace.h
@@ -556,10 +556,7 @@ static inline int rt_genid_ipv6(const struct net *net)
 }
 #endif
 
-static inline void rt_genid_bump_ipv4(struct net *net)
-{
-	atomic_inc(&net->ipv4.rt_genid);
-}
+void rt_genid_bump_ipv4(struct net *net);
 
 extern void (*__fib6_flush_trees)(struct net *net);
 static inline void rt_genid_bump_ipv6(struct net *net)
diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index 595a3b7f17830..25bebbb30bf3c 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -393,6 +393,26 @@ static inline int ip_rt_proc_init(void)
 }
 #endif /* CONFIG_PROC_FS */
 
+/*
+ * Draw rt_genid bumps from one sequence shared by all netns. A route keeps
+ * the genid of the netns it was created in, and is checked against the netns
+ * its device is in now, so distinct values make that check fail once the
+ * device has moved. Every netns starts at 0, which the sequence never hands
+ * out, and bumps on NETDEV_UP before any of its devices can carry traffic.
+ */
+static atomic_t rt_genid_seq;
+
+void rt_genid_bump_ipv4(struct net *net)
+{
+	int genid;
+
+	do {
+		genid = atomic_inc_return(&rt_genid_seq);
+	} while (!genid);
+	atomic_set(&net->ipv4.rt_genid, genid);
+}
+EXPORT_SYMBOL_GPL(rt_genid_bump_ipv4);
+
 static inline bool rt_is_expired(const struct rtable *rth)
 {
 	bool res;

-- 
2.43.0

[PATCH net-next v2 2/3] ipv6: take fib6_sernum from a sequence shared by all netns

[Qiliang Yuan]

A per-cpu IPv6 route of a nexthop object records fib6_sernum of the
netns it was created in, and rt6_is_valid() compares it with fib6_sernum
of the netns its device belongs to when the route is checked. Every netns
starts its fib6_sernum at 1 and increments it on each tree change, so
different netns often hold the same value.

When a device moves to another netns, such a route remains usable until
nexthop_free_rcu() releases it, and fib6_info_destroy_rcu() queues that
callback from inside an RCU callback, after the device has moved. If the
fib6_sernum of the new netns matches the cached one in that window,
ip6_dst_check() accepts the route and a socket in the old netns keeps
sending through a device that now belongs to another netns.

Take every fib6_sernum bump from one sequence shared by all netns, which
only hands out values from 2 up: every netns still starts at 1, and 0
means there is no sernum to check. Every netns bumps its fib6_sernum
when a route is inserted, so before a route through it can be cached,
and the new netns does so before the moved device can transmit, when
NETDEV_UP adds a route through it. A cached route therefore never
matches the fib6_sernum of the netns its device moves to.
fib6_new_sernum() callers only hold their own table's tb6_lock, so two
tables of one netns can store their values in either order, and
fib6_sernum is no longer monotonic per netns. It is only compared for
equality, so this costs at most an extra invalidation.

A connected UDP socket in netns A caches a route through an IPv6 nexthop
object on a veth. The veth is moved to netns B and brought up with one
RTM_NEWLINK, then the socket sends 200000 datagrams. A route is added to
and removed from B 0 to 20 times before the move, 63 trials:

                                before     after
  trials sending after move          1         0
  datagrams sent after move        251         0

Fixes: 8f34e53b60b3 ("ipv6: Use global sernum for dst validation with nexthop objects")
Signed-off-by: Qiliang Yuan <odys.yuan@gmail.com>
---
 net/ipv6/ip6_fib.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c
index 0dc6ebee6e568..28d20d6def401 100644
--- a/net/ipv6/ip6_fib.c
+++ b/net/ipv6/ip6_fib.c
@@ -89,13 +89,22 @@ static void fib6_walker_unlink(struct net *net, struct fib6_walker *w)
 	write_unlock_bh(&net->ipv6.fib6_walker_lock);
 }
 
+/*
+ * Draw fib6_sernum bumps from one sequence shared by all netns, so that a
+ * route cached against one netns never matches the sernum of another one,
+ * e.g. after its device moved. Every netns starts at 1 and the sequence only
+ * hands out values from 2 up: 0 means no sernum to check.
+ */
+static atomic_t fib6_sernum_seq = ATOMIC_INIT(1);
+
 static int fib6_new_sernum(struct net *net)
 {
-	int new, old = atomic_read(&net->ipv6.fib6_sernum);
+	int new, old = atomic_read(&fib6_sernum_seq);
 
 	do {
-		new = old < INT_MAX ? old + 1 : 1;
-	} while (!atomic_try_cmpxchg(&net->ipv6.fib6_sernum, &old, new));
+		new = old < INT_MAX ? old + 1 : 2;
+	} while (!atomic_try_cmpxchg(&fib6_sernum_seq, &old, new));
+	atomic_set(&net->ipv6.fib6_sernum, new);
 
 	return new;
 }

-- 
2.43.0

[PATCH net-next v2 3/3] net: remove rcu_barrier() from __dev_change_net_namespace()

[Qiliang Yuan]

Commit 6549dd43c043 ("net: dev: fix the incorrect hold of net
namespace's lo device") added rcu_barrier() before
NETDEV_UNREGISTER_FINAL in dev_change_net_namespace(), so that dst
entries released by RCU callbacks were on the dst garbage list before
dst_ifdown() moved them to the loopback device of the old netns.
Commit 070f2d7e264a ("net: Drop NETDEV_UNREGISTER_FINAL") removed the
event but kept the barrier, and dead dst entries now point to
blackhole_netdev instead of a loopback device.

The barrier waits for the RCU callbacks pending on every CPU while RTNL
is held, which takes a full grace period on each netns move. Container
and sandbox managers move one end of a veth pair for every new network
namespace, so this limits how fast such namespaces can be created on the
whole system, regardless of how many threads create them.

Remove the barrier. The RCU callbacks queued by NETDEV_UNREGISTER
handlers free memory and drop device references, which the move does not
wait for. Cached IPv4 and IPv6 routes through the moved device are
rejected once its netns changes, since they no longer match the rt_genid
or fib6_sernum of another netns.

Creating 400 netns, each with a veth pair whose host end is moved back
to the initial netns, a tap device, addresses and routes, in a 16 vCPU
guest, median of 5 runs:

                   before      after
  1 thread        69 /s      385 /s
  4 threads       74 /s      368 /s
  16 threads      76 /s      263 /s

Signed-off-by: Qiliang Yuan <odys.yuan@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@kernel.org>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
---
 net/core/dev.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 8eb9bf1ff9eb6..ef172905109ee 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -12994,7 +12994,6 @@ int __dev_change_net_namespace(struct net_device *dev, struct net *net,
 	 * the device is just moving and can keep their slaves up.
 	 */
 	call_netdevice_notifiers(NETDEV_UNREGISTER, dev);
-	rcu_barrier();
 
 	new_nsid = peernet2id_alloc(dev_net(dev), net, GFP_KERNEL);
 

-- 
2.43.0

Re: [PATCH net-next v2 1/3] ipv4: draw rt_genid from a sequence shared by all netns

[Kuniyuki Iwashima]

On Mon, 19 Oct 2026 11:31:15 +0800, Qiliang Yuan wrote:
> An IPv4 route records the rt_genid of the netns it was created in, and
> rt_is_expired() compares it with the rt_genid of the netns its device
> belongs to when the route is checked. Every netns starts its rt_genid
> at 0 and increments it on each cache flush, so different netns often
> hold the same value.
> 
> When a device moves to another netns, routes cached through it remain
> usable until dst_dev_put() points them to blackhole_netdev. For a route
> using a nexthop object this happens in nexthop_free_rcu(), which
> free_fib_info_rcu() queues from inside an RCU callback, so the
> rcu_barrier() in __dev_change_net_namespace() does not wait for it. If
> the rt_genid of the new netns matches the cached one in that window,
> ipv4_dst_check() accepts the route and a socket in the old netns keeps
> sending through a device that now belongs to another netns.
> 
> Take every rt_genid bump from one sequence shared by all netns, which
> never hands out 0, the value every netns starts with. A netns bumps its
> rt_genid on NETDEV_UP before its devices can carry traffic, so a cached
> route never matches the rt_genid of the netns its device moves to.
> Concurrent bumps of one netns can leave the older of two values, so
> rt_genid is no longer monotonic per netns. It is only compared for
> equality, so this costs at most an extra route lookup.
> 
> A connected UDP socket in netns A caches a route through a nexthop
> object on a veth. The veth is moved to netns B and brought up with one
> RTM_NEWLINK, then the socket sends 200000 datagrams. B's rt_genid is
> bumped 0 to 20 times before the move, 63 trials:
> 
>                                 before     after
>   trials sending after move          2         0
>   datagrams sent after move       2616         0
> 
> Fixes: 4c7e8084fd46 ("ipv4: Plumb support for nexthop object in a fib_info")
> Signed-off-by: Qiliang Yuan <odys.yuan@gmail.com>

Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>

Re: [PATCH net-next v2 2/3] ipv6: take fib6_sernum from a sequence shared by all netns

[Kuniyuki Iwashima]

On Mon, 19 Oct 2026 11:31:16 +0800, Qiliang Yuan wrote:
> A per-cpu IPv6 route of a nexthop object records fib6_sernum of the
> netns it was created in, and rt6_is_valid() compares it with fib6_sernum
> of the netns its device belongs to when the route is checked. Every netns
> starts its fib6_sernum at 1 and increments it on each tree change, so
> different netns often hold the same value.
> 
> When a device moves to another netns, such a route remains usable until
> nexthop_free_rcu() releases it, and fib6_info_destroy_rcu() queues that
> callback from inside an RCU callback, after the device has moved. If the
> fib6_sernum of the new netns matches the cached one in that window,
> ip6_dst_check() accepts the route and a socket in the old netns keeps
> sending through a device that now belongs to another netns.
> 
> Take every fib6_sernum bump from one sequence shared by all netns, which
> only hands out values from 2 up: every netns still starts at 1, and 0
> means there is no sernum to check. Every netns bumps its fib6_sernum
> when a route is inserted, so before a route through it can be cached,
> and the new netns does so before the moved device can transmit, when
> NETDEV_UP adds a route through it. A cached route therefore never
> matches the fib6_sernum of the netns its device moves to.
> fib6_new_sernum() callers only hold their own table's tb6_lock, so two
> tables of one netns can store their values in either order, and
> fib6_sernum is no longer monotonic per netns. It is only compared for
> equality, so this costs at most an extra invalidation.
> 
> A connected UDP socket in netns A caches a route through an IPv6 nexthop
> object on a veth. The veth is moved to netns B and brought up with one
> RTM_NEWLINK, then the socket sends 200000 datagrams. A route is added to
> and removed from B 0 to 20 times before the move, 63 trials:
> 
>                                 before     after
>   trials sending after move          1         0
>   datagrams sent after move        251         0
> 
> Fixes: 8f34e53b60b3 ("ipv6: Use global sernum for dst validation with nexthop objects")
> Signed-off-by: Qiliang Yuan <odys.yuan@gmail.com>

Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>