[PATCH net v4 1/1] ipmr: account multicast table and route memory

[PATCH net v4 1/1] ipmr: account multicast table and route memory

[Zihan Xi]

A netadmin in a user+net namespace can create many IPv4 and IPv6
multicast routing tables with MRT_TABLE and MRT6_TABLE. Each unseen
id allocates an mr_table via the shared mr_table_alloc(), links it
into the per-net list, and leaves it until netns teardown. Those
objects were not charged to memcg, so the host unreclaimable slab
grows with the table count.

Account mr_table allocations with GFP_KERNEL_ACCOUNT and mark the
IPv4/IPv6 MFC caches SLAB_ACCOUNT. This matches the established
handling of IP addresses, routes and alternate interface names.

Unresolved MFC entries are still allocated from softIRQ with
GFP_ATOMIC and are not charged. They expire after 10 seconds and are
bounded by the socket receive queue; see commit 0079ad8e8dc3
("ipmr: remove hard code cache_resolve_queue_len limit").

Fixes: f0ad0860d01e ("ipv4: ipmr: support multiple tables")
Fixes: d1db275dd3f6 ("ipv6: ip6mr: support multiple tables")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Assisted-by: LLM
Signed-off-by: Zihan Xi <zihanx@nebusec.ai>
---
changes in v4:
  - Reword the opening paragraph for IPv4 and IPv6, since
    mr_table_alloc() is shared by both families, as suggested by
    Ido Schimmel.
  - Note that unresolved MFC entries stay GFP_ATOMIC / unaccounted;
    they expire after 10 seconds and are bounded by the socket
    receive queue (commit 0079ad8e8dc3).
  - Point Fixes: at f0ad0860d01e and d1db275dd3f6, the commits that
    let user space create these tables.
  - Drop the 4e16880cb422 archaeology paragraph from the commit
    message and cover letter.
  - v3 Link:
    https://lore.kernel.org/all/cover.1788765185.git.zihanx@nebusec.ai/
changes in v3:
  - Drop the table lifetime / unpublished-alloc / empty-reclaim
    approach from v2.
  - Charge mr_table allocations with GFP_KERNEL_ACCOUNT and mark the
    IPv4/IPv6 MFC caches SLAB_ACCOUNT, as suggested by Ido Schimmel.
  - Point Fixes: at 4e16880cb422, the first GFP_KERNEL IPv6 heap
    vif6_table. 6bd521433942 only wrapped that already-heap state;
    d1db275dd3f6 only expanded the table count from 1 to N.
  - Cover: unfixed evidence is Slab/SUnreclaim growth with RET:0, not
    a host OOM. The memcg OOM log is from the patched kernel under
    64M memory.max using poc.static.
  - v2 Link:
    https://lore.kernel.org/all/cover.1788622674.git.zihanx@nebusec.ai/
changes in v2:
  - Drop the shared mr_table refcount / list_del_rcu path that broke
    the ipmr forwarding selftest.
  - Limit the v2 approach to net/ipv6/ip6mr.c.
  - v1 Link:
    https://lore.kernel.org/all/cover.1784795838.git.zihanx@nebusec.ai/

 net/ipv4/ipmr.c      | 3 ++-
 net/ipv4/ipmr_base.c | 2 +-
 net/ipv6/ip6mr.c     | 2 +-
 3 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/ipmr.c b/net/ipv4/ipmr.c
index e5f2b1c6150d2..b9c544d48c452 100644
--- a/net/ipv4/ipmr.c
+++ b/net/ipv4/ipmr.c
@@ -3376,7 +3376,8 @@ int __init ip_mr_init(void)
 {
 	int err;
 
-	mrt_cachep = KMEM_CACHE(mfc_cache, SLAB_HWCACHE_ALIGN | SLAB_PANIC);
+	mrt_cachep = KMEM_CACHE(mfc_cache,
+				SLAB_HWCACHE_ALIGN | SLAB_PANIC | SLAB_ACCOUNT);
 
 	err = register_pernet_subsys(&ipmr_net_ops);
 	if (err)
diff --git a/net/ipv4/ipmr_base.c b/net/ipv4/ipmr_base.c
index 867b24beded11..a0ec6d19a237f 100644
--- a/net/ipv4/ipmr_base.c
+++ b/net/ipv4/ipmr_base.c
@@ -52,7 +52,7 @@ mr_table_alloc(struct net *net, u32 id,
 	struct mr_table *mrt;
 	int err;
 
-	mrt = kzalloc_obj(*mrt);
+	mrt = kzalloc_obj(*mrt, GFP_KERNEL_ACCOUNT);
 	if (!mrt)
 		return ERR_PTR(-ENOMEM);
 	mrt->id = id;
diff --git a/net/ipv6/ip6mr.c b/net/ipv6/ip6mr.c
index 3f2ed9b77deb5..9d8116b5edb17 100644
--- a/net/ipv6/ip6mr.c
+++ b/net/ipv6/ip6mr.c
@@ -1427,7 +1427,7 @@ int __init ip6_mr_init(void)
 {
 	int err;
 
-	mrt_cachep = KMEM_CACHE(mfc6_cache, SLAB_HWCACHE_ALIGN);
+	mrt_cachep = KMEM_CACHE(mfc6_cache, SLAB_HWCACHE_ALIGN | SLAB_ACCOUNT);
 	if (!mrt_cachep)
 		return -ENOMEM;
 
base-commit: 641d03105cc0d2437e32fdeec164f91a4ccef6c4
-- 
2.43.0

Re: [PATCH net v4 1/1] ipmr: account multicast table and route memory

[Ido Schimmel]

On Tue, Sep 08, 2026 at 11:58:39AM +0000, Zihan Xi wrote:
> A netadmin in a user+net namespace can create many IPv4 and IPv6
> multicast routing tables with MRT_TABLE and MRT6_TABLE. Each unseen
> id allocates an mr_table via the shared mr_table_alloc(), links it
> into the per-net list, and leaves it until netns teardown. Those
> objects were not charged to memcg, so the host unreclaimable slab
> grows with the table count.
> 
> Account mr_table allocations with GFP_KERNEL_ACCOUNT and mark the
> IPv4/IPv6 MFC caches SLAB_ACCOUNT. This matches the established
> handling of IP addresses, routes and alternate interface names.
> 
> Unresolved MFC entries are still allocated from softIRQ with
> GFP_ATOMIC and are not charged. They expire after 10 seconds and are
> bounded by the socket receive queue; see commit 0079ad8e8dc3
> ("ipmr: remove hard code cache_resolve_queue_len limit").

Not worth a v5, just noting in case Sashiko nit picks on it:

Unresolved MFC entries can be allocated from process context and in this
case they are charged correctly.

> 
> Fixes: f0ad0860d01e ("ipv4: ipmr: support multiple tables")
> Fixes: d1db275dd3f6 ("ipv6: ip6mr: support multiple tables")
> Cc: stable@vger.kernel.org
> Reported-by: Vega <vega@nebusec.ai>
> Assisted-by: LLM
> Signed-off-by: Zihan Xi <zihanx@nebusec.ai>

Reviewed-by: Ido Schimmel <idosch@nvidia.com>

Re: [PATCH net v4 1/1] ipmr: account multicast table and route memory

[zihan xi]

On Wed, Sep 9, 2026 at 3:10 PM Ido Schimmel <idosch@nvidia.com> wrote:
>
> On Tue, Sep 08, 2026 at 11:58:39AM +0000, Zihan Xi wrote:
> > A netadmin in a user+net namespace can create many IPv4 and IPv6
> > multicast routing tables with MRT_TABLE and MRT6_TABLE. Each unseen
> > id allocates an mr_table via the shared mr_table_alloc(), links it
> > into the per-net list, and leaves it until netns teardown. Those
> > objects were not charged to memcg, so the host unreclaimable slab
> > grows with the table count.
> >
> > Account mr_table allocations with GFP_KERNEL_ACCOUNT and mark the
> > IPv4/IPv6 MFC caches SLAB_ACCOUNT. This matches the established
> > handling of IP addresses, routes and alternate interface names.
> >
> > Unresolved MFC entries are still allocated from softIRQ with
> > GFP_ATOMIC and are not charged. They expire after 10 seconds and are
> > bounded by the socket receive queue; see commit 0079ad8e8dc3
> > ("ipmr: remove hard code cache_resolve_queue_len limit").
>
> Not worth a v5, just noting in case Sashiko nit picks on it:
>
> Unresolved MFC entries can be allocated from process context and in this
> case they are charged correctly.

Thanks for pointing that out.
>
> >
> > Fixes: f0ad0860d01e ("ipv4: ipmr: support multiple tables")
> > Fixes: d1db275dd3f6 ("ipv6: ip6mr: support multiple tables")
> > Cc: stable@vger.kernel.org
> > Reported-by: Vega <vega@nebusec.ai>
> > Assisted-by: LLM
> > Signed-off-by: Zihan Xi <zihanx@nebusec.ai>
>
> Reviewed-by: Ido Schimmel <idosch@nvidia.com>

Thanks for the review.

Zihan Xi