Re: [PATCH net] net/packet: charge fanout groups to the memory cgroup

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Ahmad AG wrote:
> fanout_add() allocates a new group with room for max_num_members
> socket pointers. Since commit 9c661b0b8544 ("net/packet: make
> packet_fanout.arr size configurable up to 64K") that is up to 64K
> pointers, 512 KB per group, from a single PACKET_FANOUT setsockopt().
> The allocation uses GFP_KERNEL, so it is not charged to the caller's
> memory cgroup.
>
> Each packet socket can create its own group, so a process with
> CAP_NET_RAW in its own user and network namespace holds about 512 KB
> of kernel memory per open socket that its memory cgroup never sees.
> In a cgroup with memory.max set to 16M, 256 sockets each creating a
> group with max_num_members = 65536 took 264 MB of system memory while
> memory.current grew by 380 KB.
> 
> Allocate the group with GFP_KERNEL_ACCOUNT, as other allocations a

This is not the only such allocation in packet sockets that does not
account. If addressing that, probably scan all such allocations and
update them in one go.

> user can make on demand in the networking stack already do. With the
> change the same test in the 16M cgroup is stopped by the cgroup's OOM
> killer, and with memory.max at 512M the 256 groups charge 262656 KB
> for 264640 KB of system memory used.
> 
> The missing charge was found by code inspection of net/packet/ with
> the help of an LLM and then triggered with a small test program that
> creates the sockets and groups and reads memory.current; the numbers
> above are from that run. Tested in a QEMU/KVM guest on this tree
> (x86_64_defconfig plus kvm_guest.config, KASAN, MEMCG, PACKET),
> running the test as uid 1000 in a cgroup v2 memory cgroup.
> tools/testing/selftests/net/psock_fanout passes before and after.
> 
> Fixes: 9c661b0b8544 ("net/packet: make packet_fanout.arr size
> configurable up to 64K")

Your email client corrupted the patch.

> Cc: stable@vger.kernel.org
> Assisted-by: Claude Code
> Signed-off-by: Ahmad AG <ahmadag1001@gmail.com>
> ---
>  net/packet/af_packet.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> index 823e0911c91..406397acc5c 100644
> --- a/net/packet/af_packet.c
> +++ b/net/packet/af_packet.c
> @@ -1778,7 +1778,8 @@ static int fanout_add(struct sock *sk, struct
> fanout_args *args)
>   /* legacy PACKET_FANOUT_MAX */
>   args->max_num_members = 256;
>   err = -ENOMEM;
> - match = kvzalloc_flex(*match, arr, args->max_num_members);
> + match = kvzalloc_flex(*match, arr, args->max_num_members,
> +      GFP_KERNEL_ACCOUNT);
>   if (!match)
>   goto out;
>   write_pnet(&match->net, sock_net(sk));
> 
> base-commit: c339f4010dea8b0d754f84ace70cd1b089878ef8
> -- 
> 2.43.0