On Thu, Aug 14, 2025 at 2:44 PM Shakeel Butt [off-list ref] wrote:

On Thu, Aug 14, 2025 at 08:08:33PM +0000, Kuniyuki Iwashima wrote:

quoted

When sk_alloc() allocates a socket, mem_cgroup_sk_alloc() sets
sk->sk_memcg based on the current task.

MPTCP subflow socket creation is triggered from userspace or
an in-kernel worker.

In the latter case, sk->sk_memcg is not what we want.  So, we fix
it up from the parent socket's sk->sk_memcg in mptcp_attach_cgroup().

Although the code is placed under #ifdef CONFIG_MEMCG, it is buried
under #ifdef CONFIG_SOCK_CGROUP_DATA.

The two configs are orthogonal.  If CONFIG_MEMCG is enabled without
CONFIG_SOCK_CGROUP_DATA, the subflow's memory usage is not charged
correctly.

Let's wrap sock_create_kern() for subflow with set_active_memcg()
using the parent sk->sk_memcg.

Fixes: 3764b0c5651e3 ("mptcp: attach subflow socket to parent cgroup")
Suggested-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
 mm/memcontrol.c     |  5 ++++-
 net/mptcp/subflow.c | 11 +++--------
 2 files changed, 7 insertions(+), 9 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 8dd7fbed5a94..450862e7fd7a 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c

@@ -5006,8 +5006,11 @@ void mem_cgroup_sk_alloc(struct sock *sk)
      if (!in_task())
              return;

+     memcg = current->active_memcg;
+

Use active_memcg() instead of current->active_memcg and do before the
!in_task() check.

Why not reuse the !in_task() check here ?
We never use int_active_memcg for socket and also
know int_active_memcg is always NULL here.

Basically something like following:

        memcg = active_memcg();
        /* Do not associate the sock with unrelated interrupted task's memcg. */
        if (!in_task() && !memcg)
                return;