Re: [PATCH v4 net-next 01/10] mptcp: Fix up subflow's memcg when CONFIG_SOCK_CGROUP_DATA=n.

[Kuniyuki Iwashima <kuniyu@google.com>]

On Thu, Aug 14, 2025 at 2:44 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
>
> On Thu, Aug 14, 2025 at 08:08:33PM +0000, Kuniyuki Iwashima wrote:
> > 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;