Re: [mptcp-net] mptcp: pm: Defer freeing of MPTCP userspace path manager entries

[Geliang Tang <geliang@kernel.org>]

Hi Mat,

On Wed, 2025-04-09 at 19:59 -0700, Mat Martineau wrote:
> When path manager entries are deleted from the local address list,
> they
> are first unlinked from the address list using list_del_rcu(). The
> entries must not be freed until after the RCU grace period, but the
> existing code immediately frees the entry.
> 
> Use kfree_rcu_mightsleep() and adjust sk_omem_alloc in open code
> instead
> of using the sock_kfree_s() helper. This code path is only called in
> a
> netlink handler, so the "might sleep" function is preferable to
> adding
> a rarely-used rcu_head member to struct mptcp_pm_addr_entry.
> 
> Signed-off-by: Mat Martineau <martineau@kernel.org>
> ---
> 
> My first implementation did add a sock_kfree_rcu_s() function like
> Geliang suggested, but kfree_rcu() is a macro so that approach got
> complicated. sock_kfree_rcu_s_mightsleep() seemed cumbersome, so I

I thought about this again recently, and I think sock_krfree_s() is a
better name, and it matches sock_kfree_s() and sock_kzfree_s() better.
I just sent a patch named "sock: add sock_krfree_s helper" for this to
MPTCP ML, please review it for me.

Thanks,
-Geliang

> went
> ahead and open-coded it. This should be applied after Geliang's v2,
> which will delete similar code in a helper function.
> 
> ---
>  net/mptcp/pm_userspace.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/net/mptcp/pm_userspace.c b/net/mptcp/pm_userspace.c
> index 7fc19b844384..959af1e42a97 100644
> --- a/net/mptcp/pm_userspace.c
> +++ b/net/mptcp/pm_userspace.c
> @@ -337,7 +337,8 @@ int mptcp_pm_nl_remove_doit(struct sk_buff *skb,
> struct genl_info *info)
>  
>  	release_sock(sk);
>  
> -	sock_kfree_s(sk, match, sizeof(*match));
> +	kfree_rcu_mightsleep(match);
> +	atomic_sub(sizeof(*match), &sk->sk_omem_alloc);
>  
>  	err = 0;
>  out: