Re: [PATCH net-next 2/2] sock: Fix improper heuristic on raising memory

[Shakeel Butt]

On Tue, Oct 03, 2023 at 08:49:08PM +0800, Abel Wu wrote:
[...]
> > 1. minimum buffer size even under pressure.
> 
> This is required by RFC 7323 (TCP Extensions for High Performance) to
> make features like Window Scale option work as expected, and should be
> succeeded under global pressure by tcp_{r,w}mem's definition. And IMHO
> for same reason, it should also be succeeded under memcg pressure, or
> else workloads might suffer performance drop due to bottleneck on
> network.
> 
> The allocation must not be succeeded either exceed global or memcg's
> hard limit, or else a DoS attack can be taken place by spawning lots
> of sockets that are under minimum buffer size.
> 

Sounds good.

> > 
> > 2. allow allocation for a socket whose usage is below average of the
> > system.
> 
> Since 'average' is within the scope of global accounting, this one
> only makes sense under global memory pressure. Actually this exists
> before cgroup was born, hence doesn't take memcg into consideration.
> 
> While OTOH the intention of throttling under memcg pressure is to
> relief the memcg from heavy reclaim pressure, this heuristic does no
> help. And there also seems to be no reason to succeed the allocation
> when global or memcg's hard limit is exceeded.
> 

Sounds good too.

> > 
> > 3. socket is over its sndbuf.
> 
> TBH I don't get its point..
> 

So, this corresponds to following code in __sk_mem_raise_allocated()

	if (kind == SK_MEM_SEND && sk->sk_type == SOCK_STREAM) {
		sk_stream_moderate_sndbuf(sk);

		/* Fail only if socket is _under_ its sndbuf.
		 * In this case we cannot block, so that we have to fail.
		 */
		if (sk->sk_wmem_queued + size >= sk->sk_sndbuf) {
			/* Force charge with __GFP_NOFAIL */
			if (memcg_charge && !charged) {
				mem_cgroup_charge_skmem(sk->sk_memcg, amt,
					gfp_memcg_charge() | __GFP_NOFAIL);
			}
			return 1;
		}
	}

Here we moderate the sk_sndbuf possibly half of sk_wmem_queued and thus
we always succeed unless user has done SO_SNDBUF on the socket in which
case it interacts with sk_stream_wait_memory() called in sendmsg.

I am not really able to make sense of the interaction between this code
and sk_stream_wait_memory() and will punt to networking experts to shed
some light.

Other than that I think we need to answer if we want to moderate the
sndbuf on memcg charge failure.

> > 
> > Let's discuss which heuristic applies to which accounting infra and
> > under which state (under pressure or over limit).
> 
> I will follow your suggestion to post a patch to explicitly document
> the behaviors once things are cleared.
> 

Let's just post the patch and see what other folks comment as well.