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SubjectRe: Steam is broken on new kernels

On 6/21/19 3:38 PM, Eric Dumazet wrote:
> Please look at my recent patch.
>  Sorry I am travelling....
> On Fri, Jun 21, 2019, 6:19 PM Linus Torvalds
> < <>>
> wrote:
> On Fri, Jun 21, 2019 at 2:41 PM Greg Kroah-Hartman
> < <>> wrote:
> >
> > What specific commit caused the breakage?
> Both on reddit and on github there seems to be confusion about whether
> it's a problem or not. Some people have it working with the exact same
> kernel that breaks for others.
> And then some people seem to say it works intermittently for them,
> which seems to indicate a timing issue.
> Looking at the SACK patches (assuming it's one of them), I'd suspect
> the "tcp: tcp_fragment() should apply sane memory limits".
> Eric, that one does
>        if (unlikely((sk->sk_wmem_queued >> 1) > sk->sk_sndbuf)) {
>                NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPWQUEUETOOBIG);
>                return -ENOMEM;
>        }
> but I think it's *normal* for "sk_wmem_queued >> 1" to be around the
> same size as sk_sndbuf. So if there is some fragmentation, and we add
> more skb's to it, that would seem to trigger fairly easily.
> Particularly since this is all in "truesize" units, which can be a lot
> bigger than the packets themselves.
> I don't know the code, so I may be out to lunch and barking up
> completely the wrong tree, but that particular check does seem like it
> might trigger much more easily than I think the code _intended_ it to
> trigger?
> Pierre-Loup - do you guys have a test-case inside of valve? Or is this
> purely "we see some people with problems"?

Definitely the latter, although the volume of complaints clearly points
to a real problem from our experience. Reproducing locally, bisecting
and testing possible fixes is just now starting on our end.

I agree not all users seem affected; most affected people report success
by using -tcp to launch Steam, which makes it use direct TCP instead of
WebSockets, our current default connection method for Linux.

- Pierre-Loup

>                Linus

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