linux-kernel - Re: Steam is broken on new kernels

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Date:   Fri, 21 Jun 2019 20:19:04 -0400
From:   Eric Dumazet <edumazet@...gle.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Pierre-Loup A. Griffais" <pgriffais@...vesoftware.com>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: Steam is broken on new kernels

On Fri, Jun 21, 2019 at 7:54 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> Eric is talking about this patch, I think:
>
>    https://patchwork.ozlabs.org/patch/1120222/
>

That  is correct.

I am about to take a flight from Boston to Paris, so I can not really
follow discussions/tests for the following hours.

Thanks.

> I guess I'll ask people on the github thread to test that too.
>
>                   Linus
>
> On Fri, Jun 21, 2019 at 3:38 PM Eric Dumazet <edumazet@...gle.com> wrote:
> >
> > Please look at my recent patch.
> >  Sorry I am travelling....
> >
> > On Fri, Jun 21, 2019, 6:19 PM Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> >>
> >> On Fri, Jun 21, 2019 at 2:41 PM Greg Kroah-Hartman
> >> <gregkh@...uxfoundation.org> 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"?
> >>
> >>                Linus