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Subject: Re: Steam is broken on new kernels
To:     Eric Dumazet <edumazet@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
CC:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        lkml <linux-kernel@vger.kernel.org>
References: <a624ec85-ea21-c72e-f997-06273d9b9f9e@valvesoftware.com>
 <20190621214139.GA31034@kroah.com>
 <CAHk-=wgXoBMWdBahuQR9e75ri6oeVBBjoVEnk0rN1QXfSKK2Eg@mail.gmail.com>
 <CANn89iL5+x3n9H9v4O6y39W=jvQs=uuXbzOvN5mBbcj0t+wdeg@mail.gmail.com>
From:   "Pierre-Loup A. Griffais" <pgriffais@valvesoftware.com>
Message-ID: <5a012e7c-cbed-c3a9-8d84-851de34630d8@valvesoftware.com>
Date:   Fri, 21 Jun 2019 16:04:57 -0700
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On 6/21/19 3:38 PM, Eric Dumazet wrote:
> Please look at my recent patch.
>  =C2=A0Sorry I am travelling....
>=20
> On Fri, Jun 21, 2019, 6:19 PM Linus Torvalds=20
> <torvalds@linux-foundation.org <mailto:torvalds@linux-foundation.org>>=20
> wrote:
>=20
>     On Fri, Jun 21, 2019 at 2:41 PM Greg Kroah-Hartman
>     <gregkh@linuxfoundation.org <mailto:gregkh@linuxfoundation.org>> wrot=
e:
>      >
>      > What specific commit caused the breakage?
>=20
>     Both on reddit and on github there seems to be confusion about whethe=
r
>     it's a problem or not. Some people have it working with the exact sam=
e
>     kernel that breaks for others.
>=20
>     And then some people seem to say it works intermittently for them,
>     which seems to indicate a timing issue.
>=20
>     Looking at the SACK patches (assuming it's one of them), I'd suspect
>     the "tcp: tcp_fragment() should apply sane memory limits".
>=20
>     Eric, that one does
>=20
>      =C2=A0 =C2=A0 =C2=A0 =C2=A0if (unlikely((sk->sk_wmem_queued >> 1) > =
sk->sk_sndbuf)) {
>      =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0NET_INC_STATS=
(sock_net(sk), LINUX_MIB_TCPWQUEUETOOBIG);
>      =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0return -ENOME=
M;
>      =C2=A0 =C2=A0 =C2=A0 =C2=A0}
>=20
>     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 lo=
t
>     bigger than the packets themselves.
>=20
>     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 i=
t
>     might trigger much more easily than I think the code _intended_ it to
>     trigger?
>=20
>     Pierre-Loup - do you guys have a test-case inside of valve? Or is thi=
s
>     purely "we see some people with problems"?

Definitely the latter, although the volume of complaints clearly points=20
to a real problem from our experience. Reproducing locally, bisecting=20
and testing possible fixes is just now starting on our end.

I agree not all users seem affected; most affected people report success=20
by using -tcp to launch Steam, which makes it use direct TCP instead of=20
WebSockets, our current default connection method for Linux.

Thanks,
  - Pierre-Loup

>=20
>      =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0Linus
>=20