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

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Date:   Fri, 21 Jun 2019 16:54:34 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Eric Dumazet <edumazet@...gle.com>
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

Eric is talking about this patch, I think:

   https://patchwork.ozlabs.org/patch/1120222/

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