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Date:   Mon, 15 Aug 2022 13:53:30 -0700
From:   Andres Freund <andres@anarazel.de>
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     Xuan Zhuo <xuanzhuo@linux.alibaba.com>,
        Jason Wang <jasowang@redhat.com>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        virtualization@lists.linux-foundation.org, netdev@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Jens Axboe <axboe@kernel.dk>,
        James Bottomley <James.Bottomley@hansenpartnership.com>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        Guenter Roeck <linux@roeck-us.net>,
        linux-kernel@vger.kernel.org, Greg KH <gregkh@linuxfoundation.org>,
        c@redhat.com
Subject: Re: upstream kernel crashes
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Hi,

On 2022-08-15 16:21:51 -0400, Michael S. Tsirkin wrote:
> On Mon, Aug 15, 2022 at 10:46:17AM -0700, Andres Freund wrote:
> > Hi,
> >
> > On 2022-08-15 12:50:52 -0400, Michael S. Tsirkin wrote:
> > > On Mon, Aug 15, 2022 at 09:45:03AM -0700, Andres Freund wrote:
> > > > Hi,
> > > >
> > > > On 2022-08-15 11:40:59 -0400, Michael S. Tsirkin wrote:
> > > > > OK so this gives us a quick revert as a solution for now.
> > > > > Next, I would appreciate it if you just try this simple hack.
> > > > > If it crashes we either have a long standing problem in virtio
> > > > > code or more likely a gcp bug where it can't handle smaller
> > > > > rings than what device requestes.
> > > > > Thanks!
> > > >
> > > > I applied the below and the problem persists.
> > > >
> > > > [...]
> > >
> > > Okay!
> >
> > Just checking - I applied and tested this atop 6.0-rc1, correct? Or did you
> > want me to test it with the 762faee5a267 reverted? I guess what you're trying
> > to test if a smaller queue than what's requested you'd want to do so without
> > the problematic patch applied...
> >
> >
> > Either way, I did this, and there are no issues that I could observe. No
> > oopses, no broken networking. But:
> >
> > To make sure it does something I added a debugging printk - which doesn't show
> > up. I assume this is at a point at least earlyprintk should work (which I see
> > getting enabled via serial)?
> >

> Sorry if I was unclear.  I wanted to know whether the change somehow
> exposes a driver bug or a GCP bug. So what I wanted to do is to test
> this patch on top of *5.19*, not on top of the revert.

Right, the 5.19 part was clear, just the earlier test:

> > > > On 2022-08-15 11:40:59 -0400, Michael S. Tsirkin wrote:
> > > > > OK so this gives us a quick revert as a solution for now.
> > > > > Next, I would appreciate it if you just try this simple hack.
> > > > > If it crashes we either have a long standing problem in virtio
> > > > > code or more likely a gcp bug where it can't handle smaller
> > > > > Thanks!

I wasn't sure about.

After I didn't see any effect on 5.19 + your patch, I grew a bit suspicious
and added the printks.


> Yes I think printk should work here.

The reason the debug patch didn't change anything, and that my debug printk
didn't show, is that gcp uses the legacy paths...

If there were a bug in the legacy path, it'd explain why the problem only
shows on gcp, and not in other situations.

I'll queue testing the legacy path with the equivalent change.

- Andres


Greetings,

Andres Freund