Re: running out of tags in 9P (was Re: [git pull] vfs part 2)

[Andrey Ryabinin]

On 07/02/2015 10:59 AM, Al Viro wrote:
> On Thu, Jul 02, 2015 at 10:50:05AM +0300, Andrey Ryabinin wrote:
>
>>>> and see if it triggers.  I'm not sure if failing with ENOMEM is the
>>>> right response (another variant is to sleep there until the pile
>>>> gets cleaned or until we get killed), and WARN_ON_ONCE() is definitely
>>>> not for the real work, but it will do for confirming that this is what
>>>> we are hitting.
>>>
>>
>> Apparently, I'm seeing something else. That WARN_ON_ONCE didn't trigger.
>
> Summary for those who'd missed the beginning of the thread: what we are
> seeing is p9_client_write() issing TWRITE and getting RWRITE in reply
> (tags match, packets look plausible) with count in RWRITE way more than
> that in TWRITE.
>
> IOW, we are telling the server to write e.g. 93 bytes and are getting told
> that yes, the write had been successful - all 4096 bytes of it.
>
> qemu virtio-9p for server; from my reading of qemu side of things, it can't
> be sending reply with count greater than that in request.

Besides qemu, I've also tried kvmtool with the same result. IOW I'm seeing
this under kvmtool as well. It just takes a bit longer to reproduce
this in kvmtool.

> The bug I suspected to be the cause of that is in tag allocation in
> net/9p/client.c - we could end up wrapping around 2^16 with enough pending
> requests and that would have triggered that kind of mess.  However, Andrey
> doesn't see that test (tag wraparound in p9_client_prepare_req()) trigger.
> BTW, was that on the run where debugging printk in p9_client_write() *did*
> trigger?

Yes, WARN_ON_ONCE() in p9_client_prepare_req() didn't trigger,
but debug printk in p9_client_write() *did* trigger.
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