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