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Re: [Nbd] [Qemu-devel] Hibernate and qemu-nbd

>-----Original Message-----
>From: Stefan Hajnoczi [mailto:stefanha@...17...]
>Sent: Wednesday, September 18, 2013 06:12 AM
>To: 'Mark Trumpold'
>Cc: qemu-devel@...530..., 'Paul Clements', nbd-general@lists.sourceforge.net, 
>bonzini@...1390..., w@...112...
>Subject: Re: [Qemu-devel] Hibernate and qemu-nbd
>On Tue, Sep 17, 2013 at 07:10:44AM -0700, Mark Trumpold wrote:
>> I am using the kernel functionality directly with the commands:
>>     echo platform >/sys/power/disk
>>     echo disk >/sys/power/state
>> The following appears in dmesg when I attempt to hibernate:
>> ====================================================
>> [   38.881397] nbd (pid 1473: qemu-nbd) got signal 0
>> [   38.881401] block nbd0: shutting down socket
>> [   38.881404] block nbd0: Receive control failed (result -4)
>> [   38.881417] block nbd0: queue cleared
>> [   87.463133] block nbd0: Attempted send on closed socket
>> [   87.463137] end_request: I/O error, dev nbd0, sector 66824
>> ====================================================
>> My environment:
>>   Debian: 6.0.5
>>   Kernel: 3.3.1
>>   Qemu userspace: 1.2.0
>This could be a bug in the nbd client kernel module.
>drivers/block/nbd.c:sock_xmit() does the following:
>            result = kernel_recvmsg(sock, &msg, &iov, 1, size,
>                                    msg.msg_flags);
>    if (signal_pending(current)) {
>            siginfo_t info;
>            printk(KERN_WARNING "nbd (pid %d: %s) got signal %d\n",
>                    task_pid_nr(current), current->comm,
>                    dequeue_signal_lock(current, &current->blocked, &info));
>            result = -EINTR;
>            sock_shutdown(nbd, !send);
>            break;
>    }
>The signal number in the log output looks bogus, we shouldn't get 0.
>sock_xmit() actually blocks all signals except SIGKILL before calling
>kernel_recvmsg().  I guess this is an artifact of the suspend-to-disk
>operation, maybe the signal pending flag is set on the process.
>Perhaps someone with a better understanding of the kernel internals can
>check this?
>What happens next is that the nbd kernel module shuts down the NBD connection.
>As a workaround, please try running a separate nbd-client(1) process and drop
>the qemu-nbd -c command-line argument.  This way nbd-client(1) uses the
>nbd kernel module instead of the qemu-nbd process and you'll get the
>benefit of nbd-client's automatic reconnect.

Hi Stefan,

Thank you for the information.

I did some experiments per you suggestion.  Wasn't sure if the following
was what you had in mind:

1) Configured 'nbd-server' and started (/etc/nbd-server/config):
    exportname = /root/qemu/q1.img
    port = 2000

2) Started 'nbd-client':
   -> nbd-client localhost 2000 /dev/nbd0

3) Verify '/dev/nbd0' is in use (will appear in list):
   -> cat /proc/partitions

At this point I could mount '/dev/nbd0' as expected, but not necessary
to demonstrate a problem.

Now at this point if I enter S1(standby), S3(suspend to ram), or
S4(suspend to disk) I get the same dmesg as before indicating
'nbd0' caught signal 0 and exited.

When I resume I simply repeat step #3 to verify.


Also, previously before contacting the group I had modified the same
kernel source that you had identified in 'drivers/block/nbd.c:sock_xmit()'
to not take any action.  This was strictly for troubleshooting:

199            result = kernel_recvmsg(sock, &msg, &iov, 1, size,
200                                    msg.msg_flags);
202    if (signal_pending(current)) {
203            siginfo_t info;
204            printk(KERN_WARNING "nbd (pid %d: %s) got signal %d\n",
205                    task_pid_nr(current), current->comm,
206                    dequeue_signal_lock(current, &current->blocked,&info)); 207
208            //result = -EINTR;
209            //sock_shutdown(nbd, !send);
210            //break;
211    }

We then got errors ("Wrong magac ...) in the following section:

/* NULL returned = something went wrong, inform userspace */
static struct request *nbd_read_stat(struct nbd_device *lo)
        int result;
        struct nbd_reply reply;
        struct request *req;

        reply.magic = 0;
        result = sock_xmit(lo, 0, &reply, sizeof(reply), MSG_WAITALL);
        if (result <= 0) {
                        "Receive control failed (result %d)\n", result);
                goto harderror;

        if (ntohl(reply.magic) != NBD_REPLY_MAGIC) {
                dev_err(disk_to_dev(lo->disk), "Wrong magic (0x%lx)\n",
                                (unsigned long)ntohl(reply.magic));
                result = -EPROTO;
                goto harderror;

So, it seemed to me the call at line #199 above must be returning with
error after we commented out the signal action logic.

Thank you for your attention on this.
Let me know if I followed you suggestion correctly, and/or other tests
I can do.

Mark T.

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