Re: [Nbd] [Qemu-devel] Hibernate and qemu-nbd
>-----Original Message-----
>From: Stefan Hajnoczi [mailto:stefanha@...17...]
>Sent: Thursday, September 19, 2013 10:14 PM
>To: 'Mark Trumpold'
>Cc: 'qemu-devel', 'Paul Clements', nbd-general@lists.sourceforge.net,
>bonzini@...1390..., w@...112...
>Subject: Re: [Qemu-devel] Hibernate and qemu-nbd
>
>On Thu, Sep 19, 2013 at 10:44 PM, Mark Trumpold <markt@...1389...> wrote:
>>
>>>-----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, ¤t->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.
>>>
>>>Stefan
>>>
>>
>> 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):
>> [generic]
>> [export]
>> exportname = /root/qemu/q1.img
>> port = 2000
>
>You can use qemu-nbd instead of nbd-server. This way you'll be able
>to serve up qcow2 and other image formats.
>
>Just avoid the qemu-nbd -c option. This makes qemu-nbd purely run the
>NBD network protocol and skips simultaneously running the kernel NBD
>client. (Since qemu-nbd doesn't reconnect when ioctl(NBD_DO_IT) fails
>with EINTR the workaround is to use nbd-client(1) to drive the kernel
>NBD client instead.)
>
>> 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.
>
>It's expected that you get the same kernel messages. The difference
>should be that /dev/nbd0 is still accessible after resuming from disk
>because nbd-client automatically reconnects after the nbd kernel
>module bails out with EINTR.
>
>> ==================
>>
>> 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);
>> 201
>> 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, ¤t->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) {
>> dev_err(disk_to_dev(lo->disk),
>> "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.
>
>I'm not familiar enough with the code to say what is happening. As
>the next step I would print out the kernel_recvmsg() return value when
>the signal is pending and look into what happens during
>suspend-to-disk (there's some sort of process freezing that takes
>place).
>
>Sorry I can't be of more help. Hopefully someone more familiar with
>the nbd kernel module will have time to chime in.
>
>Stefan
>
Stefan,
So, I tried the following:
-> qemu-nbd -p 2000 /root/qemu/q1.img &
-> nbd-client localhost 2000 /dev/nbd0 &
At this point I can mount /dev/nbd0, etc.
-> echo platform > /sys/power/disk
-> echo disk >/sys/power/state
At this point we are 'hibernated'.
On power cycle, the OS seems to come back to the state
before hibernation with exception to QEMU:
nbd.c:nbd_receive_request():L517: read failed <-- on command line
[78979.269039] Freezing user space processes ...
[78979.269122] nbd (pid 2455: nbd-client) got signal 0
[78979.269127] block nbd0: shutting down socket
[78979.269151] block nbd0: Receive control failed (result -4)
[78979.269165] block nbd0: queue cleared
=============================
Is this the correct test you were thinking?
Thanks for your input!
Regards,
Mark T.
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