Hi Wouter, On Sun, May 15, 2016 at 02:55:39PM +0200, Wouter Verhelst wrote: > Hi Markus, > > On Thu, May 12, 2016 at 11:53:01AM +0200, Markus Pargmann wrote: > > On Thursday 28 April 2016 18:27:34 Wouter Verhelst wrote: > > > However, at some point I agreed with Paul (your predecessor) that when > > > this happens due to an error condition (as opposed to it being due to an > > > explicit disconnect), the kernel would block all reads from or writes to > > > the device, and the client may try to reconnect *from the same > > > PID* (i.e., it may not fork()). If that succeeds, the next NBD_DO_IT is > > > assumed to be connected to the same server; if instead the process > > > exits, then the block device is assumed to be dead, will be reset, and > > > all pending reads or writes would error. > > > > > > In principle, this allows for a proper reconnect from userspace if it > > > can be done. However, I'm not sure whether this ever worked well or > > > whether it was documented, so it's probably fine if you think it should > > > be replaced with something else. > > > > At least I was not aware of this possibility. As far as I know the > > previous code even had issues with the signals used to kill on timeouts > > which also killed the userspace program sometimes. > > > > Currently I can't see a code path that supports reconnects. But I may > > have removed that accidently in the past. > > Right. Like I said, I'm not sure if it ever worked well. The user space > client has a -persist option that tries to implement it, but I've been > getting some bug reports from people who've tried it (although that may > have been my fault rather than the kernel's). > > > > (obviously, userspace reconnecting the device to a different device is > > > wrong and should not be done, but that's a case of "if you break it, you > > > get to keep both pieces) > > > > > > At any rate, I think it makes sense for userspace to be given a chance > > > to *attempt* to reconnect a device when the connection drops > > > unexpectedly. > > > > Perhaps it would be better to setup the kernel driver explicitly for > > that. Perhaps some flag to let the kernel driver know that the client > > would like to keep the block device open? In that case the client could > > excplicitly use NBD_CLEAR_SOCK to cleanup everything. > > I'm not sure what you mean by this. Can you clarify?
I meant that it might be better to have a separate way for NBD_DO_IT.
Something where the client software can directly instruct the kernel to
keep everything opened in case of an error so that the client may
reconnect afterwards.
This could be a new ioctl that sets it up, for example 'NBD_PERSISTENT'.
The NBD_DO_IT afterwards would keep everything up and running in case of
a connection error so that the client could set a new socket using
NBD_SET_SOCK and reenter using NBD_DO_IT.
For all clients that are not capable of this mechanism or don't use it,
NBD_DO_IT would clean up everything properly on any error.
>
> > Or perhaps a completely new ioctl that can transmit back some more
> > information about what failures were seen and whether the blockdevice
> > was closed or not?
>
> The intent was that ioctl(NBD_DO_IT) would return an error when the
> disconnect was not requested, and would return 0 when the connection
> dropped due to userspace doing ioctl(NBD_DISCONNECT), since dropping the
> connection when userspace explicitly asks for it is not an error.
>
> drivers/block/nbd.c contains the following:
>
> static int __nbd_ioctl(struct block_device *bdev, struct nbd_device *nbd,
> unsigned int cmd, unsigned long arg)
> {
> [...]
> case NBD_DO_IT: {
> [...]
> if (nbd->disconnect) /* user requested, ignore socket errors
> */
> return 0;
> return error;
> }
> [...]
>
> so the signalling part of it is at least still there. Whether it works,
> I haven't tested.
I just looked up the kernel code from 4.0. This code was there as
well. But the socket and blockdevice were both destroyed before leaving
the NBD_DO_IT ioctl. So it seems to have never been really persistent.
Filesystems would have still been killed.
So for a persistent nbd device there is some more code necessary to do
it.
Best Regards,
Markus
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