Re: [Nbd] [PATCH] NBD: replace kill_bdev() with __invalidate_device()

[Wouter Verhelst <w@uter.be>]

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?

> 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.

-- 
< ron> I mean, the main *practical* problem with C++, is there's like a dozen
       people in the world who think they really understand all of its rules,
       and pretty much all of them are just lying to themselves too.
 -- #debian-devel, OFTC, 2016-02-12