Date:	Thu, 19 May 2016 08:35:03 +0200
From:	Markus Pargmann <mpa@...gutronix.de>
To:	Wouter Verhelst <w@...r.be>
Cc:	Ratna Manoj <manoj.br@...il.com>,
	nbd-general@...ts.sourceforge.net,
	Vinod Jayaraman <jv@...tworx.com>, jack@...e.cz,
	linux-kernel@...r.kernel.org, Gou Rao <grao@...tworx.com>,
	pbonzini@...hat.com
Subject: Re: [Nbd] [PATCH] NBD: replace kill_bdev() with __invalidate_device()

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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