Re: [Nbd] [RFC 4/4] nbd: Add support for nbd as root device
- To: Markus Pargmann <mpa@...1897...>
- Cc: nbd-general@lists.sourceforge.net, Paul Clements <Paul.Clements@...124...>, linux-kernel@...25..., kernel@...1897...
- Subject: Re: [Nbd] [RFC 4/4] nbd: Add support for nbd as root device
- From: Wouter Verhelst <w@...112...>
- Date: Fri, 30 Jan 2015 18:30:14 +0100
- Message-id: <20150130173014.GA16754@...3...>
- In-reply-to: <20150130080400.GD16879@...1897...>
- References: <1421156665-27318-1-git-send-email-mpa@...1897...> <1421156665-27318-5-git-send-email-mpa@...1897...> <20150129234254.GD15853@...3...> <20150130080400.GD16879@...1897...>
On Fri, Jan 30, 2015 at 09:04:00AM +0100, Markus Pargmann wrote:
> Hi,
>
> On Fri, Jan 30, 2015 at 12:42:54AM +0100, Wouter Verhelst wrote:
> > Not that I'm opposed to this, but you do realize that doing nbd-client
> > from initramfs or similar is possible, right? Most initramfs
> > implementations these days support it.
>
> Yes, that was the first idea how to implement a complete netboot for an
> embedded ARM device. However, an initramfs is at least around 1MB in
> size which has to be loaded using tftp. As the essential nbd-client
> connection setup and negotiation is quite small I decided to go with
> nbd-root support.
>
> Also it is quite useful to have nbd-root support much like nfsroot
> directly built-in for debugging purposes. It has the big advantage of
> booting/testing read-only filesystem images for embedded systems without
> the need for an initramfs.
Fair enough, just thought I'd point it out.
When looking at your patch set, two things pop out which you should
probably look at:
- What will happen if someone boots with root-on-NBD in your scheme and
later does a pivot_root() followed by an NBD_DISCONNECT ioctl on the
device?
- When a connection is started by nbd-client, the kernel creates a "pid"
file in sysfs, which contains the PID of the client and which the
client (when called with -c, or in other cases) uses to verify whether
a device is connected. At first glance, your patch does not do this,
which could cause confusion.
I must admit I haven't checked your patch very well (my kernel fu isn't
that advanced) so I might have missed something, but I'd rather point
it out now than have to fix the pieces afterwards ;-)
--
It is easy to love a country that is famous for chocolate and beer
-- Barack Obama, speaking in Brussels, Belgium, 2014-03-26
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