linux-kernel - Re: bug in 2.6.22-rc2: loop mount limited to one single iso image

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Message-Id: <200705212151.44667.arvidjaar@mail.ru>
Date:	Mon, 21 May 2007 21:51:28 +0400
From:	Andrey Borzenkov <arvidjaar@...l.ru>
To:	Kay Sievers <kay.sievers@...y.org>
Cc:	Uwe Bugla <uwe.bugla@....de>, Ken Chen <kenchen@...gle.com>,
	Ray Lee <ray-lk@...rabbit.org>,
	Al Viro <viro@....linux.org.uk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Piotrowski <michal.k.k.piotrowski@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: bug in 2.6.22-rc2: loop mount limited to one single iso image

On Monday 21 May 2007, Kay Sievers wrote:
> On Mon, 2007-05-21 at 18:50 +0200, Uwe Bugla wrote:
> > Am Montag, 21. Mai 2007 18:37 schrieben Sie:
> > > On 5/21/07, Ken Chen <kenchen@...gle.com> wrote:
> > > > yes and no. in that commit, I automatically create n+1 device when
> > > > loop device n is created, allergically was tested to be fine with
> > > > casual usage of "losetup" and "mount -o loop".  However, there is a
> > > > bug in that commit when loop.c was compiled as a module.  And when Al
> > > > fixed it, he also removed that magic "n+1" trick.
> > > >
> > > > Nevertheless, yes, I'm guilty of introducing the new behavior.
> > >
> > > The easiest way is to reinstate max_loop and create "max_loop" device
> > > up front at module load time.  However, that will lose all the "fancy
> > > on-demand device instantiation feature".
> > >
> > > So I propose we do the following:
> > >
> > > 1. have the module honor "max_loop" parameter and create that many
> > > device upfront on module load (max_loop will also be a hard max) iff
> > > user specify the parameter.
> > > 2. if max_loop is not specified, default create 8 loop device.  User
> > > can extent more loop device by create device node themselves and have
> > > kernel automatically instantiate loop device on-demand.
> >
> > Sorry, Ken:
> > My question on point 2 would be: Does "User can extent more loop device
> > by create device node themselves and......." correspond or conflict to
> > working with udev?
>
> Udev shouldn't care if the kernel tells udev about the new device, and
> the node with the correct dev_t is already there, it will leave it as it
> is, and only apply the configured user,group,mode values.
>
> The loop tools should probably extended to be able to request new
> devices from the kernel in a different way than open().
>

Except as already mentioned it will introduce races between tool requesting 
new device and udev creating new node. We already had this with raw devices. 
My comparison with /dev/pts was incorrect because it is using internal 
filesystem that creates devices synchronously.

May be all such cases has to be converted to use common file system.

mount -t nodefs -o device=pts none /dev/pts
mount -t nodefs -o device=loop none /dev/loop

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