linux-kernel - Re: [PATCH] sr: Ensure disk is revalidated when media changes

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Message-ID: <20110408165207.GD3871@mtj.dyndns.org>
Date:	Fri, 8 Apr 2011 09:52:07 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Stefan Hajnoczi <stefanha@...il.com>
Cc:	Amit Shah <amit.shah@...hat.com>, linux-kernel@...r.kernel.org,
	Jens Axboe <axboe@...nel.dk>,
	"James E.J. Bottomley" <James.Bottomley@...e.de>,
	linux-scsi@...r.kernel.org, Markus Armbruster <armbru@...hat.com>
Subject: Re: [PATCH] sr: Ensure disk is revalidated when media changes

Hello,

On Fri, Apr 08, 2011 at 05:43:16PM +0100, Stefan Hajnoczi wrote:
> >> I think it would make sense to refresh the inode size on media change
> >> so that even open file descriptors see the new size and a single
> >> process cannot force a stale value for all other userspace processes
> >> on the system.
> >
> > Hmmm... I don't know.  Maybe we can but I'm not sure whether there's a
> > good reason for it.  cdrom is locked while opened after all.  Are
> > there actual problems?
> 
> Yeah, sorry I didn't explain what the use case was.  With QEMU you can
> pass through the physical CD-ROM into the virtual machine.
> 
> QEMU opens /dev/cdrom with O_NONBLOCK | O_RDONLY.  The guest can test
> if the medium is present and QEMU will do ioctl(fd,
> CDROM_DRIVE_STATUS, CDSL_CURRENT).  The guest can also lock the tray
> and eject, again using the respective ioctls.  Read operations are
> serviced by performing a read on the file descriptor in QEMU.  And
> finally the medium size is queried by QEMU using lseek(fd, 0,
> SEEK_END).
> 
> Today QEMU cannot keep /dev/cdrom open across media change because it
> will have an outdated inode size returned from lseek(fd, 0, SEEK_END).
>  But if the cdrom driver (or sr) refresh the inode size on media
> change then there is no need to work around this from userspace.

Hmmm... ISTR there was some discussion about changing inode size on
the fly quite a while ago.  I didn't follow the discussion but it
seemed to have rather nasty/delicate implications.

Jens, any ideas?

Thanks.

-- 
tejun
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