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

[Amit Shah <amit.shah@redhat.com>]

On (Tue) 12 Apr 2011 [09:16:41], Stefan Hajnoczi wrote:
> On Tue, Apr 12, 2011 at 5:51 AM, Amit Shah <amit.shah@redhat.com> wrote:
> > On (Fri) 08 Apr 2011 [09:52:07], Tejun Heo wrote:
> >> 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 will have an outdated inode size returned from lseek(fd, 0, SEEK_END).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.
> >
> > I don't necessarily agree with having to modify inode sizes on the
> > fly, but the main bug here is that the inode doesn't get invalidated
> > if a CDROM is ejected while a process has an fd to the CDROM device
> > opened.  So as in the original case, if a CD is swapped with one
> > having more data, lseek continues to report the original media's size
> > in the process that keeps the fd open across eject/insert.
> >
> > I haven't tried this on physical systems, so can't count out it being
> > a qemu bug as well.
> 
> Amit,
> This sounds exactly like the bug that I described and it happens on
> bare metal without virtualization.

Oh OK -- just wanted to get the qemu passthrough out of the equation.

Thanks.

		Amit