Re: losetuping files in tmpfs fails?

[Andreas Dilger]

On Jan 03, 2002  15:10 -0800, Andrew Morton wrote:
> Andy Gaynor wrote:
> > Whilst trying to figure out why my dang stripes won't persist (a separate
> > but worrisome issue), I wrote a dittie which creates a couple junk files in
> > /tmp (tmpfs), associates loop devices with them, whoops, losetup craps out.
> > 
> > ...
> >   /tmp# mount | grep tmp                # Filesystem is ...
> >   tmpfs on /tmp type tmpfs (rw)         #   tmpfs
> >   /tmp# echo foo > foo                  # Create file foo
> >   /tmp# losetup /dev/loop/5 foo         # Give foo to /dev/loop/5
> >   ioctl: LOOP_SET_FD: Invalid argument  # DISCO!!!          <o >  <o >
> 
> Yup, tmpfs doesn't provide some of the facilities which the
> loop driver requires.   Specifically, prepare_write() and 
> commit_write().  
> 
> Probably it's not too hard to change loop to use generic_file_write(),
> and it will then permit tmpfs file-backed loop mounts.
> 
> It's not obvious that there's a burning need to support loop-on-tmpfs
> though, is there?

Well, if you are using tmpfs as your /tmp filesystem (presumably not
such a strange situation given the name ;-), then any tool which creates
a loopback in /tmp will break.  Good examples would be mkinitrd, or
anything that is generating a floppy image (e.g. Linux Router Project
floppy, boot floppy tools, etc).

Cheers, Andreas
--
Andreas Dilger
http://sourceforge.net/projects/ext2resize/
http://www-mddsp.enel.ucalgary.ca/People/adilger/