Re: Removeable Media, partitions and devfs?

[Paul Bristow <paul@xxxxxxxxxxxxxxx>]

Richard Gooch wrote:

> Because I believe that most (maybe even all) of the problems are
> solvable with a few minor tweaks. So I'd prefer to start with a few
> tweaks and see if that fixes it.


I like this approach.  The patch also explains why I was going crazy 
trying to squeeze the solution into ide-floppy.  I didn't initially 
realise the problem was more generic than just my driver.  Richard, you 
may be pleased to know that *everyone* who emailed me assumed it was my 
fault in the driver, and that devfs was perfect :-)

> > > The solution, as I see it, is to force grok_partitions() to call
> > > devfs_register_disc() when size==0 and the GENHD_FL_REMOVABLE flag is
> > > set. This will cause the "disc" entry in the
> > > /dev/ide/host*/bus*/lun*/target* directory to be created, and it will
> > > tag that directory for media revalidation.
> > >
> > > If you read that directory, or attempt a lookup of some leaf node, the
> > > media will be revalidated. If there's new media in the drive, the
> > > normal partition handling code should be invoked, and thus the
> > > appropriate leaf nodes will be created. Take the media out, and the
> > > next readdir or lookup will remove partition entries (assuming the
> > > ide-floppy driver doesn't set the size back to 0). Magic. Just the way
> > > god^H^H^HI intended.
> > Nope, grok_partitions does not cause existing entries to be removed,
> > unless the "size" is left at its original value, in which case your
> > system log will be filled with "unable to read partition table"
> > errors and errors from the ide layer trying to read non-existent
> > media.
>
> So, let me get this right (I haven't looked closely at ide-floppy, so
> bear with me): when there is no media present, the driver sets the
> size to zero at revalidate time, even if previously the size was
> non-zero?


At the moment no.  But it could :-)

> So please try the appended patch again. It doesn't claim to solve the
> unregistering problem, but that's OK. I want to take this a step at a
> time. A bit more tweaking should (I hope) take care of the remaining
> problems.


Working patch below... slightly tweaked


> > - without media in the drive at driver load time (either kernel
> > startup or module load), no entries are created, no access is
> > possible to the drive
>
> My patch should fix that.


The patch does fix this, by creating the disc entries regardless.

Now, the problem we still have is how to automatically revalidate when a 
user tries to mount the a partition.  This is the most common usage case 
for zip drives, and is not working on my system with the patch below.

i.e. attempting to do this
   mount /dev/ide/host0/bus1/target1/lun0/part4 /mnt/zip
with no disc inserted will fail, even with the 
/dev/ide/host0/bus1/target1/lun0/disc entry present

however, having a re-validation caused, which in ide-floppy does a 
grok_partitions again should create the part4 entry so that the mount 
succeeds

The following items should be future work that we will fix during 2.5. 
Kevin your expertise will be greatly appreciated.

> > - when media is ejected, invalid partition entries remain in the
> > directory
>
> Future work.
>
>
> > - when media is repartitioned, invalid partition entries remain in
> > the directory
>
> Future work, same solution as above.
>
>
> > - when media is reformatted, invalid partition entries remain in the
> > directory
>
> Future work, same solution as above.
>
>
> > - when ide-floppy (or ide-hd) modules are unloaded, the
> > disc/partition entries (and the directory) are not removed
>
> Future work.
>
>                                 Regards,
>
>                                         Richard....
> Permanent: rgooch@xxxxxxxxxxxxx
> Current:   rgooch@xxxxxxxxxxxxxxx



--

Paul

Email: 
paul@xxxxxxxxxxxxxxx
Web: 
http://paulbristow.net
ICQ: 
11965223


--- ../../../linux-2.4.16-clean/fs/partitions/check.c   Fri Oct 12 
02:25:10 2001
+++ check.c     Thu Dec  6 21:59:14 2001
@@ -377,12 +379,19 @@
        int i;
        int first_minor = drive << dev->minor_shift;
        int end_minor   = first_minor + dev->max_p;
+       int devnum = first_minor >> dev->minor_shift;

        if(!dev->sizes)
                blk_size[dev->major] = NULL;

        dev->part[first_minor].nr_sects = size;
        /* No such device or no minors to use for partitions */
+       if ( !size && dev->flags && (dev->flags[devnum] & 
GENHD_FL_REMOVABLE) )
+               devfs_register_disc (dev, first_minor);
        if (!size || minors == 1)
                return;