[Crash-utility] dev command deteriorates with new kernels

[Dave Anderson]

----- "Dave Anderson" <anderson at redhat.com> wrote:

> ----- "Bob Montgomery" <bob.montgomery at hp.com> wrote:
> 
> > On Fri, 2009-06-05 at 14:53 +0000, Dave Anderson wrote:
> >
> > >
> > > I've attached what I'm going with.  I've added the capability of getting
> > > the file_operations from the cdev_map when necessary.  The block device
> > > code was also suffering from bit-rot as well, and so I put in a new
> > > collector function that uses the bdev_map as well.
> >
> > Dave, this looks good.  Two issues:
> >
> > 1) Add "-f" to dev help?  (What does it mean to still be a "(none)" device?)
> 
> It means that a pointer to a file_operations either doesn't exist
> (or that I have no clue how to find it...)  For the hell of it I
> added that -f flag to show those devices in case somebody's
> interested.
> 
> >
> > 2) The old code found the block extended device number (a feature added
> > to the kernel by a 25 Aug 2008 patch from Tejun Heo):
> >
> >     259      blkext        (unknown)
> >
> > Also shown in /proc/devices:
> > ...
> >  Block devices:
> >   1 ramdisk
> > 259 blkext
> >   7 loop
> >  11 sr
> > 104 cciss0
> >
> > Deliberate omission?
> 
> I did see that, and I forget now how the old code found it (although the
> function still exists), but the structures being used now are bdev_map.probes[]
> and major_names[]:
> 
>   crash> whatis struct kobj_map
>   struct kobj_map {
>       struct probe *probes[255];
>       struct mutex *lock;
>   }
>   SIZE: 2048
>   crash> whatis major_names
>   struct blk_major_name *major_names[255];
>   crash>
> 
> where the kernel's kobj_map.probes[] array size is just hardwired to 255,
> and the major_names[] array size is BLKDEV_MAJOR_HASH_SIZE which is 255.
> So obviously 259 won't be found.

Correction -- it does appear in the major_names[] array, in a 2.6.30
kernel for example, like this:

  crash> p * major_names[4]
  $51 = {
    next = 0x0, 
    major = 259, 
    name = "blkext\000\000\000\000\000\000\000\000\000"
  }

where it appears to be the only major_names[] entry whose "major" value
doesn't equal the index into the array (i.e., 259 != 4).  But the
bdev_map.probes[4] entry is unused.

Dave

> 
> If you want to figure out how to show it, send me a patch.
> 
> At this point I'm about ready to deprecate the whole command...  ;-)
> 
> Dave
> 
> 
> 
> >
> > Thanks for cleaning this up,
> > Bob Montgomery