Oops when starting md multipath on a 2.4 kernel

[James Pearson <james-p@moving-picture.com>]

We have an existing system runing a 2.4.27 based kernel that uses md 
multipath and external fibre channel arrays.

We need to add more internal disks to the system, which means the 
external drives change device names.

When I tried to start the md multipath device using mdadm, the kernel 
Oops'd. Removing the new internal disks and going back the original 
setup, I can start the multipath device - as this machine is in 
production, I can't do any more tests.

However, I can reproduce the problem on test system by creating an md 
multipath device on an external SCSI disk, using /dev/sda1, stopping the 
multipath device, rmmod'ing the SCSI driver, pluging in a couple of USB 
storage devices which become /dev/sda and /dev/sdb and then modprobing 
the SCSI driver, so the original /dev/sda1 is now /dev/sdc1.

When I run 'mdadm -A -s', I get the following Oops:

  [events: 00000004]
md: bind<sdc1,1>
md: sdc1's event counter: 00000004
md0: former device sda1 is unavailable, removing from array!
md: unbind<sdc1,0>
md: export_rdev(sdc1)
md: RAID level -4 does not need chunksize! Continuing anyway.
md: multipath personality registered as nr 7
md0: max total readahead window set to 124k
md0: 1 data-disks, max readahead per data-disk: 124k
Unable to handle kernel NULL pointer dereference at virtual address 00000040
  printing eip:
e096527e
*pde = 00000000
Oops: 0000
CPU:    0
EIP:    0010:[<e096527e>]    Not tainted
EFLAGS: 00010246
eax: deb62a94   ebx: 00000000   ecx: dd65b400   edx: 00000000
esi: 0000001c   edi: deb62a94   ebp: 00000000   esp: dd5fbdbc
ds: 0018   es: 0018   ss: 0018
Process mdadm (pid: 1389, stackpage=dd5fb000)
Stack: dd4c4000 dfa96000 c035ad00 00000000 00000286 dd4c4000 00000000 
00000000
        deb62a94 dd5fbe5c dd4c6000 c02a6e10 dd65b400 c035ef1f 0000007c 
00000000
        0000000a ffffffff 00000002 00002e2e c0118b49 00002e2e 00002e2e 
00000286
Call Trace:    [<c02a6e10>] [<c0118b49>] [<c0118cc4>] [<c024a88c>] 
[<c024abb6>]
   [<c0118cc4>] [<c024907e>] [<c024b6f2>] [<c024c60c>] [<c014a326>] 
[<c013c483>]
   [<c013ca18>] [<c01375ac>] [<c013ca63>] [<c01439b6>] [<c01087c7>]

Code: 8b 45 40 85 c0 0f 84 c2 01 00 00 6a 00 ff b4 24 cc 00 00 00

Running through ksymoops gives:

Unable to handle kernel NULL pointer dereference at virtual address 00000040
e096527e
*pde = 00000000
Oops: 0000
CPU:    0
EIP:    0010:[<e096527e>]    Not tainted
Using defaults from ksymoops -t elf32-i386 -a i386
EFLAGS: 00010246
eax: deb62a94   ebx: 00000000   ecx: dd65b400   edx: 00000000
esi: 0000001c   edi: deb62a94   ebp: 00000000   esp: dd5fbdbc
ds: 0018   es: 0018   ss: 0018
Process mdadm (pid: 1389, stackpage=dd5fb000)
Stack: dd4c4000 dfa96000 c035ad00 00000000 00000286 dd4c4000 00000000 
00000000
        deb62a94 dd5fbe5c dd4c6000 c02a6e10 dd65b400 c035ef1f 0000007c 
00000000
        0000000a ffffffff 00000002 00002e2e c0118b49 00002e2e 00002e2e 
00000286
Call Trace:    [<c02a6e10>] [<c0118b49>] [<c0118cc4>] [<c024a88c>] 
[<c024abb6>]
   [<c0118cc4>] [<c024907e>] [<c024b6f2>] [<c024c60c>] [<c014a326>] 
[<c013c483>]
   [<c013ca18>] [<c01375ac>] [<c013ca63>] [<c01439b6>] [<c01087c7>]
Code: 8b 45 40 85 c0 0f 84 c2 01 00 00 6a 00 ff b4 24 cc 00 00 00

 >>EIP; e096527e <[multipath]multipath_run+2be/6c0>   <=====
Trace; c02a6e10 <vsnprintf+2e0/450>
Trace; c0118b49 <call_console_drivers+e9/f0>
Trace; c0118cc4 <printk+104/110>
Trace; c024a88c <device_size_calculation+19c/1f0>
Trace; c024abb6 <do_md_run+2d6/360>
Trace; c0118cc4 <printk+104/110>
Trace; c024907e <bind_rdev_to_array+9e/b0>
Trace; c024b6f2 <add_new_disk+132/290>
Trace; c024c60c <md_ioctl+6fc/790>
Trace; c014a326 <iput+236/240>
Trace; c013c483 <bdput+93/a0>
Trace; c013ca18 <blkdev_put+98/a0>
Trace; c01375ac <fput+bc/e0>
Trace; c013ca63 <blkdev_ioctl+23/30>
Trace; c01439b6 <sys_ioctl+216/230>
Trace; c01087c7 <system_call+33/38>
Code;  e096527e <[multipath]multipath_run+2be/6c0>
00000000 <_EIP>:
Code;  e096527e <[multipath]multipath_run+2be/6c0>   <=====
    0:   8b 45 40                  mov    0x40(%ebp),%eax   <=====
Code;  e0965281 <[multipath]multipath_run+2c1/6c0>
    3:   85 c0                     test   %eax,%eax
Code;  e0965283 <[multipath]multipath_run+2c3/6c0>
    5:   0f 84 c2 01 00 00         je     1cd <_EIP+0x1cd> e096544b 
<[multipath]m
ultipath_run+48b/6c0>
Code;  e0965289 <[multipath]multipath_run+2c9/6c0>
    b:   6a 00                     push   $0x0
Code;  e096528b <[multipath]multipath_run+2cb/6c0>
    d:   ff b4 24 cc 00 00 00      pushl  0xcc(%esp,1)

My /etc/mdadm.conf contains:

DEVICE /dev/sd?1
ARRAY /dev/md0 level=multipath num-devices=1
   UUID=277e4ba5:6c23c087:e17c877c:da642955


Should md multipath be able to handle changes like this with the 
underlying devices?


Thanks

James Pearson