Re: Oops when starting md multipath on a 2.4 kernel

[Mike Tran <mhtran@us.ibm.com>]

James Pearson wrote:

> 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
>
Hi James,

My co-worker and I just happened to run into this problem a few days 
ago. So, I would like to share with you what we know.

The device major/minor numbers no longer match up values recorded in the 
descriptor array in the md superblock. Because of the exception made in 
the current code, the descriptor entries are removed and although the 
real devices are present and accounted for, they are kicked out from the 
array. This leaves the array with zero devices. When multipath_run() is 
invoked, it blows up expecting to have had some disks.

Lars Marowsky-Brée suggested some patches for md multipath in 2002 but 
never made it to mainline 2.4 kernel:
http://marc.theaimsgroup.com/?l=linux-kernel&m=103355467608953&w=2

That patch is large and most of it is not requried for this particular 
problem.  The section that reinitializes the descriptor array from 
current rdevs for the case of multipath will resolve this issue of 
device names shift.

Lars, Is it ok with you if I compose a patch from your original patch 
and post it here?

--
Regards,
Mike T.


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