On Fri, Sep 04, 2015 at 01:13:02PM -0400, Tejun Heo wrote:
> > [    6.784689] BUG: unable to handle kernel NULL pointer dereference at 0000000000000bb8
> > [    6.787605] IP: [<ffffffff814107a0>] blk_throtl_drain+0x80/0x220
> 
> The only struct which is large enough for 0xbb8 offset is
> request_queue.  Hmm.... can you please try the brute force debug patch
> below and report the kernel log after the crash?

I'll test your patch very soon, after I've recompiled the kernel with it.

I just wanted to say that I was working on a better reproducer using a
newer kernel, and now I have got one.  It is this:

  guestfish -v -x <<EOF
    disk-create test1.img raw 500M
    add-drive test1.img format:raw cachemode:unsafe
    run
    part-disk /dev/sda mbr
    pvcreate /dev/sda1
    vgcreate VG /dev/sda1
    lvcreate LV1 VG 50
    lvcreate LV2 VG 50
    lvremove /dev/VG/LV1
  EOF

The output of this command (when it fails, which is not 100% reliably,
but is probably > 50%) is attached.

To explain what the command above does:

(1) It creates a 500 MB sparse file called 'test1.img'.

(2) It creates a small, short-lived VM, using qemu, adding 'test1.img'
to the qemu command line.  Setting 'cachemode:unsafe' seems to be
either important, or makes the bug reproduce much more often.  This
corresponds to the qemu option '-drive file=test1.img,cache=unsafe'
and should be below the level that the kernel sees, so should make no
difference.

(3) It boots the VM.

(4) It runs some parted and LVM commands -- see the attached log file
for precisely what commands are run.

(5) The final command (lvremove) fails, with the stacktrace seen in
the attachment.

Rich.

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