On Wed, May 28 2014 at  2:22am -0400,
Li, Zhen-Hua [off-list ref] wrote:

This patch is trying to fix a kernel crash bug.

When kernel boots on a HP large system, it crashes.
The reason is when blk_rq_init is called, the second parameter rq , which
is a member as q->flush_rq, is NULL. Kernel does not allocate space for it.

This fix adds an alloc for flush_rq member when request_queue is created in
struct mapped_device *alloc_dev(int minor);

Bug Details:
Error message:

[   62.931942] BUG: unable to handle kernel NULL pointer dereference at (null)
[   62.931949] IP: [<ffffffff812b3f30>] blk_rq_init+0x40/0x160^M
[   62.931949] PGD 0 ^M
[   62.931951] Oops: 0002 [#1] SMP

You didn't specify which kernel you're running.  But this was fixed for
v3.15-rc6 via linux.git commit 7982e90c3a5 ("block: fix q->flush_rq NULL
pointer crash on dm-mpath flush").  And then there was the follow-on fix
from linux.git commit 708f04d2ab ("block: free q->flush_rq in
blk_init_allocated_queue error paths")

So all this is to say: Nack to your patch, we've already fixed the issue
differently.