Thanks, will try raising it and see how it go. 
________________________________________
From: Keith Busch [keith.busch@intel.com]
Sent: Wednesday, January 6, 2016 2:54 PM
To: Xuehua Chen
Cc: linux-nvme at lists.infradead.org
Subject: Re: Maximum NVMe IO command size > 1MB?

On Wed, Jan 06, 2016@09:56:24PM +0000, Xuehua Chen wrote:

Hi, Keith,

I wonder whether this could be caused by BIO_MAX_PAGES defined as 256, which means 1MB at most.
What do you think?

I think you got it. You're running O_DIRECT, and fs/direct-io.c,
dio_new_bio() allocates up to BIO_MAX_PAGES.

I can't tell where the value for came from (looks like it was there from
the very first git commit), but maybe you can propose raising it if you
set BIO_MAX_PAGES higher without issue.