[PATCH 08/10] block: provide a generic_make_request_fast helper

[hch@lst.de (Christoph Hellwig)]

On Mon, Aug 28, 2017@02:01:16PM +0300, Sagi Grimberg wrote:
>> That being said ->make_request basically doesn't care about actual
>> limits at all, it mostly care about support features (e.g. discard, fua,
>> etc).  So I think a lot of these limits could porbably be lifted,
>> but I'd need to add the check to generic_make_request_checks back.
>
> Different virt_boundary capabilities will trigger bio splits which
> can make make_request blocking (due to lack of tags).

All the bio splitting is done in blk_queue_split, and other things
related to limits are done even later in blk_mq_make_request when
building the request.  For normal make_request based stacking drivers
nothing of this matters, although a few drivers t call blk_queue_split
manually from their make_request method.