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

[Christoph Hellwig]

On Mon, Aug 28, 2017 at 10:00:35AM +0300, Sagi Grimberg wrote:
>> This helper allows reinserting a bio into a new queue without much
>> overhead, but requires all queue limits to be the same for the upper
>> and lower queues, and it does not provide any recursion preventions,
>> which requires the caller to not split the bio.
>
> Isn't the same limits constraint too restrictive?
>
> Say I have two paths to the same namespace via two different HBAs, each
> with its own virt_boundary capability for example? That would require us
> to split failover bio wouldn't it?

Uh oh - different transports for the same subsystem will be intereting.
For one it's not specified anywhere so I'd like to kick off a discussion
on the working group mailing list about it.

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.

>> +/*
>> + * Fast-path version of generic_make_request.
>
> generic_make_request is also called in the fast-path, maybe reword it
> to: "Fast version of generic_make_request"

Yeah.  Maybe generic_make_request_direct or direct_make_request
is a better name as it describes the recursion avoidance bypassing
a little better.