Re: [PATCH V2 2/2] blk-mq: make sure elevator callbacks aren't called for passthrough request

[Christoph Hellwig <hch@lst.de>]

On Tue, May 16, 2023 at 09:20:55AM +0800, Ming Lei wrote:
> > That sounds like a good idea. It changes more behavior than what Ming is
> > targeting here, but after looking through each use for RQF_ELV, I think
> > not having that set really is the right thing to do in all cases for
> > passthrough requests.
> 
> I did consider that approach. But:
> 
> - RQF_ELV actually means that the request & its tag is allocated from sched tags.
> 
> - if RQF_ELV is cleared for passthrough request, request may be
>   allocated from sched tags(normal IO) and driver tags(passthrough) at the same time.
>   This way may cause other problem, such as, breaking blk_mq_hctx_has_requests().
>   Meantime it becomes not likely to optimize tags resource utilization in future,
>   at least for single LUN/NS, no need to keep sched tags & driver tags
>   in memory at the same time.

Then make that obvious.  That is:

 - rename RQF_ELV to RQV_SCHED_TAGS
 - add the RQV_SCHED_TAGS check to your blk_mq_bypass_sched helper.
   I'd also invert the return value and rename it to someting like
   blk_rq_use_sched.