Re: [PATCH 0/2] block: Add done_split_bio() for chained bio post-processing

[Christoph Hellwig]

On Fri, Oct 30, 2026 at 02:59:43PM +0800, Li kunyu wrote:
> 
> patch 1:  Introduce a new abstract function 'done_split_bio' and call it
> from bio_remaining_done to resolve this issue.
> 
> patch 2: Implement the blkcg_iolatency_done_split_bio callback in
> blk-iolatency to correctly decrement the inflight count when the other
> bio headers, which have not called rq_qos_done_bio(bio), complete.

Why are you not simplify calling rq_qos_done_bio before
bio_remaining_done?  __bi_remaining is only incremented for clones of
various types, and the qos processing should probably happen before
calling back into the submitter anyway.

As far as I can tell all the code here is duplicating the existing
done code without much of an explanation of why we need another copy.

While we're at, the existing code in rq_qos_done_bio added in commit
aa1b46dcdc7b ("block: fix rq-qos breakage from skipping rq_qos_done_bio()")
is extremly wrong and dangerous.  Once a bio completes, bi_bdev
is invalid, the driver could have even used it for something that is not
a block device like md raid5 does, but it certainly does not have any
use for the qos/iocost code.  So it really needs to derive anything
it needs to I/O processing from the bio itself.