[dm-devel] [PATCH] blk-mq: enforce op-specific segment limits in blk_insert_cloned_request

[Christoph Hellwig]

On Thu, Feb 16, 2023 at 12:27:02PM -0700, Uday Shankar wrote:
>  * Description:
>  *    @rq may have been made based on weaker limitations of upper-level queues
>  *    in request stacking drivers, and it may violate the limitation of @q.
>  *    Since the block layer and the underlying device driver trust @rq
>  *    after it is inserted to @q, it should be checked against @q before
>  *    the insertion using this generic function.
>  *
>  *    Request stacking drivers like request-based dm may change the queue
>  *    limits when retrying requests on other queues. Those requests need
>  *    to be checked against the new queue limits again during dispatch.
>  */.
> 
> Is this concern no longer relevant?

The concern is still valid, but it does not refer to the debug check.
It refers to recalculating nr_phys_segments using
blk_recalc_rq_segments, and the fact that any driver using this
interface needs to stack its limits properly.