Re: blk-mq: enforce op-specific segment limits in blk_insert_cloned_request

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On Fri, Feb 17 2023 at 11:16P -0500,
Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:

> 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.

It is a negative check that you're calling a debug check.

Much easier to address a bug in a driver when seeing this message than
figuring it out after more elaborate hunting. Not seeing the downside
of preserving/fixing given it is a quick limit check. *shrug*

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