[PATCH] nvme-pci: add missing condition check for existence of mapped data

[Keith Busch]

On Tue, Jul 30, 2024 at 07:21:11PM +0200, Christoph Hellwig wrote:
> On Tue, Jul 30, 2024 at 11:09:04AM -0600, Keith Busch wrote:
> > > +	if (blk_rq_nr_phys_segments(req))
> > > +		nvme_unmap_data(dev, req);
> > 
> > This is already applied, but it is kind of strange. We get here only if
> > metadata mapping fails. Is there actually a command that has metadata
> > without data?
> 
> Well, passthrough can always set metadata to map without data even
> if there is no NVMe defined command that works that way, so we should
> handle the error.

That's what I initially thought too, but nvme passthrough maps metadata
only if there's also user data. It doesn't look like you can even build
a request to have metadata if it doesn't have user data: where does the
bio come from in that case?

And generic block stack maps it only for READ or WRITE commands, which
must have payloads too, so I didn't find a path to reach this condition.

Not a big deal, the patch is fine, but I was wondering if we need to
change something else to allow the conditions it proposes to fix.