Re: [PATCH 2/4] nvme: use bio_integrity_map_user

[Christoph Hellwig <hch@lst.de>]

On Wed, Oct 18, 2023 at 08:18:41AM -0700, Keith Busch wrote:
> From: Keith Busch <kbusch@kernel.org>
> 
> Map user metadata buffers directly instead of maintaining a complicated
> copy buffer.
> 
> Now that the bio tracks the metadata through its bip, nvme doesn't need
> special metadata handling, callbacks, or additional fields in the pdu.
> This greatly simplifies passthrough handling and avoids a "might_fault"
> copy_to_user in the completion path. This also creates pdu space to
> track the original request separately from its bio, further simplifying
> polling without relying on special iouring fields.
> 
> The downside is that nvme requires the metadata buffer be physically
> contiguous, so user space will need to utilize huge pages if the buffer
> needs to span multiple pages. In practice, metadata payload sizes are a
> small fraction of the main payload, so this shouldn't be a problem.

We can't just remove the old path.  We might still need bounce
buffering to due misalignment and/or because it is notcontiguous.
Same as we have a direct map and a copy path for data.