[PATCH v7 16/21] block: add check when merging zone device pages

[Christoph Hellwig]

On Wed, Jun 15, 2022 at 10:12:28AM -0600, Logan Gunthorpe wrote:
> Consecutive zone device pages should not be merged into the same sgl
> or bvec segment with other types of pages or if they belong to different
> pgmaps. Otherwise getting the pgmap of a given segment is not possible
> without scanning the entire segment. This helper returns true either if
> both pages are not zone device pages or both pages are zone device
> pages with the same pgmap.
> 
> Add a helper to determine if zone device pages are mergeable and use
> this helper in page_is_mergeable().

Any reason not to simply set REQ_NOMERGE for these requests?  We
can't merge for passthrough requests anyway, and genrally don't merge
for direct I/O either, so adding all this overhead seems a bit pointless.