[PATCH v4 17/23] block: add check when merging zone device pages

[Christoph Hellwig]

> +/*
> + * 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.
> + */
> +static inline bool zone_device_pages_are_mergeable(const struct page *a,
> +						   const struct page *b)

Merging is only really a use case here.  This really checks if they
belong to the same pgmap, so I suspect that should be in the name.