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CONFIG_ARCH_WANT_OPTIMIZE_DAX_VMEMMAP would change whether or not we got
compound pages by default, just that if we did some of the (tail?) pages may
refer to the same physical struct page.

> Might as well fix up device-dax refcounts in this series too, but I
> won't ask you to do that, will send you something to include.

Eh. That should be relatively straight forward. But then I thought that about FS
DAX too :-)