Alistair Popple wrote:

The procfs mmu files such as smaps and pagemap currently ignore devdax and
fsdax pages because these pages are considered special. A future change
will start treating these as normal pages, meaning they can be exposed via
smaps and pagemap.

The only difference is that devdax and fsdax pages can never be pinned for
DMA via FOLL_LONGTERM, so add an explicit check in pte_is_pinned() to
reflect that.

I don't understand this patch.

pin_user_pages() is also used for Direct-I/O page pinning, so the
comment about FOLL_LONGTERM is wrong, and I otherwise do not understand
what goes wrong if the only pte_is_pinned() user correctly detects the
pin state?