fs/proc/task_mmu: indicate PM_FILE for PMD-mapped file THP

[ Upstream commit 3f9f022e975d930709848a86a1c79775b0585202 ]

Patch series "fs/proc: move page_mapcount() to fs/proc/internal.h".

With all other page_mapcount() users in the tree gone, move
page_mapcount() to fs/proc/internal.h, rename it and extend the
documentation to prevent future (ab)use.

... of course, I find some issues while working on that code that I sort
first ;)

We'll now only end up calling page_mapcount() [now
folio_precise_page_mapcount()] on pages mapped via present page table
entries.  Except for /proc/kpagecount, that still does questionable
things, but we'll leave that legacy interface as is for now.

Did a quick sanity check.  Likely we would want some better selfestest for
/proc/$/pagemap + smaps.  I'll see if I can find some time to write some
more.

This patch (of 6):

Looks like we never taught pagemap_pmd_range() about the existence of
PMD-mapped file THPs.  Seems to date back to the times when we first added
support for non-anon THPs in the form of shmem THP.

Link: https://lkml.kernel.org/r/20240607122357.115423-1-david@redhat.com
Link: https://lkml.kernel.org/r/20240607122357.115423-2-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Fixes: 800d8c63b2e9 ("shmem: add huge pages support")
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: Lance Yang <ioworker0@gmail.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Cc: David Hildenbrand <david@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

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