On Wed, Nov 15, 2023 at 08:29:02PM -0500, Peter Xu wrote:

Hugepd format is only used in PowerPC with hugetlbfs.  In commit
a6e79df92e4a ("mm/gup: disallow FOLL_LONGTERM GUP-fast writing to
file-backed mappings"), we added a check to fail gup-fast if there's
potential risk of violating GUP over writeback file systems.  That should
never apply to hugepd.

Drop that check, not only because it'll never be true for hugepd, but also
it paves way for reusing the function outside fast-gup.

What prevents us from ever using hugepd with file mappings?  I think
it would naturally fit in with how large folios for the pagecache work.

So keeping this check and generalizing it seems like the better idea to
me.