Peter Xu [off-list ref] writes:

On Thu, Nov 23, 2023 at 06:22:33PM +0000, Christophe Leroy wrote:

quoted

quoted

For fast-gup I think the hugepd code is in use, however for walk_page_*
apis hugepd code shouldn't be reached iiuc as we have the hugetlb specific
handling (walk_hugetlb_range()), so anything within walk_pgd_range() to hit
a hugepd can be dead code to me (but note that this "dead code" is good
stuff to me, if one would like to merge hugetlb instead into generic mm).

Not sure what you mean here. What do you mean by "dead code" ?
A hugepage directory can be plugged at any page level, from PGD to PMD.
So the following bit in walk_pgd_range() is valid and not dead:

		if (is_hugepd(__hugepd(pgd_val(*pgd))))
			err = walk_hugepd_range((hugepd_t *)pgd, addr, next, walk, PGDIR_SHIFT);

IMHO it boils down to the question on whether hugepd is only used in
hugetlbfs.  I think I already mentioned that above, but I can be more
explicit; what I see is that from higher stack in __walk_page_range():

	if (is_vm_hugetlb_page(vma)) {
		if (ops->hugetlb_entry)
			err = walk_hugetlb_range(start, end, walk);
	} else
		err = walk_pgd_range(start, end, walk);

It means to me as long as the vma is hugetlb, it'll not trigger any code in
walk_pgd_range(), but only walk_hugetlb_range().  Do you perhaps mean
hugepd is used outside hugetlbfs?

walk_pgd_range also get called from walk_page_range_novma().
IIRC commit e17eae2b839937817d771e2f5d2b30e5e2b81bb7 added the hugepd
details to pagewalk code to handle ptdump.

There is also a desire to use hugepd format in vmap mappings. 
https://lore.kernel.org/linuxppc-dev/cover.1620795204.git.christophe.leroy@csgroup.eu (local)

-aneesh