[PATCH v1 2/2] mm/hugetlb: fix uffd-wp handling for migration entries in hugetlb_change_protection()

[David Hildenbrand]

We have to update the uffd-wp SWP PTE bit independent of the type of
migration entry. Currently, if we're unlucky and we want to install/clear
the uffd-wp bit just while we're migrating a read-only mapped hugetlb page,
we would miss to set/clear the uffd-wp bit.

Further, if we're processing a readable-exclusive
migration entry and neither want to set or clear the uffd-wp bit, we
could currently end up losing the uffd-wp bit. Note that the same would
hold for writable migrating entries, however, having a writable
migration entry with the uffd-wp bit set would already mean that
something went wrong.

Note that the change from !is_readable_migration_entry ->
writable_migration_entry is harmless and actually cleaner, as raised by
Miaohe Lin and discussed in [1].

[1] https://lkml.kernel.org/r/90dd6a93-4500-e0de-2bf0-bf522c311b0c@xxxxxxxxxx

Fixes: 60dfaad65aa9 ("mm/hugetlb: allow uffd wr-protect none ptes")
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
 mm/hugetlb.c | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 3a94f519304f..9552a6d1a281 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -6516,10 +6516,9 @@ unsigned long hugetlb_change_protection(struct vm_area_struct *vma,
 		} else if (unlikely(is_hugetlb_entry_migration(pte))) {
 			swp_entry_t entry = pte_to_swp_entry(pte);
 			struct page *page = pfn_swap_entry_to_page(entry);
+			pte_t newpte = pte;
 
-			if (!is_readable_migration_entry(entry)) {
-				pte_t newpte;
-
+			if (is_writable_migration_entry(entry)) {
 				if (PageAnon(page))
 					entry = make_readable_exclusive_migration_entry(
 								swp_offset(entry));
@@ -6527,13 +6526,15 @@ unsigned long hugetlb_change_protection(struct vm_area_struct *vma,
 					entry = make_readable_migration_entry(
 								swp_offset(entry));
 				newpte = swp_entry_to_pte(entry);
-				if (uffd_wp)
-					newpte = pte_swp_mkuffd_wp(newpte);
-				else if (uffd_wp_resolve)
-					newpte = pte_swp_clear_uffd_wp(newpte);
-				set_huge_pte_at(mm, address, ptep, newpte);
 				pages++;
 			}
+
+			if (uffd_wp)
+				newpte = pte_swp_mkuffd_wp(newpte);
+			else if (uffd_wp_resolve)
+				newpte = pte_swp_clear_uffd_wp(newpte);
+			if (!pte_same(pte, newpte))
+				set_huge_pte_at(mm, address, ptep, newpte);
 		} else if (unlikely(is_pte_marker(pte))) {
 			/* No other markers apply for now. */
 			WARN_ON_ONCE(!pte_marker_uffd_wp(pte));
-- 
2.38.1