[PATCH 1/3] mm/hugetlb: Pre-allocate pgtable pages for uffd wr-protects

[Peter Xu]

Userfaultfd-wp uses pte markers to mark wr-protected pages for both shmem
and hugetlb.  Shmem has pre-allocation ready for markers, but hugetlb path
was overlooked.

Doing so by calling huge_pte_alloc() if the initial pgtable walk fails to
find the huge ptep.  It's possible that huge_pte_alloc() can fail with high
memory pressure, in that case stop the loop immediately and fail silently.
This is not the most ideal solution but it matches with what we do with
shmem meanwhile it avoids the splat in dmesg.

Cc: linux-stable <stable@xxxxxxxxxxxxxxx> # 5.19+
Fixes: 60dfaad65aa9 ("mm/hugetlb: allow uffd wr-protect none ptes")
Reported-by: James Houghton <jthoughton@xxxxxxxxxx>
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
---
 mm/hugetlb.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index bf7a1f628357..017d9159cddf 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -6649,8 +6649,17 @@ unsigned long hugetlb_change_protection(struct vm_area_struct *vma,
 		spinlock_t *ptl;
 		ptep = hugetlb_walk(vma, address, psize);
 		if (!ptep) {
-			address |= last_addr_mask;
-			continue;
+			if (!uffd_wp) {
+				address |= last_addr_mask;
+				continue;
+			}
+			/*
+			 * Userfaultfd wr-protect requires pgtable
+			 * pre-allocations to install pte markers.
+			 */
+			ptep = huge_pte_alloc(mm, vma, address, psize);
+			if (!ptep)
+				break;
 		}
 		ptl = huge_pte_lock(h, mm, ptep);
 		if (huge_pmd_unshare(mm, vma, address, ptep)) {
-- 
2.37.3