[PATCH v3 5/8] mm: userfaultfd: support UFFDIO_POISON for hugetlbfs

[Axel Rasmussen <axelrasmussen@google.com>]

The behavior here is the same as it is for anon/shmem. This is done
separately because hugetlb pte marker handling is a bit different.

Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
---
 mm/hugetlb.c     | 19 +++++++++++++++++++
 mm/userfaultfd.c |  3 +--
 2 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 934e129d9939..20c5f6a5420a 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -6263,6 +6263,25 @@ int hugetlb_mfill_atomic_pte(pte_t *dst_pte,
 	int writable;
 	bool folio_in_pagecache = false;
 
+	if (uffd_flags_mode_is(flags, MFILL_ATOMIC_POISON)) {
+		ptl = huge_pte_lock(h, dst_mm, dst_pte);
+
+		/* Don't overwrite any existing PTEs (even markers) */
+		if (!huge_pte_none(huge_ptep_get(dst_pte))) {
+			spin_unlock(ptl);
+			return -EEXIST;
+		}
+
+		_dst_pte = make_pte_marker(PTE_MARKER_ERROR);
+		set_huge_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte);
+
+		/* No need to invalidate - it was non-present before */
+		update_mmu_cache(dst_vma, dst_addr, dst_pte);
+
+		spin_unlock(ptl);
+		return 0;
+	}
+
 	if (is_continue) {
 		ret = -EFAULT;
 		folio = filemap_lock_folio(mapping, idx);
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 899aa621d7c1..9ce129fdd596 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -373,8 +373,7 @@ static __always_inline ssize_t mfill_atomic_hugetlb(
 	 * by THP.  Since we can not reliably insert a zero page, this
 	 * feature is not supported.
 	 */
-	if (uffd_flags_mode_is(flags, MFILL_ATOMIC_ZEROPAGE) ||
-	    uffd_flags_mode_is(flags, MFILL_ATOMIC_POISON)) {
+	if (uffd_flags_mode_is(flags, MFILL_ATOMIC_ZEROPAGE)) {
 		mmap_read_unlock(dst_mm);
 		return -EINVAL;
 	}
-- 
2.41.0.255.g8b1d071c50-goog