[PATCH v6 3/7] userfaultfd: disable huge PMD sharing for minor fault registered VMAs

[Axel Rasmussen]

As the comment says: for the minor fault use case, although the page
might be present and populated in the other (non-UFFD-registered) half
of the mapping, it may be out of date, and we explicitly want userspace
to get a minor fault so it can check and potentially update the page's
contents.

Huge PMD sharing would prevent these faults from occurring for
suitably aligned areas, so disable it upon UFFD registration.

Signed-off-by: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
---
 include/linux/userfaultfd_k.h | 26 +++++++++++++++++---------
 1 file changed, 17 insertions(+), 9 deletions(-)

diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index 4e03268c65ec..98cb6260b4b4 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h
@@ -62,15 +62,6 @@ static inline bool is_mergeable_vm_userfaultfd_ctx(struct vm_area_struct *vma,
 	return vma->vm_userfaultfd_ctx.ctx == vm_ctx.ctx;
 }
 
-/*
- * Never enable huge pmd sharing on uffd-wp registered vmas, because uffd-wp
- * protect information is per pgtable entry.
- */
-static inline bool uffd_disable_huge_pmd_share(struct vm_area_struct *vma)
-{
-	return vma->vm_flags & VM_UFFD_WP;
-}
-
 static inline bool userfaultfd_missing(struct vm_area_struct *vma)
 {
 	return vma->vm_flags & VM_UFFD_MISSING;
@@ -83,6 +74,23 @@ static inline bool userfaultfd_wp(struct vm_area_struct *vma)
 
 bool userfaultfd_minor(struct vm_area_struct *vma);
 
+/*
+ * Never enable huge pmd sharing on some uffd registered vmas:
+ *
+ * - VM_UFFD_WP VMAs, because write protect information is per pgtable entry.
+ *
+ * - VM_UFFD_MISSING VMAs with UFFD_FEATURE_MINOR_HUGETLBFS, because otherwise
+ *   we would never get minor faults for VMAs which share huge pmds. (If you
+ *   have two mappings to the same underlying pages, and fault in the
+ *   non-UFFD-registered one with a write, with huge pmd sharing this would
+ *   *also* setup the second UFFD-registered mapping, and we'd not get minor
+ *   faults.)
+ */
+static inline bool uffd_disable_huge_pmd_share(struct vm_area_struct *vma)
+{
+	return userfaultfd_wp(vma) || userfaultfd_minor(vma);
+}
+
 static inline bool userfaultfd_pte_wp(struct vm_area_struct *vma,
 				      pte_t pte)
 {
-- 
2.30.0.478.g8a0d178c01-goog