[PATCH v2 6/9] userfaultfd: disable huge PMD sharing for MINOR 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of the shared mapping, it may be out of date, and we explicitly want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.

Reviewed-by: Peter Xu <peterx@xxxxxxxxxx>
Signed-off-by: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
---
 include/linux/userfaultfd_k.h | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index 0390e5ac63b3..fb9abaeb4194 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h
@@ -56,12 +56,18 @@ static inline bool is_mergeable_vm_userfaultfd_ctx(struct vm_area_struct *vma,
 }
 
 /*
- * Never enable huge pmd sharing on uffd-wp registered vmas, because uffd-wp
- * protect information is per pgtable entry.
+ * Never enable huge pmd sharing on some uffd registered vmas:
+ *
+ * - VM_UFFD_WP VMAs, because write protect information is per pgtable entry.
+ *
+ * - VM_UFFD_MINOR VMAs, because we explicitly want minor faults to occur even
+ *   when the other half of a shared mapping is populated (even though the page
+ *   is there, in our use case it may be out of date, so userspace needs to
+ *   check for this and possibly update it).
  */
 static inline bool uffd_disable_huge_pmd_share(struct vm_area_struct *vma)
 {
-	return vma->vm_flags & VM_UFFD_WP;
+	return vma->vm_flags & (VM_UFFD_WP | VM_UFFD_MINOR);
 }
 
 static inline bool userfaultfd_missing(struct vm_area_struct *vma)
-- 
2.30.0.280.ga3ce27912f-goog