On Wed, Jun 09, 2021 at 11:06:32PM +1000, Alistair Popple wrote:

On Friday, 28 May 2021 6:19:04 AM AEST Peter Xu wrote:

[...]

quoted

diff --git a/include/asm-generic/pgtable_uffd.h b/include/asm-generic/pgtable_uffd.h
index 828966d4c281..95e9811ce9d1 100644
--- a/include/asm-generic/pgtable_uffd.h
+++ b/include/asm-generic/pgtable_uffd.h

@@ -2,6 +2,9 @@
 #define _ASM_GENERIC_PGTABLE_UFFD_H
 
 #ifndef CONFIG_HAVE_ARCH_USERFAULTFD_WP
+
+#define  UFFD_WP_SWP_PTE_SPECIAL  __pte(0)
+
 static __always_inline int pte_uffd_wp(pte_t pte)
 {
 	return 0;

diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index 331d2ccf0bcc..93f932b53a71 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h

@@ -145,6 +145,17 @@ extern int userfaultfd_unmap_prep(struct vm_area_struct *vma,
 extern void userfaultfd_unmap_complete(struct mm_struct *mm,
 				       struct list_head *uf);
 
+static inline pte_t pte_swp_mkuffd_wp_special(struct vm_area_struct *vma)
+{
+	WARN_ON_ONCE(vma_is_anonymous(vma));
+	return UFFD_WP_SWP_PTE_SPECIAL;
+}
+
+static inline bool pte_swp_uffd_wp_special(pte_t pte)
+{
+	return pte_same(pte, UFFD_WP_SWP_PTE_SPECIAL);
+}
+

Sorry, only just noticed this but do we need to define a different version of
this helper that returns false for CONFIG_HAVE_ARCH_USERFAULTFD_WP=n to avoid
spurious matches with __pte(0) on architectures supporting userfaultfd but not
userfaultfd-wp?

Good point.. Yes we definitely don't want the empty pte to be recognized as the
special pte..  I'll squash below into the same patch:

----8<----

diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index 489fb375e66c..23ca449240d1 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h

@@ -177,7 +177,11 @@ static inline pte_t pte_swp_mkuffd_wp_special(struct vm_area_struct *vma)
 
 static inline bool pte_swp_uffd_wp_special(pte_t pte)
 {
+#ifdef CONFIG_HAVE_ARCH_USERFAULTFD_WP
        return pte_same(pte, UFFD_WP_SWP_PTE_SPECIAL);
+#else
+       return false;
+#fi
 }
 
 #else /* CONFIG_USERFAULTFD */
----8<----

I'll see whether I can give some dry run without HAVE_ARCH_USERFAULTFD_WP but
with USERFAULTFD.

Thanks for spotting that!

-- 
Peter Xu