This patch only introduces the special pte and its operators.  It's not yet
applied to have any functional difference.

Link: https://lore.kernel.org/lkml/20201126222359.8120-1-peterx@redhat.com/ (local)
Link: https://lore.kernel.org/lkml/20201130230603.46187-1-peterx@redhat.com/ (local)
Suggested-by: Andrea Arcangeli <redacted>
Suggested-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
---
 arch/x86/include/asm/pgtable.h     | 28 ++++++++++++++++++++++++++++
 include/asm-generic/pgtable_uffd.h |  3 +++
 include/linux/userfaultfd_k.h      | 21 +++++++++++++++++++++
 3 files changed, 52 insertions(+)

diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index b1099f2d9800..9781ba2da049 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h

@@ -1329,6 +1329,34 @@ static inline pmd_t pmd_swp_clear_soft_dirty(pmd_t

pmd) #endif

 #ifdef CONFIG_HAVE_ARCH_USERFAULTFD_WP
+
+/*
+ * This is a very special swap-like pte that marks this pte as
"wr-protected" + * by userfaultfd-wp.  It should only exist for file-backed
memory where the + * page (previously got wr-protected) has been unmapped
or swapped out. + *
+ * For anonymous memories, the userfaultfd-wp _PAGE_SWP_UFFD_WP bit is kept
+ * along with a real swp entry instead.
+ *
+ * Let's make some rules for this special pte:
+ *
+ * (1) pte_none()==false, so that it'll not trigger a missing page fault.
+ *
+ * (2) pte_present()==false, so that it's recognized as swap (is_swap_pte).
+ *
+ * (3) pte_swp_uffd_wp()==true, so it can be tested just like a swap pte
that + *     contains a valid swap entry, so that we can check a swap pte
always + *     using "is_swap_pte() && pte_swp_uffd_wp()" without caring
about whether + *     there's one swap entry inside of the pte.
+ *
+ * (4) It should not be a valid swap pte anywhere, so that when we see this
pte + *     we know it does not contain a swap entry.
+ *
+ * For x86, the simplest special pte which satisfies all of above should be
the + * pte with only _PAGE_SWP_UFFD_WP bit set (where
swp_type==swp_offset==0). + */
+#define  UFFD_WP_SWP_PTE_SPECIAL  __pte(_PAGE_SWP_UFFD_WP)
+
 static inline pte_t pte_swp_mkuffd_wp(pte_t pte)
 {
 	return pte_set_flags(pte, _PAGE_SWP_UFFD_WP);

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);
+}
+
 #else /* CONFIG_USERFAULTFD */

 /* mm helpers */

@@ -234,6 +245,16 @@ static inline void userfaultfd_unmap_complete(struct

mm_struct *mm, {
 }

+static inline pte_t pte_swp_mkuffd_wp_special(struct vm_area_struct *vma)
+{
+	return __pte(0);
+}
+
+static inline bool pte_swp_uffd_wp_special(pte_t pte)
+{
+	return false;
+}
+
 #endif /* CONFIG_USERFAULTFD */

 #endif /* _LINUX_USERFAULTFD_K_H */