mm/uffd: PTE_MARKER_UFFD_WP

This patch introduces the 1st user of pte marker: the uffd-wp marker.

When the pte marker is installed with the uffd-wp bit set, it means this pte
was wr-protected by uffd.

We will use this special pte to arm the ptes that got either unmapped or
swapped out for a file-backed region that was previously wr-protected.  This
special pte could trigger a page fault just like swap entries.

This idea is greatly inspired by Hugh and Andrea in the discussion, which is
referenced in the links below.

Some helpers are introduced to detect whether a swap pte is uffd wr-protected.
After the pte marker introduced, one swap pte can be wr-protected in two forms:
either it is a normal swap pte and it has _PAGE_SWP_UFFD_WP set, or it's a pte
marker that has PTE_MARKER_UFFD_WP set.

Link: https://lore.kernel.org/lkml/20201126222359.8120-1-peterx@redhat.com/
Link: https://lore.kernel.org/lkml/20201130230603.46187-1-peterx@redhat.com/
Suggested-by: Andrea Arcangeli <aarcange@redhat.com>
Suggested-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Peter Xu <peterx@redhat.com>

include/linux/swapops.h

@@ -276,7 +276,8 @@ static inline int is_readable_migration_entry(swp_entry_t entry)
 
 typedef unsigned long pte_marker;
 
-#define  PTE_MARKER_MASK     (0)
+#define  PTE_MARKER_UFFD_WP  BIT(0)
+#define  PTE_MARKER_MASK     (PTE_MARKER_UFFD_WP)
 
 #ifdef CONFIG_PTE_MARKER
 

include/linux/userfaultfd_k.h

@@ -15,6 +15,8 @@
 
 #include <linux/fcntl.h>
 #include <linux/mm.h>
+#include <linux/swap.h>
+#include <linux/swapops.h>
 #include <asm-generic/pgtable_uffd.h>
 
 /* The set of all possible UFFD-related VM flags. */
@@ -236,4 +238,45 @@ static inline void userfaultfd_unmap_complete(struct mm_struct *mm,
 
 #endif /* CONFIG_USERFAULTFD */
 
+static inline bool pte_marker_entry_uffd_wp(swp_entry_t entry)
+{
+	return is_pte_marker_entry(entry) &&
+	    (pte_marker_get(entry) & PTE_MARKER_UFFD_WP);
+}
+
+static inline bool pte_marker_uffd_wp(pte_t pte)
+{
+#ifdef CONFIG_PTE_MARKER_UFFD_WP
+	swp_entry_t entry;
+
+	if (!is_swap_pte(pte))
+		return false;
+
+	entry = pte_to_swp_entry(pte);
+
+	return pte_marker_entry_uffd_wp(entry);
+#else
+	return false;
+#endif
+}
+
+/*
+ * Returns true if this is a swap pte and was uffd-wp wr-protected in either
+ * forms (pte marker or a normal swap pte), false otherwise.
+ */
+static inline bool pte_swp_uffd_wp_any(pte_t pte)
+{
+#ifdef CONFIG_PTE_MARKER_UFFD_WP
+	if (!is_swap_pte(pte))
+		return false;
+
+	if (pte_swp_uffd_wp(pte))
+		return true;
+
+	if (pte_marker_uffd_wp(pte))
+		return true;
+#endif
+	return false;
+}
+
 #endif /* _LINUX_USERFAULTFD_K_H */

mm/Kconfig

@@ -915,6 +915,15 @@ config PTE_MARKER
 	help
 	  Allows to create marker PTEs for file-backed memory.
 
+config PTE_MARKER_UFFD_WP
+	bool "Marker PTEs support for userfaultfd write protection"
+	depends on PTE_MARKER && HAVE_ARCH_USERFAULTFD_WP
+
+	help
+	  Allows to create marker PTEs for userfaultfd write protection
+	  purposes.  It is required to enable userfaultfd write protection on
+	  file-backed memory types like shmem and hugetlbfs.
+
 source "mm/damon/Kconfig"
 
 endmenu