mm: merge folio_is_secretmem() and folio_fast_pin_allowed() into gup_fast_folio_allowed()

folio_is_secretmem() is currently only used during GUP-fast. Nowadays,
folio_fast_pin_allowed() performs similar checks during GUP-fast and
contains a lot of careful handling -- READ_ONCE() -- , sanity checks --
lockdep_assert_irqs_disabled() -- and helpful comments on how this
handling is safe and correct.

So let's merge folio_is_secretmem() into folio_fast_pin_allowed(). Rename
folio_fast_pin_allowed() to gup_fast_folio_allowed(), to better match the
new semantics.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: David Hildenbrand <[e-mail]>
Reviewed-by: Mike Rapoport (IBM) <[e-mail]>
Cc: David Hildenbrand <[e-mail]>
Cc: Lorenzo Stoakes <[e-mail]>
Cc: Miklos Szeredi <[e-mail]>
Cc: xingwei lee <[e-mail]>
Cc: yue sun <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

include/linux/secretmem.h

···
 
 extern const struct address_space_operations secretmem_aops;
 
-static inline bool folio_is_secretmem(struct folio *folio)
+static inline bool secretmem_mapping(struct address_space *mapping)
 {
-	struct address_space *mapping;
-
-	/*
-	 * Using folio_mapping() is quite slow because of the actual call
-	 * instruction.
-	 * We know that secretmem pages are not compound, so we can
-	 * save a couple of cycles here.
-	 */
-	if (folio_test_large(folio))
-		return false;
-
-	mapping = (struct address_space *)
-		((unsigned long)folio->mapping & ~PAGE_MAPPING_FLAGS);
-
-	if (!mapping || mapping != folio->mapping)
-		return false;
-
 	return mapping->a_ops == &secretmem_aops;
 }
 
···
 	return false;
 }
 
-static inline bool folio_is_secretmem(struct folio *folio)
+static inline bool secretmem_mapping(struct address_space *mapping)
 {
 	return false;
 }

mm/gup.c

···
 #ifdef CONFIG_HAVE_FAST_GUP
 
 /*
- * Used in the GUP-fast path to determine whether a pin is permitted for a
- * specific folio.
+ * Used in the GUP-fast path to determine whether GUP is permitted to work on
+ * a specific folio.
  *
  * This call assumes the caller has pinned the folio, that the lowest page table
  * level still points to this folio, and that interrupts have been disabled.
+ *
+ * GUP-fast must reject all secretmem folios.
  *
  * Writing to pinned file-backed dirty tracked folios is inherently problematic
  * (see comment describing the writable_file_mapping_allowed() function). We
···
  * in the fast path, so instead we whitelist known good cases and if in doubt,
  * fall back to the slow path.
  */
-static bool folio_fast_pin_allowed(struct folio *folio, unsigned int flags)
+static bool gup_fast_folio_allowed(struct folio *folio, unsigned int flags)
 {
+	bool reject_file_backed = false;
 	struct address_space *mapping;
+	bool check_secretmem = false;
 	unsigned long mapping_flags;
 
 	/*
 	 * If we aren't pinning then no problematic write can occur. A long term
 	 * pin is the most egregious case so this is the one we disallow.
 	 */
-	if ((flags & (FOLL_PIN | FOLL_LONGTERM | FOLL_WRITE)) !=
+	if ((flags & (FOLL_PIN | FOLL_LONGTERM | FOLL_WRITE)) ==
 	    (FOLL_PIN | FOLL_LONGTERM | FOLL_WRITE))
-		return true;
+		reject_file_backed = true;
 
-	/* The folio is pinned, so we can safely access folio fields. */
+	/* We hold a folio reference, so we can safely access folio fields. */
+
+	/* secretmem folios are always order-0 folios. */
+	if (IS_ENABLED(CONFIG_SECRETMEM) && !folio_test_large(folio))
+		check_secretmem = true;
+
+	if (!reject_file_backed && !check_secretmem)
+		return true;
 
 	if (WARN_ON_ONCE(folio_test_slab(folio)))
 		return false;
 
-	/* hugetlb mappings do not require dirty-tracking. */
+	/* hugetlb neither requires dirty-tracking nor can be secretmem. */
 	if (folio_test_hugetlb(folio))
 		return true;
 
···
 
 	/*
 	 * At this point, we know the mapping is non-null and points to an
-	 * address_space object. The only remaining whitelisted file system is
-	 * shmem.
+	 * address_space object.
 	 */
-	return shmem_mapping(mapping);
+	if (check_secretmem && secretmem_mapping(mapping))
+		return false;
+	/* The only remaining allowed file system is shmem. */
+	return !reject_file_backed || shmem_mapping(mapping);
 }
 
 static void __maybe_unused undo_dev_pagemap(int *nr, int nr_start,
···
 		if (!folio)
 			goto pte_unmap;
 
-		if (unlikely(folio_is_secretmem(folio))) {
-			gup_put_folio(folio, 1, flags);
-			goto pte_unmap;
-		}
-
 		if (unlikely(pmd_val(pmd) != pmd_val(*pmdp)) ||
 		    unlikely(pte_val(pte) != pte_val(ptep_get(ptep)))) {
 			gup_put_folio(folio, 1, flags);
 			goto pte_unmap;
 		}
 
-		if (!folio_fast_pin_allowed(folio, flags)) {
+		if (!gup_fast_folio_allowed(folio, flags)) {
 			gup_put_folio(folio, 1, flags);
 			goto pte_unmap;
 		}
···
 		return 0;
 	}
 
-	if (!folio_fast_pin_allowed(folio, flags)) {
+	if (!gup_fast_folio_allowed(folio, flags)) {
 		gup_put_folio(folio, refs, flags);
 		return 0;
 	}
···
 		return 0;
 	}
 
-	if (!folio_fast_pin_allowed(folio, flags)) {
+	if (!gup_fast_folio_allowed(folio, flags)) {
 		gup_put_folio(folio, refs, flags);
 		return 0;
 	}
···
 		return 0;
 	}
 
-	if (!folio_fast_pin_allowed(folio, flags)) {
+	if (!gup_fast_folio_allowed(folio, flags)) {
 		gup_put_folio(folio, refs, flags);
 		return 0;
 	}
···
 		return 0;
 	}
 
-	if (!folio_fast_pin_allowed(folio, flags)) {
+	if (!gup_fast_folio_allowed(folio, flags)) {
 		gup_put_folio(folio, refs, flags);
 		return 0;
 	}