mm/migrate.c: cleanup expected_page_refs()

Andrea has noted that page migration code propagates page_mapping(page)
through the whole migration stack down to migrate_page() function so it
seems stupid to then use page_mapping(page) in expected_page_refs()
instead of passed down 'mapping' argument.  I agree so let's make
expected_page_refs() more in line with the rest of the migration stack.

Link: http://lkml.kernel.org/r/20190207112314.24872-1-jack@suse.czSigned-off-by: Jan Kara <jack@suse.cz>
Suggested-by: Andrea Arcangeli <aarcange@redhat.com>
Reviewed-by: Andrea Arcangeli <aarcange@redhat.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/migrate.c

@@ -374,7 +374,7 @@ void pmd_migration_entry_wait(struct mm_struct *mm, pmd_t *pmd)
 }
 #endif
 
-static int expected_page_refs(struct page *page)
+static int expected_page_refs(struct address_space *mapping, struct page *page)
 {
 	int expected_count = 1;
 
@@ -384,7 +384,7 @@ static int expected_page_refs(struct page *page)
 	 */
 	expected_count += is_device_private_page(page);
 	expected_count += is_device_public_page(page);
-	if (page_mapping(page))
+	if (mapping)
 		expected_count += hpage_nr_pages(page) + page_has_private(page);
 
 	return expected_count;
@@ -405,7 +405,7 @@ int migrate_page_move_mapping(struct address_space *mapping,
 	XA_STATE(xas, &mapping->i_pages, page_index(page));
 	struct zone *oldzone, *newzone;
 	int dirty;
-	int expected_count = expected_page_refs(page) + extra_count;
+	int expected_count = expected_page_refs(mapping, page) + extra_count;
 
 	if (!mapping) {
 		/* Anonymous page without mapping */
@@ -750,7 +750,7 @@ static int __buffer_migrate_page(struct address_space *mapping,
 		return migrate_page(mapping, newpage, page, mode);
 
 	/* Check whether page does not have extra refs before we do more work */
-	expected_count = expected_page_refs(page);
+	expected_count = expected_page_refs(mapping, page);
 	if (page_count(page) != expected_count)
 		return -EAGAIN;