mm: page migration remove_migration_ptes at lock+unlock level

Clean up page migration a little more by calling remove_migration_ptes()
from the same level, on success or on failure, from __unmap_and_move() or
from unmap_and_move_huge_page().

Don't reset page->mapping of a PageAnon old page in move_to_new_page(),
leave that to when the page is freed.  Except for here in page migration,
it has been an invariant that a PageAnon (bit set in page->mapping) page
stays PageAnon until it is freed, and I think we're safer to keep to that.

And with the above rearrangement, it's necessary because zap_pte_range()
wants to identify whether a migration entry represents a file or an anon
page, to update the appropriate rss stats without waiting on it.

Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Sasha Levin <sasha.levin@oracle.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/migrate.c

@@ -722,7 +722,7 @@ static int fallback_migrate_page(struct address_space *mapping,
  *  MIGRATEPAGE_SUCCESS - success
  */
 static int move_to_new_page(struct page *newpage, struct page *page,
-				int page_was_mapped, enum migrate_mode mode)
+				enum migrate_mode mode)
 {
 	struct address_space *mapping;
 	int rc;
@@ -755,19 +755,21 @@ static int move_to_new_page(struct page *newpage, struct page *page,
 		 * space which also has its own migratepage callback. This
 		 * is the most common path for page migration.
 		 */
-		rc = mapping->a_ops->migratepage(mapping,
-						newpage, page, mode);
+		rc = mapping->a_ops->migratepage(mapping, newpage, page, mode);
 	else
 		rc = fallback_migrate_page(mapping, newpage, page, mode);
 
-	if (rc != MIGRATEPAGE_SUCCESS) {
+	/*
+	 * When successful, old pagecache page->mapping must be cleared before
+	 * page is freed; but stats require that PageAnon be left as PageAnon.
+	 */
+	if (rc == MIGRATEPAGE_SUCCESS) {
+		set_page_memcg(page, NULL);
+		if (!PageAnon(page))
+			page->mapping = NULL;
+	} else {
 		set_page_memcg(newpage, NULL);
 		newpage->mapping = NULL;
-	} else {
-		set_page_memcg(page, NULL);
-		if (page_was_mapped)
-			remove_migration_ptes(page, newpage);
-		page->mapping = NULL;
 	}
 	return rc;
 }
@@ -902,10 +904,11 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
 	}
 
 	if (!page_mapped(page))
-		rc = move_to_new_page(newpage, page, page_was_mapped, mode);
+		rc = move_to_new_page(newpage, page, mode);
 
-	if (rc && page_was_mapped)
-		remove_migration_ptes(page, page);
+	if (page_was_mapped)
+		remove_migration_ptes(page,
+			rc == MIGRATEPAGE_SUCCESS ? newpage : page);
 
 out_unlock_both:
 	unlock_page(newpage);
@@ -1066,10 +1069,11 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
 	}
 
 	if (!page_mapped(hpage))
-		rc = move_to_new_page(new_hpage, hpage, page_was_mapped, mode);
+		rc = move_to_new_page(new_hpage, hpage, mode);
 
-	if (rc != MIGRATEPAGE_SUCCESS && page_was_mapped)
-		remove_migration_ptes(hpage, hpage);
+	if (page_was_mapped)
+		remove_migration_ptes(hpage,
+			rc == MIGRATEPAGE_SUCCESS ? new_hpage : hpage);
 
 	unlock_page(new_hpage);