mm: migrate: lock buffers before migrate_page_move_mapping()

Lock buffers before calling into migrate_page_move_mapping() so that that
function doesn't have to know about buffers (which is somewhat unexpected
anyway) and all the buffer head logic is in buffer_migrate_page().

Link: http://lkml.kernel.org/r/20181211172143.7358-3-jack@suse.czSigned-off-by: Jan Kara <jack@suse.cz>
Acked-by: Mel Gorman <mgorman@suse.de>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/migrate.c

@@ -486,20 +486,6 @@ int migrate_page_move_mapping(struct address_space *mapping,
 		return -EAGAIN;
 	}
 
-	/*
-	 * In the async migration case of moving a page with buffers, lock the
-	 * buffers using trylock before the mapping is moved. If the mapping
-	 * was moved, we later failed to lock the buffers and could not move
-	 * the mapping back due to an elevated page count, we would have to
-	 * block waiting on other references to be dropped.
-	 */
-	if (mode == MIGRATE_ASYNC && head &&
-			!buffer_migrate_lock_buffers(head, mode)) {
-		page_ref_unfreeze(page, expected_count);
-		xas_unlock_irq(&xas);
-		return -EAGAIN;
-	}
-
 	/*
 	 * Now we know that no one else is looking at the page:
 	 * no turning back from here.
@@ -775,24 +761,23 @@ int buffer_migrate_page(struct address_space *mapping,
 {
 	struct buffer_head *bh, *head;
 	int rc;
+	int expected_count;
 
 	if (!page_has_buffers(page))
 		return migrate_page(mapping, newpage, page, mode);
 
-	head = page_buffers(page);
+	/* Check whether page does not have extra refs before we do more work */
+	expected_count = expected_page_refs(page);
+	if (page_count(page) != expected_count)
+		return -EAGAIN;
 
-	rc = migrate_page_move_mapping(mapping, newpage, page, head, mode, 0);
+	head = page_buffers(page);
+	if (!buffer_migrate_lock_buffers(head, mode))
+		return -EAGAIN;
 
+	rc = migrate_page_move_mapping(mapping, newpage, page, NULL, mode, 0);
 	if (rc != MIGRATEPAGE_SUCCESS)
-		return rc;
-
-	/*
-	 * In the async case, migrate_page_move_mapping locked the buffers
-	 * with an IRQ-safe spinlock held. In the sync case, the buffers
-	 * need to be locked now
-	 */
-	if (mode != MIGRATE_ASYNC)
-		BUG_ON(!buffer_migrate_lock_buffers(head, mode));
+		goto unlock_buffers;
 
 	ClearPagePrivate(page);
 	set_page_private(newpage, page_private(page));
@@ -814,6 +799,8 @@ int buffer_migrate_page(struct address_space *mapping,
 	else
 		migrate_page_states(newpage, page);
 
+	rc = MIGRATEPAGE_SUCCESS;
+unlock_buffers:
 	bh = head;
 	do {
 		unlock_buffer(bh);
@@ -822,7 +809,7 @@ int buffer_migrate_page(struct address_space *mapping,
 
 	} while (bh != head);
 
-	return MIGRATEPAGE_SUCCESS;
+	return rc;
 }
 EXPORT_SYMBOL(buffer_migrate_page);
 #endif