mm: fix the NULL mapping case in __isolate_lru_page()

commit 145e1a7 upstream.

George Boole would have noticed a slight error in 4.16 commit
69d763f ("mm: pin address_space before dereferencing it while
isolating an LRU page").  Fix it, to match both the comment above it,
and the original behaviour.

Although anonymous pages are not marked PageDirty at first, we have an
old habit of calling SetPageDirty when a page is removed from swap
cache: so there's a category of ex-swap pages that are easily
migratable, but were inadvertently excluded from compaction's async
migration in 4.16.

Link: http://lkml.kernel.org/r/[email protected]
Fixes: 69d763f ("mm: pin address_space before dereferencing it while isolating an LRU page")
Signed-off-by: Hugh Dickins <[email protected]>
Acked-by: Minchan Kim <[email protected]>
Acked-by: Mel Gorman <[email protected]>
Reported-by:  Ivan Kalvachev <[email protected]>
Cc: "Huang, Ying" <[email protected]>
Cc: Jan Kara <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
Signed-off-by: Ben Hutchings <[email protected]>

Author: Hugh Dickins

mm/vmscan.c

@@ -1225,7 +1225,7 @@ int __isolate_lru_page(struct page *page, isolate_mode_t mode)
 				return ret;
 
 			mapping = page_mapping(page);
-			migrate_dirty = mapping && mapping->a_ops->migratepage;
+			migrate_dirty = !mapping || mapping->a_ops->migratepage;
 			unlock_page(page);
 			if (!migrate_dirty)
 				return ret;