mm: fix warning in __set_page_dirty_nobuffers

New tmpfs use of !PageUptodate pages for fallocate() is triggering the
WARNING: at mm/page-writeback.c:1990 when __set_page_dirty_nobuffers()
is called from migrate_page_copy() for compaction.

It is anomalous that migration should use __set_page_dirty_nobuffers()
on an address_space that does not participate in dirty and writeback
accounting; and this has also been observed to insert surprising dirty
tags into a tmpfs radix_tree, despite tmpfs not using tags at all.

We should probably give migrate_page_copy() a better way to preserve the
tag and migrate accounting info, when mapping_cap_account_dirty(). But
that needs some more work: so in the interim, avoid the warning by using
a simple SetPageDirty on PageSwapBacked pages.

Reported-and-tested-by: Dave Jones <[e-mail]>
Signed-off-by: Hugh Dickins <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/migrate.c

···
 		 * is actually a signal that all of the page has become dirty.
 		 * Whereas only part of our page may be dirty.
 		 */
-		__set_page_dirty_nobuffers(newpage);
+		if (PageSwapBacked(page))
+			SetPageDirty(newpage);
+		else
+			__set_page_dirty_nobuffers(newpage);
  	}
 
 	mlock_migrate_page(newpage, page);