[PATCH] Special-case fail_writepage() in page reclaim

Pages from memory-backed filesystems are supposed to be moved up onto
the active list, but that's not working because fail_writepage() is
called when the page is not on the LRU.

So look for this case in page reclaim and handle it there.

And it's more efficient, the VM knows more about what is going on and
it later leads to the removal of fail_writepage().

mm/vmscan.c

@@ -309,6 +309,8 @@ shrink_list(struct list_head *page_list, unsigned int gfp_mask,
 				goto keep_locked;
 			if (!mapping)
 				goto keep_locked;
+			if (mapping->a_ops->writepage == fail_writepage)
+				goto activate_locked;
 			if (!may_enter_fs)
 				goto keep_locked;
 			bdi = mapping->backing_dev_info;