[S390] reference bit testing for unmapped pages

On x86 a page without a mapper is by definition not referenced / old.
The s390 architecture keeps the reference bit in the storage key and
the current code will check the storage key for page without a mapper.
This leads to an interesting effect: the first time an s390 system
needs to write pages to swap it only finds referenced pages. This
causes a lot of pages to get added and written to the swap device.
To avoid this behaviour change page_referenced to query the storage
key only if there is a mapper of the page.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

diff --git a/mm/rmap.c b/mm/rmap.c
index 2540a39eea4a02c9fc7463409edbc14258933d51..9701574bb67a2501ea15ec0c82601c0cb6e9a987 100644 (file)
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -869,11 +869,11 @@ int page_referenced(struct page *page,
                                                                vm_flags);
                if (we_locked)
                        unlock_page(page);
+
+               if (page_test_and_clear_young(page_to_pfn(page)))
+                       referenced++;
        }
 out:
-       if (page_test_and_clear_young(page_to_pfn(page)))
-               referenced++;
-
        return referenced;
 }