[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>

1 files changed, 3 insertions, 3 deletions

diff --git a/mm/rmap.c b/mm/rmap.c
index 2540a39eea4a..9701574bb67a 100644
--- 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;
 }