[S390] Optimize storage key handling for anonymous pages

page_mkclean used to call page_clear_dirty for every given page. This
is different to all other architectures, where the dirty bit in the
PTEs is only resetted, if page_mapping() returns a non-NULL pointer.
We can move the page_test_dirty/page_clear_dirty sequence into the
2nd if to avoid unnecessary iske/sske sequences, which are expensive.

This change also helps kvm for s390 as the host must transfer the
dirty bit into the guest status bits. By moving the page_clear_dirty
operation into the 2nd if, the vm will only call page_clear_dirty
for pages where it walks the mapping anyway. There it calls
ptep_clear_flush for writable ptes, so we can transfer the dirty bit
to the guest.

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

1 files changed, 5 insertions, 4 deletions

diff --git a/mm/rmap.c b/mm/rmap.c
index dc3be5f5b0d..dbc2ca2057a 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -471,11 +471,12 @@ int page_mkclean(struct page *page)
 
         if (page_mapped(page)) {
                 struct address_space *mapping = page_mapping(page);
-                if (mapping)
+                if (mapping) {
                         ret = page_mkclean_file(mapping, page);
-                if (page_test_dirty(page)) {
-                        page_clear_dirty(page);
-                        ret = 1;
+                        if (page_test_dirty(page)) {
+                                page_clear_dirty(page);
+                                ret = 1;
+                        }
                 }
         }