[PATCH] do_wp_page: cannot share file page

A small optimization to do_wp_page's check for whether to avoid copy by
reusing the page already mapped.  It can never share a cached file page,
nor can it share a reserved page (often the empty zero page), so it's a
waste of time to lock and unlock in those cases.  Which nowadays can both
be neatly excluded by a preliminary PageAnon test.

Christoph has reported that a preliminary page_count test proved valuable
for scalability here, but PageAnon covers more common cases all at once.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

diff --git a/mm/memory.c b/mm/memory.c
index b8846cf2358a54fb9903568fc50dbd69ab3d2bce..1c0a3db78a052e2ad0e166b9b0c1af44109623da 100644 (file)
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1239,7 +1239,7 @@ static int do_wp_page(struct mm_struct *mm, struct vm_area_struct * vma,
        }
        old_page = pfn_to_page(pfn);
 
-       if (!TestSetPageLocked(old_page)) {
+       if (PageAnon(old_page) && !TestSetPageLocked(old_page)) {
                int reuse = can_share_swap_page(old_page);
                unlock_page(old_page);
                if (reuse) {