[PATCH 1/2] do_wp_page: cannot share file page

[Hugh Dickins]

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@xxxxxxxxxxx>
---

 mm/memory.c |    2 +-
 1 files changed, 1 insertion(+), 1 deletion(-)

--- 2.6.12-rc6/mm/memory.c	2005-05-25 18:09:21.000000000 +0100
+++ linux/mm/memory.c	2005-06-04 20:41:55.000000000 +0100
@@ -1264,7 +1264,7 @@ static int do_wp_page(struct mm_struct *
 	}
 	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) {
-
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