[PATCH] speed up fork performance

Rik van Riel
2004-08-27T14:09:38Z

OK, this patch is _completely_ untested, but since I'm about to run
off to a conference I guess I should get it to you anyway.

Basically in 2.6 lru_cache_add_active() takes an extra reference to
the page, but do_wp_page() and friends don't expect a private anonymous
page to have 2 references instead of 1. This little patchlet changes
can_share_swap_page() and exclusive_swap_page() to expect the extra
reference.

Note that we cannot test for PageLRU(page) since lru_cache_add_active()
uses a delayed insertion onto the LRU, so the PG_lru might not get set
for a while...

Use at your own risk.

--- linux-2.6.9-rc1/mm/swapfile.c.blah 2004-08-27 09:51:54.000000000 -0400
+++ linux-2.6.9-rc1/mm/swapfile.c 2004-08-27 09:55:38.000000000 -0400
@@ -291,7 +291,8 @@ static int exclusive_swap_page(struct pa
if (p->swap_map[swp_offset(entry)] == 1) {
/* Recheck the page count with the swapcache lock held.. */
spin_lock_irq(&swapper_space.tree_lock);
- if (page_count(page) == 2)
+ /* Process, swapcache and LRU each have a reference. */
+ if (page_count(page) == 3)
retval = 1;
spin_unlock_irq(&swapper_space.tree_lock);
}
@@ -315,15 +316,17 @@ int can_share_swap_page(struct page *pag
if (!PageLocked(page))
BUG();
switch (page_count(page)) {
- case 3:
+ case 4:
if (!PagePrivate(page))
break;
/* Fallthrough */
- case 2:
+ case 3:
if (!PageSwapCache(page))
break;
retval = exclusive_swap_page(page);
break;
+ case 2:
+ /* The LRU takes 1 reference */
case 1:
if (PageReserved(page))
break;