[RFC][PATCH 7/8] memcg: remove redundant checks

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

This patch needs review ;)

Because of remove refcnt patch, it's very rare case to that
mem_cgroup_charge_common() is called against a page which is accounted.

mem_cgroup_charge_common() is called when.
 1. a page is added into file cache.
 2. an anon page is newly mapped.
 3. a file-cache page is newly mapped.

To rise a racy condition, above action against a page should occur
at once.

And, for case 3, we don't account it because it's already file cache.
For avoiding accounting "File cache but not mapped" page, checking
page->mapping is better than checking page_mapped().

Signed-off-by : KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

---
 mm/memcontrol.c |   39 ++++++++++-----------------------------
 1 file changed, 10 insertions(+), 29 deletions(-)

Index: mm-2.6.25-mm1/mm/memcontrol.c
===================================================================
--- mm-2.6.25-mm1.orig/mm/memcontrol.c
+++ mm-2.6.25-mm1/mm/memcontrol.c
@@ -527,28 +527,6 @@ static int mem_cgroup_charge_common(stru
 	if (mem_cgroup_subsys.disabled)
 		return 0;
 
-	/*
-	 * Should page_cgroup's go to their own slab?
-	 * One could optimize the performance of the charging routine
-	 * by saving a bit in the page_flags and using it as a lock
-	 * to see if the cgroup page already has a page_cgroup associated
-	 * with it
-	 */
-retry:
-	lock_page_cgroup(page);
-	pc = page_get_page_cgroup(page);
-	/*
-	 * The page_cgroup exists and
-	 * the page has already been accounted.
-	 */
-	if (unlikely(pc)) {
-		VM_BUG_ON(pc->page != page);
-		VM_BUG_ON(!pc->mem_cgroup);
-		unlock_page_cgroup(page);
-		goto done;
-	}
-	unlock_page_cgroup(page);
-
 	pc = kmem_cache_alloc(page_cgroup_cache, gfp_mask);
 	if (unlikely(!pc))
 		goto err;
@@ -604,15 +582,10 @@ retry:
 	lock_page_cgroup(page);
 	if (unlikely(page_get_page_cgroup(page))) {
 		unlock_page_cgroup(page);
-		/*
-		 * Another charge has been added to this page already.
-		 * We take lock_page_cgroup(page) again and read
-		 * page->cgroup, increment refcnt.... just retry is OK.
-		 */
 		res_counter_uncharge(&mem->res, PAGE_SIZE);
 		css_put(&mem->css);
 		kmem_cache_free(page_cgroup_cache, pc);
-		goto retry;
+		goto done;
 	}
 	page_assign_page_cgroup(page, pc);
 
@@ -633,7 +606,15 @@ err:
 
 int mem_cgroup_charge(struct page *page, struct mm_struct *mm, gfp_t gfp_mask)
 {
-	if (page_mapped(page))
+	/*
+	 * If a page is file cache, it's already charged. If a page is anon
+	 * and mapped, it's already charged. We accounts only *new* anon
+	 * page here. We cannot use !PageAnon() here because a new page for
+	 * anon is not marked as an anon page. we just checks a page has some
+	 * objrmap context or not.
+	 * Note that swap-cache's page->mapping is NULL. So we have no problem.
+	 */
+	if (page->mapping)
 		return 0;
 	if (unlikely(!mm))
 		mm = &init_mm;

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