memcg: remove a redundant check

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.

A racy case is that a newly-swapped-in anonymous page is referred from
prural threads in do_swap_page() at the same time.
(a page is not Locked when mem_cgroup_charge() is called from do_swap_page.)

Another case is shmem. It charges its page before calling add_to_page_cache().
Then, mem_cgroup_charge_cache() is called twice. This case is handled in
mem_cgroup_cache_charge(). But this check may be too hacky...

Signed-off-by : KAMEZAWA Hiroyuki <[e-mail]>
Cc: Balbir Singh <[e-mail]>
Cc: "Eric W. Biederman" <[e-mail]>
Cc: Pavel Emelyanov <[e-mail]>
Cc: Li Zefan <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Cc: YAMAMOTO Takashi <[e-mail]>
Cc: Paul Menage <[e-mail]>
Cc: David Rientjes <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/memcontrol.c

···
 	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 == NULL))
 		goto err;
···
 	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);
 
···
 int mem_cgroup_cache_charge(struct page *page, struct mm_struct *mm,
 				gfp_t gfp_mask)
 {
+	/*
+	 * Corner case handling. This is called from add_to_page_cache()
+	 * in usual. But some FS (shmem) precharges this page before calling it
+	 * and call add_to_page_cache() with GFP_NOWAIT.
+	 *
+	 * For GFP_NOWAIT case, the page may be pre-charged before calling
+	 * add_to_page_cache(). (See shmem.c) check it here and avoid to call
+	 * charge twice. (It works but has to pay a bit larger cost.)
+	 */
+	if (!(gfp_mask & __GFP_WAIT)) {
+		struct page_cgroup *pc;
+
+		lock_page_cgroup(page);
+		pc = page_get_page_cgroup(page);
+		if (pc) {
+			VM_BUG_ON(pc->page != page);
+			VM_BUG_ON(!pc->mem_cgroup);
+			unlock_page_cgroup(page);
+			return 0;
+		}
+		unlock_page_cgroup(page);
+	}
+
 	if (unlikely(!mm))
 		mm = &init_mm;
+
 	return mem_cgroup_charge_common(page, mm, gfp_mask,
 				MEM_CGROUP_CHARGE_TYPE_CACHE, NULL);
 }