mm: memcg: lookup_page_cgroup (almost) never returns NULL

Pages have their corresponding page_cgroup descriptors set up before
they are used in userspace, and thus managed by a memory cgroup.

The only time where lookup_page_cgroup() can return NULL is in the
CONFIG_DEBUG_VM-only page sanity checking code that executes while
feeding pages into the page allocator for the first time.

Remove the NULL checks against lookup_page_cgroup() results from all
callsites where we know that corresponding page_cgroup descriptors must
be allocated, and add a comment to the callsite that actually does have
to check the return value.

[hughd@google.com: stop oops in mem_cgroup_update_page_stat()]
Signed-off-by: NJohannes Weiner <jweiner@redhat.com>
Acked-by: NKAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: NMichal Hocko <mhocko@suse.cz>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: NHugh Dickins <hughd@google.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -1960,7 +1960,7 @@ void mem_cgroup_update_page_stat(struct page *page,
 	bool need_unlock = false;
 	unsigned long uninitialized_var(flags);
 
-	if (unlikely(!pc))
+	if (mem_cgroup_disabled())
 		return;
 
 	rcu_read_lock();
@@ -2735,8 +2735,6 @@ static int mem_cgroup_charge_common(struct page *page, struct mm_struct *mm,
 	}
 
 	pc = lookup_page_cgroup(page);
-	BUG_ON(!pc); /* XXX: remove this and move pc lookup into commit */
-
 	ret = __mem_cgroup_try_charge(mm, gfp_mask, nr_pages, &memcg, oom);
 	if (ret || !memcg)
 		return ret;
@@ -3008,7 +3006,7 @@ __mem_cgroup_uncharge_common(struct page *page, enum charge_type ctype)
 	 * Check if our page_cgroup is valid
 	 */
 	pc = lookup_page_cgroup(page);
-	if (unlikely(!pc || !PageCgroupUsed(pc)))
+	if (unlikely(!PageCgroupUsed(pc)))
 		return NULL;
 
 	lock_page_cgroup(pc);
@@ -3436,6 +3434,11 @@ static struct page_cgroup *lookup_page_cgroup_used(struct page *page)
 	struct page_cgroup *pc;
 
 	pc = lookup_page_cgroup(page);
+	/*
+	 * Can be NULL while feeding pages into the page allocator for
+	 * the first time, i.e. during boot or memory hotplug;
+	 * or when mem_cgroup_disabled().
+	 */
 	if (likely(pc) && PageCgroupUsed(pc))
 		return pc;
 	return NULL;