[patch 4/7] mm: memcg: lookup_page_cgroup (almost) never returns NULL

[Johannes Weiner]

From: Johannes Weiner <jweiner@xxxxxxxxxx>

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.

Signed-off-by: Johannes Weiner <jweiner@xxxxxxxxxx>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxx>
---
 mm/memcontrol.c |   11 +++++------
 1 files changed, 5 insertions(+), 6 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index d825af9..8ccb342 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1894,9 +1894,6 @@ void mem_cgroup_update_page_stat(struct page *page,
 	bool need_unlock = false;
 	unsigned long uninitialized_var(flags);
 
-	if (unlikely(!pc))
-		return;
-
 	rcu_read_lock();
 	memcg = pc->mem_cgroup;
 	if (unlikely(!memcg || !PageCgroupUsed(pc)))
@@ -2669,8 +2666,6 @@ 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;
@@ -2942,7 +2937,7 @@ __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);
@@ -3326,6 +3321,10 @@ 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.
+	 */
 	if (likely(pc) && PageCgroupUsed(pc))
 		return pc;
 	return NULL;
-- 
1.7.6.4

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