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Subject[patch 4/8] mm: memcg: lookup_page_cgroup (almost) never returns NULL
Date
From: Johannes Weiner <jweiner@redhat.com>

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
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.

Signed-off-by: Johannes Weiner <jweiner@redhat.com>
---
mm/memcontrol.c | 8 ++------
1 files changed, 2 insertions(+), 6 deletions(-)
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index d825af9..d4d139a 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 +3321,7 @@ static struct page_cgroup *lookup_page_cgroup_used(struct page *page)
struct page_cgroup *pc;

pc = lookup_page_cgroup(page);
+ /* Can be NULL while bootstrapping the page allocator */
if (likely(pc) && PageCgroupUsed(pc))
return pc;
return NULL;
--
1.7.6.4


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