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From:   Roman Gushchin <guro@fb.com>
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To:     <linux-mm@kvack.org>
CC:     Michal Hocko <mhocko@kernel.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        <linux-kernel@vger.kernel.org>, <kernel-team@fb.com>,
        Roman Gushchin <guro@fb.com>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Shakeel Butt <shakeelb@google.com>
Smtp-Origin-Cluster: prn2c23
Subject: [PATCH] mm: slab: make page_cgroup_ino() to recognize non-compound slab pages properly
Date:   Fri, 25 Oct 2019 16:27:10 -0700
Message-ID: <20191025232710.4081957-1-guro@fb.com>
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Content-Type: text/plain
Sender: linux-kernel-owner@vger.kernel.org
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page_cgroup_ino() doesn't return a valid memcg pointer for non-compund
slab pages, because it depends on PgHead AND PgSlab flags to be set
to determine the memory cgroup from the kmem_cache.
It's correct for compound pages, but not for generic small pages. Those
don't have PgHead set, so it ends up returning zero.

Fix this by replacing the condition to PageSlab() && !PageTail().

Before this patch:
[root@localhost ~]# ./page-types -c /sys/fs/cgroup/user.slice/user-0.slice/user@0.service/ | grep slab
0x0000000000000080	        38        0  _______S___________________________________	slab

After this patch:
[root@localhost ~]# ./page-types -c /sys/fs/cgroup/user.slice/user-0.slice/user@0.service/ | grep slab
0x0000000000000080	       147        0  _______S___________________________________	slab

Fixes: 4d96ba353075 ("mm: memcg/slab: stop setting page->mem_cgroup pointer for slab pages")
Signed-off-by: Roman Gushchin <guro@fb.com>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: Shakeel Butt <shakeelb@google.com>
---
 mm/memcontrol.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index ea085877c548..00b4188b1bed 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -476,7 +476,7 @@ ino_t page_cgroup_ino(struct page *page)
 	unsigned long ino = 0;
 
 	rcu_read_lock();
-	if (PageHead(page) && PageSlab(page))
+	if (PageSlab(page) && !PageTail(page))
 		memcg = memcg_from_slab_page(page);
 	else
 		memcg = READ_ONCE(page->mem_cgroup);
-- 
2.17.1