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Date:   Wed, 30 Oct 2019 21:16:08 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Roman Gushchin <guro@fb.com>
Cc:     <linux-mm@kvack.org>, Michal Hocko <mhocko@kernel.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        <linux-kernel@vger.kernel.org>, <kernel-team@fb.com>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Daniel Jordan <daniel.m.jordan@oracle.com>,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Subject: Re: [PATCH v2] mm: slab: make page_cgroup_ino() to recognize
 non-compound slab pages properly
Message-Id: <20191030211608.29f8fc92e07fd2ac2ef4d1d3@linux-foundation.org>
In-Reply-To: <20191031012151.2722280-1-guro@fb.com>
References: <20191031012151.2722280-1-guro@fb.com>
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On Wed, 30 Oct 2019 18:21:51 -0700 Roman Gushchin <guro@fb.com> wrote:

> page_cgroup_ino() doesn't return a valid memcg pointer for non-compound
> 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>
> Reviewed-by: Shakeel Butt <shakeelb@google.com>
> Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
> Cc: Daniel Jordan <daniel.m.jordan@oracle.com>

Affects /proc/kpagecgroup, but page_cgroup_ino() is also used in the
memory-failure code - I wonder what effect this bug has there?

IOW, should we backport this into -stable?