linux-kernel - Re: [PATCH] mm: slab: make page_cgroup_ino() to recognize non-compound slab pages properly

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Message-ID: <20191030210314.2el7wysojucqypoq@ca-dmjordan1.us.oracle.com>
Date:   Wed, 30 Oct 2019 17:03:14 -0400
From:   Daniel Jordan <daniel.m.jordan@...cle.com>
To:     Roman Gushchin <guro@...com>
Cc:     linux-mm@...ck.org, Michal Hocko <mhocko@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        linux-kernel@...r.kernel.org, kernel-team@...com,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Shakeel Butt <shakeelb@...gle.com>
Subject: Re: [PATCH] mm: slab: make page_cgroup_ino() to recognize
 non-compound slab pages properly

Hi Roman,

On Fri, Oct 25, 2019 at 04:27:10PM -0700, Roman Gushchin wrote:
> 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().

You may also want to update the comment above memcg_from_slab_page():

 * So this function assumes that the page can pass PageHead() and PageSlab()
 * checks.