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

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Message-ID: <20191030213513.GA15706@castle.DHCP.thefacebook.com>
Date:   Wed, 30 Oct 2019 21:35:19 +0000
From:   Roman Gushchin <guro@...com>
To:     Daniel Jordan <daniel.m.jordan@...cle.com>
CC:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Michal Hocko <mhocko@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kernel Team <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

On Wed, Oct 30, 2019 at 05:03:14PM -0400, Daniel Jordan wrote:
> 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.

Good catch, thank you! Will send v2 in no time.