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

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Message-ID: <20191030194027.GA6679@castle.DHCP.thefacebook.com>
Date:   Wed, 30 Oct 2019 19:40:34 +0000
From:   Roman Gushchin <guro@...com>
To:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
CC:     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 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().
> 
> Before this patch:
> [root@...alhost ~]# ./page-types -c /sys/fs/cgroup/user.slice/user-0.slice/user@...ervice/ | grep slab
> 0x0000000000000080	        38        0  _______S___________________________________	slab
> 
> After this patch:
> [root@...alhost ~]# ./page-types -c /sys/fs/cgroup/user.slice/user-0.slice/user@...ervice/ | 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@...com>
> Cc: Vladimir Davydov <vdavydov.dev@...il.com>
> Cc: Shakeel Butt <shakeelb@...gle.com>

Ping?

> ---
>  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
>