linux-kernel - Re: [PATCH] mm: memcg: keep root group unchanged if fail to create new

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Date:	Sun, 11 Dec 2011 15:39:43 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	Hillf Danton <dhillf@...il.com>
cc:	Balbir Singh <bsingharora@...il.com>,
	David Rientjes <rientjes@...gle.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: memcg: keep root group unchanged if fail to create
 new

On Sun, 11 Dec 2011, Hillf Danton wrote:

> If the request is not to create root group and we fail to meet it,
> we'd leave the root unchanged.

I didn't understand that at first: please say "we should" rather
than "we'd", which I take to be an abbreviation for "we would".

> 
> Signed-off-by: Hillf Danton <dhillf@...il.com>

Yes indeed, well caught:
Acked-by: Hugh Dickins <hughd@...gle.com>

I wonder what was going through the author's mind when he wrote it
that way?  I wonder if it's one of those bugs that creeps in when
you start from a perfectly functional patch, then make refinements
to suit feedback from reviewers.

On which topic: wouldn't this patch be better just to move the
"root_mem_cgroup = memcg;" two lines lower down (and of course
remove free_out's "root_mem_cgroup = NULL;" as you already did)?
I can't see mem_cgroup_soft_limit_tree_init() relying on
root_mem_cgroup at all.

Should your patch go to stable, even to 2.6.32-longterm?
Matter of taste, really: while it's not quite impossible for
alloc_mem_cgroup_per_zone_info() to fail, it is very unlikely.

> ---
> 
> --- a/mm/memcontrol.c	Fri Dec  9 21:57:40 2011
> +++ b/mm/memcontrol.c	Sun Dec 11 09:04:48 2011
> @@ -4849,8 +4849,10 @@ mem_cgroup_create(struct cgroup_subsys *
>  		enable_swap_cgroup();
>  		parent = NULL;
>  		root_mem_cgroup = memcg;
> -		if (mem_cgroup_soft_limit_tree_init())
> +		if (mem_cgroup_soft_limit_tree_init()) {
> +			root_mem_cgroup = NULL;
>  			goto free_out;
> +		}
>  		for_each_possible_cpu(cpu) {
>  			struct memcg_stock_pcp *stock =
>  						&per_cpu(memcg_stock, cpu);
> @@ -4888,7 +4890,6 @@ mem_cgroup_create(struct cgroup_subsys *
>  	return &memcg->css;
>  free_out:
>  	__mem_cgroup_free(memcg);
> -	root_mem_cgroup = NULL;
>  	return ERR_PTR(error);
>  }
> 
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