Re: [PATCH 2/3] mm: Charge active memcg when no mm is set

From: "Michal Koutný" <mkoutny@suse.com>
To: Dan Schatzberg <schatzberg.dan@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	"open list:BLOCK LAYER" <linux-block@vger.kernel.org>,
	open list <linux-kernel@vger.kernel.org>,
	"open list:CONTROL GROUP (CGROUP)" <cgroups@vger.kernel.org>,
	"open list:MEMORY MANAGEMENT" <linux-mm@kvack.org>,
	Johannes Weiner <hannes@cmpxchg.org>, Tejun Heo <tj@kernel.org>,
	Chris Down <chris@chrisdown.name>, Jens Axboe <axboe@kernel.dk>,
	Shakeel Butt <shakeelb@google.com>
Subject: Re: [PATCH 2/3] mm: Charge active memcg when no mm is set
Date: Fri, 25 Jun 2021 16:47:37 +0200	[thread overview]
Message-ID: <YNXsid0Wg5X9/hAC@blackbook> (raw)
In-Reply-To: <20210610173944.1203706-3-schatzberg.dan@gmail.com>

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On Thu, Jun 10, 2021 at 10:39:43AM -0700, Dan Schatzberg <schatzberg.dan@gmail.com> wrote:
> @@ -926,8 +937,17 @@ struct mem_cgroup *get_mem_cgroup_from_mm(struct mm_struct *mm)
>  	 * counting is disabled on the root level in the
>  	 * cgroup core. See CSS_NO_REF.
>  	 */
> -	if (unlikely(!mm))
> -		return root_mem_cgroup;
> +	if (unlikely(!mm)) {
> +		memcg = active_memcg();
> +		if (unlikely(memcg)) {
> +			/* remote memcg must hold a ref */
> +			css_get(&memcg->css);
> +			return memcg;
> +		}
> +		mm = current->mm;
> +		if (unlikely(!mm))
> +			return root_mem_cgroup;
> +	}

With the change in __add_to_page_cache_locked() all page cache charges
will supply null mm, so the first !mm unlikely hint may not be warranted
anymore. Just an interesting point, generally, I'm adding

Reviewed-by: Michal Koutný <mkoutny@suse.com>

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