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

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Message-ID: <YNXsid0Wg5X9/hAC@blackbook>
Date:   Fri, 25 Jun 2021 16:47:37 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Dan Schatzberg <schatzberg.dan@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "open list:BLOCK LAYER" <linux-block@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:CONTROL GROUP (CGROUP)" <cgroups@...r.kernel.org>,
        "open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Tejun Heo <tj@...nel.org>, Chris Down <chris@...isdown.name>,
        Jens Axboe <axboe@...nel.dk>,
        Shakeel Butt <shakeelb@...gle.com>
Subject: Re: [PATCH 2/3] mm: Charge active memcg when no mm is set

On Thu, Jun 10, 2021 at 10:39:43AM -0700, Dan Schatzberg <schatzberg.dan@...il.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@...e.com>

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