linux-kernel - Re: [PATCH 3/3] loop: Charge i/o to mem and blk cg

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Date:   Fri, 25 Jun 2021 17:01:03 +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>,
        Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH 3/3] loop: Charge i/o to mem and blk cg

Hi.

On Thu, Jun 10, 2021 at 10:39:44AM -0700, Dan Schatzberg <schatzberg.dan@...il.com> wrote:
> The current code only associates with the existing blkcg when aio is
> used to access the backing file. This patch covers all types of i/o to
> the backing file and also associates the memcg so if the backing file is
> on tmpfs, memory is charged appropriately.
> 
> This patch also exports cgroup_get_e_css and int_active_memcg so it
> can be used by the loop module.

Wouldn't it be clearer to export (not explicitly inlined anymore)
set_active_memcg() instead of the int_active_memcg that's rather an
implementation detail?

> @@ -2111,13 +2112,18 @@ static blk_status_t loop_queue_rq(struct blk_mq_hw_ctx *hctx,
>  	}
>  
>  	/* always use the first bio's css */
> +	cmd->blkcg_css = NULL;
> +	cmd->memcg_css = NULL;
>  #ifdef CONFIG_BLK_CGROUP
> -	if (cmd->use_aio && rq->bio && rq->bio->bi_blkg) {
> -		cmd->css = &bio_blkcg(rq->bio)->css;
> -		css_get(cmd->css);
> -	} else
> +	if (rq->bio && rq->bio->bi_blkg) {
> +		cmd->blkcg_css = &bio_blkcg(rq->bio)->css;
> +#ifdef CONFIG_MEMCG
> +		cmd->memcg_css =
> +			cgroup_get_e_css(cmd->blkcg_css->cgroup,
> +					&memory_cgrp_subsys);
> +#endif
> +	}
>  #endif
> -		cmd->css = NULL;
>  	loop_queue_work(lo, cmd);

I see you dropped the cmd->blkcg_css reference (while rq is handled). Is
it intentional? 

Thanks,
Michal

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