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Wed, 30 Jun 2021 07:50:02 -0700 (PDT) Received: from dschatzberg-fedora-PC0Y6AEN ([2620:10d:c091:480::1:1008]) by smtp.gmail.com with ESMTPSA id g4sm5047069qtb.50.2021.06.30.07.50.00 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 30 Jun 2021 07:50:01 -0700 (PDT) Date: Wed, 30 Jun 2021 10:49:58 -0400 From: Dan Schatzberg To: Michal =?iso-8859-1?Q?Koutn=FD?= Cc: Andrew Morton , "open list:BLOCK LAYER" , open list , "open list:CONTROL GROUP (CGROUP)" , "open list:MEMORY MANAGEMENT" , Johannes Weiner , Jens Axboe Subject: Re: [PATCH 3/3] loop: Charge i/o to mem and blk cg Message-ID: References: <20210610173944.1203706-1-schatzberg.dan@gmail.com> <20210610173944.1203706-4-schatzberg.dan@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Authentication-Results: imf09.hostedemail.com; dkim=pass header.d=gmail.com header.s=20161025 header.b=gGD9BAtp; dmarc=pass (policy=none) header.from=gmail.com; spf=pass (imf09.hostedemail.com: domain of schatzbergdan@gmail.com designates 209.85.219.53 as permitted sender) smtp.mailfrom=schatzbergdan@gmail.com X-Stat-Signature: q89pmwnuosyhu7ziddwj5zhr5bjoysmu X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: E27E0300028A X-HE-Tag: 1625064602-890272 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: > This is how I understand it: > > --- a/drivers/block/loop.c > +++ b/drivers/block/loop.c > @@ -996,6 +996,7 @@ static void loop_queue_work(struct loop_device *lo, struct loop_cmd *cmd) > rb_insert_color(&worker->rb_node, &lo->worker_tree); > queue_work: > if (worker) { > + WARN_ON_ONCE(worker->blkcg_css != cmd->blkcg_css); Yes, this is correct. Though the check here seems a bit obvious to me - it must be correct because we assign worker above: if (cur_worker->blkcg_css == cmd->blkcg_css) { worker = cur_worker; break; or when we construct the worker: worker->blkcg_css = cmd->blkcg_css; I think this WARN_ON_ONCE check might be more interesting in loop_process_work which invokes loop_handle_cmd and actually uses cmd->blkcg_css. In any event, your understanding is correct here. > /* > * We need to remove from the idle list here while > * holding the lock so that the idle timer doesn't > @@ -2106,6 +2107,8 @@ static blk_status_t loop_queue_rq(struct blk_mq_hw_ctx *hctx, > cmd->memcg_css = NULL; > #ifdef CONFIG_BLK_CGROUP > if (rq->bio && rq->bio->bi_blkg) { > + /* reference to blkcg_css will be held by loop_worker (outlives > + * cmd) or it is the eternal root css */ Yes, this is correct. Feel free to add my Acked-by to such a patch