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Date: Tue, 15 Sep 2026 08:49:50 +0200
From: Christoph Hellwig <hch@lst.de>
To: Yu Kuai <yukuai@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>, Josef Bacik <josef@toxicpanda.com>,
	Tejun Heo <tj@kernel.org>, Johannes Weiner <hannes@cmpxchg.org>,
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Subject: Re: [PATCH v2 2/3] blk-cgroup: store blkcg in bio instead of blkg
Message-ID: <20260915064950.GA13417@lst.de>
References: <cover.1789237876.git.yukuai@fygo.io> <ef547fdb7a6c8e39a25a1b9473c444f24a0f03be.1789237876.git.yukuai@fygo.io>
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In-Reply-To: <ef547fdb7a6c8e39a25a1b9473c444f24a0f03be.1789237876.git.yukuai@fygo.io>
User-Agent: Mutt/1.5.17 (2007-11-01)

On Sun, Sep 13, 2026 at 02:54:39PM +0800, Yu Kuai wrote:
> @@ -6243,10 +6245,17 @@ static void bfq_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
>  	bool idle_timer_disabled = false;
>  	blk_opf_t cmd_flags;
>  	LIST_HEAD(free);
>  
>  #ifdef CONFIG_BFQ_GROUP_IOSCHED
> +	/*
> +	 * Pin the blkg used to look up bfqg.  If this is the first IO for
> +	 * the blkcg on this queue, create the bfqg before holding bfqd->lock.
> +	 */
> +	if (rq->bio && !bio_flagged(rq->bio, BIO_BLKG_REF))
> +		bio_blkg(rq->bio);

Can rq->bio be NULL here?  flush or empty passthrough request should
never enter the scheuler.

>  	if (!cgroup_subsys_on_dfl(io_cgrp_subsys) && rq->bio)
>  		bfqg_stats_update_legacy_io(q, rq);

But I guess given that the code below checks it, we should at least
be consistent, so I guess it is better to keep it for now.

>  
>  static inline void dm_bio_record(struct dm_bio_details *bd, struct bio *bio)
>  {
>  	bd->bi_bdev = bio->bi_bdev;
> -	bd->bi_flags = bio->bi_flags;
> +	/* Restoring an ownership bit cannot recreate the associated reference. */
> +	bd->bi_flags = bio->bi_flags & ~(1U << BIO_BLKG_REF);

I have a bit of a hard time parsing this, the line also is overly
long.