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Date:   Sat, 18 Mar 2023 07:33:14 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Jan Kara <jack@suse.cz>
Cc:     Bart Van Assche <bvanassche@acm.org>, Jens Axboe <axboe@kernel.dk>,
        linux-block@vger.kernel.org, Jaegeuk Kim <jaegeuk@kernel.org>,
        Christoph Hellwig <hch@lst.de>, Ming Lei <ming.lei@redhat.com>,
        Damien Le Moal <damien.lemoal@opensource.wdc.com>,
        Johannes Thumshirn <johannes.thumshirn@wdc.com>
Subject: Re: [PATCH 2/2] block: Split and submit bios in LBA order
Message-ID: <20230318063314.GC24880@lst.de>
References: <20230317195938.1745318-1-bvanassche@acm.org> <20230317195938.1745318-3-bvanassche@acm.org> <20230317222813.xkjia2cottjwzls7@quack3>
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On Fri, Mar 17, 2023 at 11:28:13PM +0100, Jan Kara wrote:
> > -	rq_list_add(&plug->mq_list, rq);
> > +	last_p = &plug->mq_list;
> > +	while (*last_p)
> > +		last_p = &(*last_p)->rq_next;
> > +	rq_list_add_tail(&last_p, rq);
> >  	plug->rq_count++;
> >  }
> 
> Uh, I don't think we want to traverse the whole plug list each time we are
> adding a request to it. We have just recently managed to avoid that at
> least for single-device cases and apparently it was a win for fast devices
> (see commit d38a9c04c0d5 ("block: only check previous entry for plug merge
> attempt")).

REQ_OP_WRITE request for zoned devices are never added to the plug
list, so all of this actually is dead (and probably untested?) code.

See blk_mq_plug() and bdev_op_is_zoned_write().