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Date:   Fri, 12 Nov 2021 09:21:40 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Ming Lei <ming.lei@redhat.com>
Cc:     Jens Axboe <axboe@kernel.dk>, linux-block@vger.kernel.org,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH] blk-mq: setup blk_mq_alloc_data.cmd_flags after
 submit_bio_checks() is done
Message-ID: <20211112082140.GA30681@lst.de>
References: <20211112081137.406930-1-ming.lei@redhat.com>
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On Fri, Nov 12, 2021 at 04:11:37PM +0800, Ming Lei wrote:
> @@ -2564,13 +2564,15 @@ static inline struct request *blk_mq_get_request(struct request_queue *q,
>  			if (blk_mq_attempt_bio_merge(q, bio, nsegs,
>  						same_queue_rq))
>  				return NULL;
> +			if (bio->bi_opf != rq->cmd_flags)
> +				goto fallback;

I think this deserves a comment, as this means a read prealloc
can only be used for reads, and no fua can be set if the preallocating
I/O didn't use fua, etc.

What are the pitfalls of just chanigng cmd_flags?