From mboxrd@z Thu Jan  1 00:00:00 1970
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Date: Thu, 28 Jun 2018 07:50:06 +0800
From: Ming Lei <ming.lei@redhat.com>
To: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Jens Axboe <axboe@kernel.dk>, linux-block@vger.kernel.org,
	Christoph Hellwig <hch@lst.de>, Mike Snitzer <snitzer@redhat.com>,
	Hannes Reinecke <hare@suse.com>,
	Johannes Thumshirn <jthumshirn@suse.de>
Subject: Re: [PATCH] block: Make __bio_clone_fast() copy bi_vcnt
Message-ID: <20180627235005.GE7583@ming.t460p>
References: <20180627201231.15641-1-bart.vanassche@wdc.com>
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In-Reply-To: <20180627201231.15641-1-bart.vanassche@wdc.com>
List-ID: <linux-block@vger.kernel.org>

On Wed, Jun 27, 2018 at 01:12:31PM -0700, Bart Van Assche wrote:
> Although __bio_clone_fast() copies bi_io_vec, it does not copy bi_vcnt,
> the number of elements in bi_io_vec[] that contains data. Copy bi_vcnt
> such that code that needs this member behaves identically for original
> and for cloned requests.
> 
> Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Mike Snitzer <snitzer@redhat.com>
> Cc: Ming Lei <ming.lei@redhat.com>
> Cc: Hannes Reinecke <hare@suse.com>
> Cc: Johannes Thumshirn <jthumshirn@suse.de>
> ---
>  block/bio.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/block/bio.c b/block/bio.c
> index f7e3d88bd0b6..55f8e0dedd69 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -605,6 +605,7 @@ void __bio_clone_fast(struct bio *bio, struct bio *bio_src)
>  	bio->bi_opf = bio_src->bi_opf;
>  	bio->bi_write_hint = bio_src->bi_write_hint;
>  	bio->bi_iter = bio_src->bi_iter;
> +	bio->bi_vcnt = bio_src->bi_vcnt;
>  	bio->bi_io_vec = bio_src->bi_io_vec;

No, don't do that.

For a cloned bio, both .bi_vcnt and .bi_io_vec can't be used directly,
and we have done huge such cleanup for supporting multipage bvec, that is
the great idea of immutable bvecs invented by Kent.

Thanks,
Ming