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Date:   Fri, 11 Aug 2023 14:33:35 -0700
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Subject: Re: [dm-devel] [PATCH v14 03/11] block: add copy offload support
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To:     Nitesh Shetty <nj.shetty@samsung.com>,
        Jens Axboe <axboe@kernel.dk>, Jonathan Corbet <corbet@lwn.net>,
        Alasdair Kergon <agk@redhat.com>,
        Mike Snitzer <snitzer@kernel.org>, dm-devel@redhat.com,
        Keith Busch <kbusch@kernel.org>,
        Christoph Hellwig <hch@lst.de>,
        Sagi Grimberg <sagi@grimberg.me>,
        Chaitanya Kulkarni <kch@nvidia.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <brauner@kernel.org>
Cc:     martin.petersen@oracle.com, linux-doc@vger.kernel.org,
        gost.dev@samsung.com, Anuj Gupta <anuj20.g@samsung.com>,
        linux-kernel@vger.kernel.org, linux-nvme@lists.infradead.org,
        linux-block@vger.kernel.org, mcgrof@kernel.org, dlemoal@kernel.org,
        linux-fsdevel@vger.kernel.org
References: <20230811105300.15889-1-nj.shetty@samsung.com>
 <CGME20230811105659epcas5p1982eeaeb580c4cb9b23a29270945be08@epcas5p1.samsung.com>
 <20230811105300.15889-4-nj.shetty@samsung.com>
From:   Bart Van Assche <bvanassche@acm.org>
In-Reply-To: <20230811105300.15889-4-nj.shetty@samsung.com>
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Precedence: bulk

On 8/11/23 03:52, Nitesh Shetty wrote:
> + * Description:
> + *	Copy source offset to destination offset within block device, using
> + *	device's native copy offload feature.

Offloading the copy operation is not guaranteed so I think that needs to 
be reflected in the above comment.

> + *	We perform copy operation by sending 2 bio's.
> + *	1. We take a plug and send a REQ_OP_COPY_SRC bio along with source
> + *	sector and length. Once this bio reaches request layer, we form a
> + *	request and wait for dst bio to arrive.

What will happen if the queue depth of the request queue at the bottom 
is one?

> +		blk_start_plug(&plug);
> +		dst_bio = blk_next_bio(src_bio, bdev, 0, REQ_OP_COPY_DST, gfp);

blk_next_bio() can return NULL so its return value should be checked.

> +		dst_bio->bi_iter.bi_size = chunk;
> +		dst_bio->bi_iter.bi_sector = pos_out >> SECTOR_SHIFT;
> +		dst_bio->bi_end_io = blkdev_copy_offload_dst_endio;
> +		dst_bio->bi_private = offload_io;

Thanks,

Bart.