linux-kernel - Re: [dm-devel] [PATCH v14 03/11] block: add copy offload support

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Message-ID: <0899ddc3-d9c1-3d9a-3649-2b1add9b2a7f@acm.org>
Date:   Fri, 11 Aug 2023 14:33:35 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Nitesh Shetty <nj.shetty@...sung.com>,
        Jens Axboe <axboe@...nel.dk>, Jonathan Corbet <corbet@....net>,
        Alasdair Kergon <agk@...hat.com>,
        Mike Snitzer <snitzer@...nel.org>, dm-devel@...hat.com,
        Keith Busch <kbusch@...nel.org>,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>,
        Chaitanya Kulkarni <kch@...dia.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>
Cc:     martin.petersen@...cle.com, linux-doc@...r.kernel.org,
        gost.dev@...sung.com, Anuj Gupta <anuj20.g@...sung.com>,
        linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-block@...r.kernel.org, mcgrof@...nel.org, dlemoal@...nel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [dm-devel] [PATCH v14 03/11] block: add copy offload support

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.