linux-kernel - Re: [PATCH v6 3/9] block: add emulation for copy

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Message-ID: <bfec42d1-a1bf-3b3a-10dd-8d3db0a6e6a0@suse.de>
Date:   Thu, 12 Jan 2023 15:46:16 +0100
From:   Hannes Reinecke <hare@...e.de>
To:     Nitesh Shetty <nj.shetty@...sung.com>,
        Jens Axboe <axboe@...nel.dk>, 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>,
        James Smart <james.smart@...adcom.com>,
        Chaitanya Kulkarni <kch@...dia.com>,
        Alexander Viro <viro@...iv.linux.org.uk>
Cc:     anuj20.g@...sung.com, joshi.k@...sung.com, p.raghav@...sung.com,
        nitheshshetty@...il.com, gost.dev@...sung.com,
        Vincent Fu <vincent.fu@...sung.com>,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v6 3/9] block: add emulation for copy

On 1/12/23 12:58, Nitesh Shetty wrote:
> For the devices which does not support copy, copy emulation is
> added. Copy-emulation is implemented by reading from source ranges
> into memory and writing to the corresponding destination asynchronously.
> For zoned device we maintain a linked list of read submission and try to
> submit corresponding write in same order.
> Also emulation is used, if copy offload fails or partially completes.
> 
> Signed-off-by: Nitesh Shetty <nj.shetty@...sung.com>
> Signed-off-by: Vincent Fu <vincent.fu@...sung.com>
> Signed-off-by: Anuj Gupta <anuj20.g@...sung.com>
> ---
>   block/blk-lib.c        | 241 ++++++++++++++++++++++++++++++++++++++++-
>   block/blk-map.c        |   4 +-
>   include/linux/blkdev.h |   3 +
>   3 files changed, 245 insertions(+), 3 deletions(-)
> 
I'm not sure if I agree with this one.

You just submitted a patch for device-mapper to implement copy offload, 
which (to all intents and purposes) _is_ an emulation.

So why do we need to implement it in the block layer as an emulation?
Or, if we have to, why do we need the device-mapper emulation?
This emulation will be doing the same thing, no?

Cheers,

Hannes