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Date: Wed, 22 May 2024 08:22:35 +0200
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Subject: Re: [PATCH v20 09/12] dm: Add support for copy offload
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To: Nitesh Shetty <nj.shetty@samsung.com>
Cc: Jens Axboe <axboe@kernel.dk>, Jonathan Corbet <corbet@lwn.net>,
 Alasdair Kergon <agk@redhat.com>, Mike Snitzer <snitzer@kernel.org>,
 Mikulas Patocka <mpatocka@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>,
 Jan Kara <jack@suse.cz>, martin.petersen@oracle.com, bvanassche@acm.org,
 david@fromorbit.com, damien.lemoal@opensource.wdc.com, anuj20.g@samsung.com,
 joshi.k@samsung.com, nitheshshetty@gmail.com, gost.dev@samsung.com,
 linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
 linux-doc@vger.kernel.org, dm-devel@lists.linux.dev,
 linux-nvme@lists.infradead.org, linux-fsdevel@vger.kernel.org
References: <20240520102033.9361-1-nj.shetty@samsung.com>
 <CGME20240520103004epcas5p4a18f3f6ba0f218d57b0ab4bb84c6ff18@epcas5p4.samsung.com>
 <20240520102033.9361-10-nj.shetty@samsung.com>
 <41228a01-9d0c-415d-9fef-a3d2600b1dfa@suse.de>
 <20240521140850.m6ppy2sxv457gxgs@green245>
From: Hannes Reinecke <hare@suse.de>
In-Reply-To: <20240521140850.m6ppy2sxv457gxgs@green245>
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On 5/21/24 16:08, Nitesh Shetty wrote:
> On 21/05/24 09:11AM, Hannes Reinecke wrote:
>> On 5/20/24 12:20, Nitesh Shetty wrote:
>>> Before enabling copy for dm target, check if underlying devices and
>>> dm target support copy. Avoid split happening inside dm target.
>>> Fail early if the request needs split, currently splitting copy
>>> request is not supported.
>>>
>>> Signed-off-by: Nitesh Shetty <nj.shetty@samsung.com>
>>> ---
>>> @@ -397,6 +397,9 @@ struct dm_target {
>>>       * bio_set_dev(). NOTE: ideally a target should _not_ need this.
>>>       */
>>>      bool needs_bio_set_dev:1;
>>> +
>>> +    /* copy offload is supported */
>>> +    bool copy_offload_supported:1;
>>>  };
>>>  void *dm_per_bio_data(struct bio *bio, size_t data_size);
>>
>> Errm. Not sure this will work. DM tables might be arbitrarily, 
>> requiring us to _split_ the copy offload request according to the 
>> underlying component devices. But we explicitly disallowed a split in 
>> one of the earlier patches.
>> Or am I wrong?
>>
> Yes you are right w.r.to split, we disallow split.
> But this flag indicates whether we support copy offload in dm-target or
> not. At present we support copy offload only in dm-linear.
> For other dm-target, eventhough underlaying device supports copy
> offload, dm-target based on it wont support copy offload.
> If the present series get merged, we can test and integrate more
> targets.
> 
But dm-linear can be concatenated, too; you can easily use dm-linear
to tie several devices together.
Which again would require a copy-offload range to be split.
Hmm?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@suse.de                                +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
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