linux-kernel - Re: [PATCH v20 09/12] dm: Add support for copy offload

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Date: Wed, 22 May 2024 08:22:35 +0200
From: Hannes Reinecke <hare@...e.de>
To: Nitesh Shetty <nj.shetty@...sung.com>
Cc: Jens Axboe <axboe@...nel.dk>, Jonathan Corbet <corbet@....net>,
 Alasdair Kergon <agk@...hat.com>, Mike Snitzer <snitzer@...nel.org>,
 Mikulas Patocka <mpatocka@...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>,
 Jan Kara <jack@...e.cz>, martin.petersen@...cle.com, bvanassche@....org,
 david@...morbit.com, damien.lemoal@...nsource.wdc.com, anuj20.g@...sung.com,
 joshi.k@...sung.com, nitheshshetty@...il.com, gost.dev@...sung.com,
 linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-doc@...r.kernel.org, dm-devel@...ts.linux.dev,
 linux-nvme@...ts.infradead.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v20 09/12] dm: Add support for copy offload

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@...sung.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@...e.de                                +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
HRB 36809 (AG Nürnberg), GF: I. Totev, A. McDonald, W. Knoblich