linux-kernel - Re: [PATCH v20 12/12] null_blk: add support for copy offload

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Date: Thu, 23 May 2024 06:55:14 +0000
From: Nitesh Shetty <nj.shetty@...sung.com>
To: Bart Van Assche <bvanassche@....org>
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, david@...morbit.com, hare@...e.de,
	damien.lemoal@...nsource.wdc.com, anuj20.g@...sung.com, joshi.k@...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-doc@...r.kernel.org,
	dm-devel@...ts.linux.dev, linux-nvme@...ts.infradead.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v20 12/12] null_blk: add support for copy offload

On 22/05/24 10:52AM, Bart Van Assche wrote:
>On 5/21/24 07:46, Nitesh Shetty wrote:
>>On 20/05/24 04:42PM, Bart Van Assche wrote:
>>>On 5/20/24 03:20, Nitesh Shetty wrote:
>>>>+    __rq_for_each_bio(bio, req) {
>>>>+        if (seg == blk_rq_nr_phys_segments(req)) {
>>>>+            sector_in = bio->bi_iter.bi_sector;
>>>>+            if (rem != bio->bi_iter.bi_size)
>>>>+                return status;
>>>>+        } else {
>>>>+            sector_out = bio->bi_iter.bi_sector;
>>>>+            rem = bio->bi_iter.bi_size;
>>>>+        }
>>>>+        seg++;
>>>>+    }
>>>
>>>_rq_for_each_bio() iterates over the bios in a request. Does a copy
>>>offload request always have two bios - one copy destination bio and
>>>one copy source bio? If so, is 'seg' a bio counter? Why is that bio
>>>counter compared with the number of physical segments in the request?
>>>
>>Yes, your observation is right. We are treating first bio as dst and
>>second as src. If not for that comparision, we might need to store the
>>index in a temporary variable and parse based on index value.
>
>I'm still wondering why 'seg' is compared with blk_rq_nr_phys_segments(req).
>
In this case blk_rq_nr_phys_segments is used as counter value(==2), which tells
its a src IO. But using a macro instead of this comparison will avoid this
confusion. We will change this in next version to make it explicit.

Thank you,
Nitesh Shetty