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

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Date: Wed, 22 May 2024 10:52:10 -0700
From: Bart Van Assche <bvanassche@....org>
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, 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 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).

Thanks,

Bart.