linux-kernel - Re: [PATCH v20 02/12] Add infrastructure for copy offload in block and request layer.

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Date: Tue, 21 May 2024 16:47:56 +0530
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, 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 02/12] Add infrastructure for copy offload in block
 and request layer.

On 20/05/24 04:00PM, Bart Van Assche wrote:
>On 5/20/24 03:20, Nitesh Shetty wrote:
>>Upon arrival of source bio we merge these two bio's and send
>>corresponding request down to device driver.
>
>bios with different operation types must not be merged.
>
Copy is a composite operation which has two operation read and
write combined, so we chose two operation types which reflects that.

>>+static enum bio_merge_status bio_attempt_copy_offload_merge(struct request *req,
>>+							    struct bio *bio)
>>+{
>>+	if (req->__data_len != bio->bi_iter.bi_size)
>>+		return BIO_MERGE_FAILED;
>>+
>>+	req->biotail->bi_next = bio;
>>+	req->biotail = bio;
>>+	req->nr_phys_segments++;
>>+	req->__data_len += bio->bi_iter.bi_size;
>>+
>>+	return BIO_MERGE_OK;
>>+}
>
>This function appends a bio to a request. Hence, the name of this function is
>wrong.
>
We followed the naming convention from discard(bio_attempt_discard_merge)
which does similar thing.
But we are open to renaming, if overall community also feels the same.

>>@@ -1085,6 +1124,8 @@ static enum bio_merge_status blk_attempt_bio_merge(struct request_queue *q,
>>  		break;
>>  	case ELEVATOR_DISCARD_MERGE:
>>  		return bio_attempt_discard_merge(q, rq, bio);
>>+	case ELEVATOR_COPY_OFFLOAD_MERGE:
>>+		return bio_attempt_copy_offload_merge(rq, bio);
>>  	default:
>>  		return BIO_MERGE_NONE;
>>  	}
>
>Is any code added in this patch series that causes an I/O scheduler to return
>ELEVATOR_COPY_OFFLOAD_MERGE?
>
yes, blk_try_merge returns ELEVATOR_COPY_OFFLOAD_MERGE.

>>+static inline bool blk_copy_offload_mergable(struct request *req,
>>+					     struct bio *bio)
>>+{
>>+	return (req_op(req) == REQ_OP_COPY_DST &&
>>+		bio_op(bio) == REQ_OP_COPY_SRC);
>>+}
>
>bios with different operation types must not be merged. Please rename this function.
>
As described above we need two different opcodes.
As far as function renaming, we followed discard's naming. But open to
any suggestion.

>>+static inline bool op_is_copy(blk_opf_t op)
>>+{
>>+	return ((op & REQ_OP_MASK) == REQ_OP_COPY_SRC ||
>>+		(op & REQ_OP_MASK) == REQ_OP_COPY_DST);
>>+}
>
>The above function is not used in this patch. Please introduce new functions in the
>patch in which these are used for the first time.
>
Acked

Thank You
Nitesh Shetty