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Message-ID: <0d3b0a6b-825d-1b01-094a-911f81f5f354@opensource.wdc.com>
Date:   Thu, 28 Apr 2022 07:04:13 +0900
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Subject: Re: [PATCH v4 02/10] block: Add copy offload support infrastructure
Content-Language: en-US
To:     Nitesh Shetty <nj.shetty@samsung.com>
Cc:     linux-block@vger.kernel.org, linux-scsi@vger.kernel.org,
        dm-devel@redhat.com, linux-nvme@lists.infradead.org,
        linux-fsdevel@vger.kernel.org, nitheshshetty@gmail.com,
        linux-kernel@vger.kernel.org
References: <20220426101241.30100-1-nj.shetty@samsung.com>
 <CGME20220426101921epcas5p341707619b5e836490284a42c92762083@epcas5p3.samsung.com>
 <20220426101241.30100-3-nj.shetty@samsung.com>
 <7d1fdd1e-c854-4744-8bec-7d222fb9be76@opensource.wdc.com>
 <20220427151535.GC9558@test-zns>
From:   Damien Le Moal <damien.lemoal@opensource.wdc.com>
Organization: Western Digital Research
In-Reply-To: <20220427151535.GC9558@test-zns>
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Precedence: bulk

On 4/28/22 00:15, Nitesh Shetty wrote:
> On Wed, Apr 27, 2022 at 11:45:26AM +0900, Damien Le Moal wrote:
>> On 4/26/22 19:12, Nitesh Shetty wrote:
>>> Introduce blkdev_issue_copy which supports source and destination bdevs,
>>> and an array of (source, destination and copy length) tuples.
>>> Introduce REQ_COPY copy offload operation flag. Create a read-write
>>> bio pair with a token as payload and submitted to the device in order.
>>> Read request populates token with source specific information which
>>> is then passed with write request.
>>> This design is courtesy Mikulas Patocka's token based copy
>>>
>>> Larger copy will be divided, based on max_copy_sectors,
>>> max_copy_range_sector limits.
>>>
>>> Signed-off-by: Nitesh Shetty <nj.shetty@samsung.com>
>>> Signed-off-by: Arnav Dawn <arnav.dawn@samsung.com>
>>> ---
>>>  block/blk-lib.c           | 232 ++++++++++++++++++++++++++++++++++++++
>>>  block/blk.h               |   2 +
>>>  include/linux/blk_types.h |  21 ++++
>>>  include/linux/blkdev.h    |   2 +
>>>  include/uapi/linux/fs.h   |  14 +++
>>>  5 files changed, 271 insertions(+)
>>>
>>> diff --git a/block/blk-lib.c b/block/blk-lib.c
>>> index 09b7e1200c0f..ba9da2d2f429 100644
>>> --- a/block/blk-lib.c
>>> +++ b/block/blk-lib.c
>>> @@ -117,6 +117,238 @@ int blkdev_issue_discard(struct block_device *bdev, sector_t sector,
>>>  }
>>>  EXPORT_SYMBOL(blkdev_issue_discard);
>>>  
>>> +/*
>>> + * Wait on and process all in-flight BIOs.  This must only be called once
>>> + * all bios have been issued so that the refcount can only decrease.
>>> + * This just waits for all bios to make it through bio_copy_end_io. IO
>>> + * errors are propagated through cio->io_error.
>>> + */
>>> +static int cio_await_completion(struct cio *cio)
>>> +{
>>> +	int ret = 0;
>>> +	unsigned long flags;
>>> +
>>> +	spin_lock_irqsave(&cio->lock, flags);
>>> +	if (cio->refcount) {
>>> +		cio->waiter = current;
>>> +		__set_current_state(TASK_UNINTERRUPTIBLE);
>>> +		spin_unlock_irqrestore(&cio->lock, flags);
>>> +		blk_io_schedule();
>>> +		/* wake up sets us TASK_RUNNING */
>>> +		spin_lock_irqsave(&cio->lock, flags);
>>> +		cio->waiter = NULL;
>>> +		ret = cio->io_err;
>>> +	}
>>> +	spin_unlock_irqrestore(&cio->lock, flags);
>>> +	kvfree(cio);
>>
>> cio is allocated with kzalloc() == kmalloc(). So why the kvfree() here ?
>>
> 
> acked.
> 
>>> +
>>> +	return ret;
>>> +}
>>> +
>>> +static void bio_copy_end_io(struct bio *bio)
>>> +{
>>> +	struct copy_ctx *ctx = bio->bi_private;
>>> +	struct cio *cio = ctx->cio;
>>> +	sector_t clen;
>>> +	int ri = ctx->range_idx;
>>> +	unsigned long flags;
>>> +	bool wake = false;
>>> +
>>> +	if (bio->bi_status) {
>>> +		cio->io_err = bio->bi_status;
>>> +		clen = (bio->bi_iter.bi_sector << SECTOR_SHIFT) - ctx->start_sec;
>>> +		cio->rlist[ri].comp_len = min_t(sector_t, clen, cio->rlist[ri].comp_len);
>>
>> long line.
> 
> Is it because line is more than 80 character, I thought limit is 100 now, so
> went with longer lines ?

When it is easy to wrap the lines without readability loss, please do to
keep things under 80 char per line.


>>> +{
>>> +	struct request_queue *src_q = bdev_get_queue(src_bdev);
>>> +	struct request_queue *dest_q = bdev_get_queue(dest_bdev);
>>> +	int ret = -EINVAL;
>>> +
>>> +	if (!src_q || !dest_q)
>>> +		return -ENXIO;
>>> +
>>> +	if (!nr)
>>> +		return -EINVAL;
>>> +
>>> +	if (nr >= MAX_COPY_NR_RANGE)
>>> +		return -EINVAL;
>>
>> Where do you check the number of ranges against what the device can do ?
>>
> 
> The present implementation submits only one range at a time. This was done to 
> make copy offload generic, so that other types of copy implementation such as
> XCOPY should be able to use same infrastructure. Downside at present being
> NVMe copy offload is not optimal.

If you issue one range at a time without checking the number of ranges,
what is the point of the nr ranges queue limit ? The user can submit a
copy ioctl request exceeding it. Please use that limit and enforce it or
remove it entirely.


-- 
Damien Le Moal
Western Digital Research