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Date:   Fri, 3 Jul 2020 12:23:11 +0530
From:   Kanchan Joshi <joshi.k@samsung.com>
To:     Damien Le Moal <Damien.LeMoal@wdc.com>
Cc:     "axboe@kernel.dk" <axboe@kernel.dk>,
        "kbusch@kernel.org" <kbusch@kernel.org>,
        "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "hch@lst.de" <hch@lst.de>, Selvakumar S <selvakuma.s1@samsung.com>,
        Nitesh Shetty <nj.shetty@samsung.com>,
        Javier Gonzalez <javier.gonz@samsung.com>
Subject: Re: [PATCH 2/2] block: enable zone-append for iov_iter of bvec type
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On Fri, Jul 03, 2020 at 05:32:56AM +0000, Damien Le Moal wrote:
>On 2020/07/03 0:42, Kanchan Joshi wrote:
>> zone-append with bvec iov_iter gives WARN_ON, and returns -EINVAL.
>> Add new helper to process such iov_iter and add pages in bio honoring
>> zone-append specific constraints.
>>
>> Signed-off-by: Kanchan Joshi <joshi.k@samsung.com>
>> Signed-off-by: Selvakumar S <selvakuma.s1@samsung.com>
>> Signed-off-by: Nitesh Shetty <nj.shetty@samsung.com>
>> Signed-off-by: Javier Gonzalez <javier.gonz@samsung.com>
>> ---
>>  block/bio.c | 31 ++++++++++++++++++++++++++++---
>>  1 file changed, 28 insertions(+), 3 deletions(-)
>>
>> diff --git a/block/bio.c b/block/bio.c
>> index 0cecdbc..ade9da7 100644
>> --- a/block/bio.c
>> +++ b/block/bio.c
>> @@ -975,6 +975,30 @@ static int __bio_iov_bvec_add_pages(struct bio *bio, struct iov_iter *iter)
>>  	iov_iter_advance(iter, size);
>>  	return 0;
>>  }
>> +static int __bio_iov_bvec_append_add_pages(struct bio *bio, struct iov_iter *iter)
>> +{
>> +	const struct bio_vec *bv = iter->bvec;
>> +	unsigned int len;
>> +	size_t size;
>> +	struct request_queue *q = bio->bi_disk->queue;
>> +	unsigned int max_append_sectors = queue_max_zone_append_sectors(q);
>> +	bool same_page = false;
>> +
>> +	if (WARN_ON_ONCE(!max_append_sectors))
>> +		return -EINVAL;
>> +
>> +	if (WARN_ON_ONCE(iter->iov_offset > bv->bv_len))
>> +		return -EINVAL;
>> +
>> +	len = min_t(size_t, bv->bv_len - iter->iov_offset, iter->count);
>> +	size = bio_add_hw_page(q, bio, bv->bv_page, len,
>> +				bv->bv_offset + iter->iov_offset,
>> +				max_append_sectors, &same_page);
>> +	if (unlikely(size != len))
>> +		return -EINVAL;
>> +	iov_iter_advance(iter, size);
>> +	return 0;
>> +}
>>
>>  #define PAGE_PTRS_PER_BVEC     (sizeof(struct bio_vec) / sizeof(struct page *))
>>
>> @@ -1105,9 +1129,10 @@ int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
>>
>>  	do {
>>  		if (bio_op(bio) == REQ_OP_ZONE_APPEND) {
>> -			if (WARN_ON_ONCE(is_bvec))
>> -				return -EINVAL;
>> -			ret = __bio_iov_append_get_pages(bio, iter);
>> +			if (is_bvec)
>> +				ret = __bio_iov_bvec_append_add_pages(bio, iter);
>> +			else
>> +				ret = __bio_iov_append_get_pages(bio, iter);
>>  		} else {
>>  			if (is_bvec)
>>  				ret = __bio_iov_bvec_add_pages(bio, iter);
>>
>
>The only user of this function that issues zone append requests is zonefs. The
>issued requests are not using bvec iter but a user direct IO buffer iter. So
>this change would have no user at all as far as I can see. Am I missing
>something ? What IO path makes this change necessary ?

Yes, zonefs does not use bvec iter. But while enabling io-uring path for
zone-append, I hit into this condition returning -EINVAL. 

Reference (from user zone-append series cover letter):
"Append using io_uring fixed-buffer --->
This is flagged as not-supported at the moment. Reason being, for fixed-buffer
io-uring sends iov_iter of bvec type. But current append-infra in block-layer
does not support such iov_iter."

And zone-append doesn't have a problem in using bvec iter as well, so
thought that this may make infra more complete/future-proof?

>-- 
>Damien Le Moal
>Western Digital Research
>

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