linux-kernel - Re: [PATCH 2/2] block: enable zone-append for iov

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Date:   Fri, 3 Jul 2020 08:07:38 +0000
From:   Damien Le Moal <Damien.LeMoal@....com>
To:     Kanchan Joshi <joshi.k@...sung.com>
CC:     "axboe@...nel.dk" <axboe@...nel.dk>,
        "kbusch@...nel.org" <kbusch@...nel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "hch@....de" <hch@....de>, Selvakumar S <selvakuma.s1@...sung.com>,
        Nitesh Shetty <nj.shetty@...sung.com>,
        Javier Gonzalez <javier.gonz@...sung.com>
Subject: Re: [PATCH 2/2] block: enable zone-append for iov_iter of bvec type

On 2020/07/03 15:56, Kanchan Joshi wrote:
> 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@...sung.com>
>>> Signed-off-by: Selvakumar S <selvakuma.s1@...sung.com>
>>> Signed-off-by: Nitesh Shetty <nj.shetty@...sung.com>
>>> Signed-off-by: Javier Gonzalez <javier.gonz@...sung.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?

As long as there is no possible user for this change, I do not see the point in
adding it.


-- 
Damien Le Moal
Western Digital Research