Re: [PATCH V4 13/45] block: blk-merge: try to make front segments in full size

[Dmitry Osipenko <digetx@gmail.com>]

On 09.01.2018 05:34, Ming Lei wrote:
> On Tue, Jan 09, 2018 at 12:09:27AM +0300, Dmitry Osipenko wrote:
>> On 18.12.2017 15:22, Ming Lei wrote:
>>> When merging one bvec into segment, if the bvec is too big
>>> to merge, current policy is to move the whole bvec into another
>>> new segment.
>>>
>>> This patchset changes the policy into trying to maximize size of
>>> front segments, that means in above situation, part of bvec
>>> is merged into current segment, and the remainder is put
>>> into next segment.
>>>
>>> This patch prepares for support multipage bvec because
>>> it can be quite common to see this case and we should try
>>> to make front segments in full size.
>>>
>>> Signed-off-by: Ming Lei <ming.lei@redhat.com>
>>> ---
>>>  block/blk-merge.c | 54 +++++++++++++++++++++++++++++++++++++++++++++++++-----
>>>  1 file changed, 49 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/block/blk-merge.c b/block/blk-merge.c
>>> index a476337a8ff4..42ceb89bc566 100644
>>> --- a/block/blk-merge.c
>>> +++ b/block/blk-merge.c
>>> @@ -109,6 +109,7 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
>>>  	bool do_split = true;
>>>  	struct bio *new = NULL;
>>>  	const unsigned max_sectors = get_max_io_size(q, bio);
>>> +	unsigned advance = 0;
>>>  
>>>  	bio_for_each_segment(bv, bio, iter) {
>>>  		/*
>>> @@ -134,12 +135,32 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
>>>  		}
>>>  
>>>  		if (bvprvp && blk_queue_cluster(q)) {
>>> -			if (seg_size + bv.bv_len > queue_max_segment_size(q))
>>> -				goto new_segment;
>>>  			if (!BIOVEC_PHYS_MERGEABLE(bvprvp, &bv))
>>>  				goto new_segment;
>>>  			if (!BIOVEC_SEG_BOUNDARY(q, bvprvp, &bv))
>>>  				goto new_segment;
>>> +			if (seg_size + bv.bv_len > queue_max_segment_size(q)) {
>>> +				/*
>>> +				 * On assumption is that initial value of
>>> +				 * @seg_size(equals to bv.bv_len) won't be
>>> +				 * bigger than max segment size, but will
>>> +				 * becomes false after multipage bvec comes.
>>> +				 */
>>> +				advance = queue_max_segment_size(q) - seg_size;
>>> +
>>> +				if (advance > 0) {
>>> +					seg_size += advance;
>>> +					sectors += advance >> 9;
>>> +					bv.bv_len -= advance;
>>> +					bv.bv_offset += advance;
>>> +				}
>>> +
>>> +				/*
>>> +				 * Still need to put remainder of current
>>> +				 * bvec into a new segment.
>>> +				 */
>>> +				goto new_segment;
>>> +			}
>>>  
>>>  			seg_size += bv.bv_len;
>>>  			bvprv = bv;
>>> @@ -161,6 +182,12 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
>>>  		seg_size = bv.bv_len;
>>>  		sectors += bv.bv_len >> 9;
>>>  
>>> +		/* restore the bvec for iterator */
>>> +		if (advance) {
>>> +			bv.bv_len += advance;
>>> +			bv.bv_offset -= advance;
>>> +			advance = 0;
>>> +		}
>>>  	}
>>>  
>>>  	do_split = false;
>>> @@ -361,16 +388,29 @@ __blk_segment_map_sg(struct request_queue *q, struct bio_vec *bvec,
>>>  {
>>>  
>>>  	int nbytes = bvec->bv_len;
>>> +	unsigned advance = 0;
>>>  
>>>  	if (*sg && *cluster) {
>>> -		if ((*sg)->length + nbytes > queue_max_segment_size(q))
>>> -			goto new_segment;
>>> -
>>>  		if (!BIOVEC_PHYS_MERGEABLE(bvprv, bvec))
>>>  			goto new_segment;
>>>  		if (!BIOVEC_SEG_BOUNDARY(q, bvprv, bvec))
>>>  			goto new_segment;
>>>  
>>> +		/*
>>> +		 * try best to merge part of the bvec into previous
>>> +		 * segment and follow same policy with
>>> +		 * blk_bio_segment_split()
>>> +		 */
>>> +		if ((*sg)->length + nbytes > queue_max_segment_size(q)) {
>>> +			advance = queue_max_segment_size(q) - (*sg)->length;
>>> +			if (advance) {
>>> +				(*sg)->length += advance;
>>> +				bvec->bv_offset += advance;
>>> +				bvec->bv_len -= advance;
>>> +			}
>>> +			goto new_segment;
>>> +		}
>>> +
>>>  		(*sg)->length += nbytes;
>>>  	} else {
>>>  new_segment:
>>> @@ -393,6 +433,10 @@ __blk_segment_map_sg(struct request_queue *q, struct bio_vec *bvec,
>>>  
>>>  		sg_set_page(*sg, bvec->bv_page, nbytes, bvec->bv_offset);
>>>  		(*nsegs)++;
>>> +
>>> +		/* for making iterator happy */
>>> +		bvec->bv_offset -= advance;
>>> +		bvec->bv_len += advance;
>>>  	}
>>>  	*bvprv = *bvec;
>>>  }
>>>
>>
>> Hello,
>>
>> This patch breaks MMC on next-20180108, in particular MMC doesn't work anymore
>> with this patch on NVIDIA Tegra20:
>>
>> <3>[   36.622253] print_req_error: I/O error, dev mmcblk1, sector 512
>> <3>[   36.671233] print_req_error: I/O error, dev mmcblk2, sector 128
>> <3>[   36.711308] print_req_error: I/O error, dev mmcblk1, sector 31325304
>> <3>[   36.749232] print_req_error: I/O error, dev mmcblk2, sector 512
>> <3>[   36.761235] print_req_error: I/O error, dev mmcblk1, sector 31325816
>> <3>[   36.832039] print_req_error: I/O error, dev mmcblk2, sector 31259768
>> <3>[   99.793248] print_req_error: I/O error, dev mmcblk1, sector 31323136
>> <3>[   99.982043] print_req_error: I/O error, dev mmcblk1, sector 929792
>> <3>[   99.986301] print_req_error: I/O error, dev mmcblk1, sector 930816
>> <3>[  100.293624] print_req_error: I/O error, dev mmcblk1, sector 932864
>> <3>[  100.466839] print_req_error: I/O error, dev mmcblk1, sector 947200
>> <3>[  100.642955] print_req_error: I/O error, dev mmcblk1, sector 949248
>> <3>[  100.818838] print_req_error: I/O error, dev mmcblk1, sector 230400
>>
>> Any attempt of mounting MMC block dev ends with a kernel crash. Reverting this
>> patch fixes the issue.
> 
> Hi Dmitry,
> 
> Thanks for your report!
> 
> Could you share us what the segment limits are on your MMC?
> 
> 	cat /sys/block/mmcN/queue/max_segment_size
> 	cat /sys/block/mmcN/queue/max_segments
> 
> Please test the following patch to see if your issue can be fixed?
> 
> ---
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index 446f63e076aa..cfab36c26608 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -431,12 +431,14 @@ __blk_segment_map_sg(struct request_queue *q, struct bio_vec *bvec,
>  
>  		sg_set_page(*sg, bvec->bv_page, nbytes, bvec->bv_offset);
>  		(*nsegs)++;
> +	}
>  
> +	*bvprv = *bvec;
> +	if (advance) {
>  		/* for making iterator happy */
>  		bvec->bv_offset -= advance;
>  		bvec->bv_len += advance;
>  	}
> -	*bvprv = *bvec;
>  }
>  
>  static inline int __blk_bvec_map_sg(struct request_queue *q, struct bio_vec bv,

Hi Ming,

I've tried your patch and unfortunately it doesn't help with the issue.

Here are the segment limits:

# cat /sys/block/mmc*/queue/max_segment_size
65535
65535
65535
65535
# cat /sys/block/mmc*/queue/max_segments
128
128
128
128

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