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

Dmitry Osipenko
2018-01-09T13:18:46Z

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