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

Dmitry Osipenko
2018-01-10T15:41:24Z

On 10.01.2018 05:40, Ming Lei wrote:
> On Tue, Jan 09, 2018 at 08:02:53PM +0300, Dmitry Osipenko wrote:
>> On 09.01.2018 17:33, Ming Lei wrote:
>>> On Tue, Jan 09, 2018 at 04:18:39PM +0300, Dmitry Osipenko wrote:
>>>> 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
>>>
>>> Hi Dmitry,
>>>
>>> The 'max_segment_size' of 65535 should be the reason, could you test the
>>> following patch?
>>>
>>> ---
>>> diff --git a/block/blk-merge.c b/block/blk-merge.c
>>> index 446f63e076aa..38a66e3e678e 100644
>>> --- a/block/blk-merge.c
>>> +++ b/block/blk-merge.c
>>> @@ -12,6 +12,8 @@
>>>
>>> #include "blk.h"
>>>
>>> +#define sector_align(x) ALIGN_DOWN(x, 512)
>>> +
>>> static struct bio *blk_bio_discard_split(struct request_queue *q,
>>> struct bio *bio,
>>> struct bio_set *bs,
>>> @@ -109,7 +111,7 @@ 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;
>>> + int advance = 0;
>>>
>>> bio_for_each_segment(bv, bio, iter) {
>>> /*
>>> @@ -144,8 +146,9 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
>>> * bigger than max segment size, but this
>>> * becomes false after multipage bvecs.
>>> */
>>> - advance = queue_max_segment_size(q) - seg_size;
>>> -
>>> + advance = sector_align(
>>> + queue_max_segment_size(q) -
>>> + seg_size);
>>> if (advance > 0) {
>>> seg_size += advance;
>>> sectors += advance >> 9;
>>> @@ -386,7 +389,7 @@ __blk_segment_map_sg(struct request_queue *q, struct bio_vec *bvec,
>>> {
>>>
>>> int nbytes = bvec->bv_len;
>>> - unsigned advance = 0;
>>> + int advance = 0;
>>>
>>> if (*sg && *cluster) {
>>> if (!BIOVEC_PHYS_MERGEABLE(bvprv, bvec))
>>> @@ -400,8 +403,9 @@ __blk_segment_map_sg(struct request_queue *q, struct bio_vec *bvec,
>>> * blk_bio_segment_split()
>>> */
>>> if ((*sg)->length + nbytes > queue_max_segment_size(q)) {
>>> - advance = queue_max_segment_size(q) - (*sg)->length;
>>> - if (advance) {
>>> + advance = sector_align(queue_max_segment_size(q) -
>>> + (*sg)->length);
>>> + if (advance > 0) {
>>> (*sg)->length += advance;
>>> bvec->bv_offset += advance;
>>> bvec->bv_len -= advance;
>>> @@ -431,12 +435,14 @@ __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 > 0) {
>>> /* 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,
>>
>> This patch doesn't help either.
>
> OK, I will send a revert later.
>
> Thinking of the patch further, we don't need this kind of logic for
> multipage bvec at all since almost all bvecs won't be contiguous if
> bio_add_page() is used after multipage bvec is enabled.

Revert works for me, thanks.