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

[Ming Lei <ming.lei@redhat.com>]

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,

Thanks,
Ming