making single-command write

src/block/bio.c

@@ -852,7 +852,23 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
 	size_t offset;
 	ssize_t size;
 	if (bio->bi_disk && (bio->bi_disk->major == 8)) { // sda, sda*
-		pr_debug(" *A* nr_pages=0x%04x,bio->bi_max_vecs=0x%04x, bio->bi_vcnt==0x%04x", nr_pages, bio->bi_max_vecs, bio->bi_vcnt); //  *A* nr_pages=0x009f,bio->bi_max_vecs=0x009f, bio->bi_vcnt==0x0000
+		pr_debug(" *A* nr_pages=0x%04x,bio->bi_max_vecs=0x%04x, bio->bi_vcnt=0x%04x", nr_pages, bio->bi_max_vecs, bio->bi_vcnt); //  *A* nr_pages=0x009f,bio->bi_max_vecs=0x009f, bio->bi_vcnt==0x0000
+		pr_debug(" *A0* max_hw_sectors=%d,  max_dev_sectors=%d, max_sectors=%d, cluster=%d, max_segment_size=0x%08x",\
+				bio->bi_disk->queue->limits.max_hw_sectors, bio->bi_disk->queue->limits.max_dev_sectors, bio->bi_disk->queue->limits.max_sectors, bio->bi_disk->queue->limits.cluster, bio->bi_disk->queue->limits.max_segment_size);
+// For now - just testing, not clear who/what sets 	// bio:__bio_iov_iter_get_pages:857:  *A0* max_hw_sectors=65535,  max_dev_sectors=65535, max_sectors=2560
+// limited by BLK_DEF_MAX_SECTORS = 2560!
+/* blk_queue_cluster(q) return q->limits.cluster; q->limits.max_segment_size
+https://patchwork.kernel.org/project/linux-block/patch/21cf85d32278bbe5acbc3def0a6db75db98a2670.1459269590.git.shli@fb.com/
+bio_alloc_bioset() allocates bvecs from bvec_slabs which can only
+allocate maximum 256 bvec (eg, 1M for 4k pages). We can't bump
+BLK_DEF_MAX_SECTORS to exceed this value otherwise bio_alloc_bioset will
+fail.
+*/
+		blk_queue_max_segment_size(bio->bi_disk->queue, 0x100000);
+//		blk_queue_max_hw_sectors(bio->bi_disk->queue, bio->bi_disk->queue->limits.max_hw_sectors); // still used constant
+		bio->bi_disk->queue->limits.max_sectors = min(bio->bi_disk->queue->limits.max_hw_sectors, bio->bi_disk->queue->limits.max_dev_sectors);
+		pr_debug(" *A1* max_hw_sectors=%d,  max_dev_sectors=%d, max_sectors=%d, cluster=%d, max_segment_size=0x%08x",\
+				bio->bi_disk->queue->limits.max_hw_sectors, bio->bi_disk->queue->limits.max_dev_sectors, bio->bi_disk->queue->limits.max_sectors, bio->bi_disk->queue->limits.cluster, bio->bi_disk->queue->limits.max_segment_size);
 		pr_debug(" *B* niter->type=%d, iter->iov_offset=0x%08x, iter->count=0x%08x, iter->nr_segs=0x%08lx",\
 				iter->type, iter->iov_offset,iter->count,iter->nr_segs); // *B* niter->type=1, iter->iov_offset=0x00000000, iter->count=0x0009f000, iter->nr_segs=0x00000001
 	}

src/block/blk-merge.c

+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Functions related to segment and merge handling
+ */
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/bio.h>
+#include <linux/blkdev.h>
+#include <linux/scatterlist.h>
+
+#include <trace/events/block.h>
+
+#include "blk.h"
+
+static struct bio *blk_bio_discard_split(struct request_queue *q,
+					 struct bio *bio,
+					 struct bio_set *bs,
+					 unsigned *nsegs)
+{
+	unsigned int max_discard_sectors, granularity;
+	int alignment;
+	sector_t tmp;
+	unsigned split_sectors;
+
+	*nsegs = 1;
+
+	/* Zero-sector (unknown) and one-sector granularities are the same.  */
+	granularity = max(q->limits.discard_granularity >> 9, 1U);
+
+	max_discard_sectors = min(q->limits.max_discard_sectors, UINT_MAX >> 9);
+	max_discard_sectors -= max_discard_sectors % granularity;
+
+	if (unlikely(!max_discard_sectors)) {
+		/* XXX: warn */
+		return NULL;
+	}
+
+	if (bio_sectors(bio) <= max_discard_sectors)
+		return NULL;
+
+	split_sectors = max_discard_sectors;
+
+	/*
+	 * If the next starting sector would be misaligned, stop the discard at
+	 * the previous aligned sector.
+	 */
+	alignment = (q->limits.discard_alignment >> 9) % granularity;
+
+	tmp = bio->bi_iter.bi_sector + split_sectors - alignment;
+	tmp = sector_div(tmp, granularity);
+
+	if (split_sectors > tmp)
+		split_sectors -= tmp;
+
+	return bio_split(bio, split_sectors, GFP_NOIO, bs);
+}
+
+static struct bio *blk_bio_write_zeroes_split(struct request_queue *q,
+		struct bio *bio, struct bio_set *bs, unsigned *nsegs)
+{
+	*nsegs = 1;
+
+	if (!q->limits.max_write_zeroes_sectors)
+		return NULL;
+
+	if (bio_sectors(bio) <= q->limits.max_write_zeroes_sectors)
+		return NULL;
+
+	return bio_split(bio, q->limits.max_write_zeroes_sectors, GFP_NOIO, bs);
+}
+
+static struct bio *blk_bio_write_same_split(struct request_queue *q,
+					    struct bio *bio,
+					    struct bio_set *bs,
+					    unsigned *nsegs)
+{
+	*nsegs = 1;
+
+	if (!q->limits.max_write_same_sectors)
+		return NULL;
+
+	if (bio_sectors(bio) <= q->limits.max_write_same_sectors)
+		return NULL;
+
+	return bio_split(bio, q->limits.max_write_same_sectors, GFP_NOIO, bs);
+}
+
+static inline unsigned get_max_io_size(struct request_queue *q,
+				       struct bio *bio)
+{
+	unsigned sectors = blk_max_size_offset(q, bio->bi_iter.bi_sector);
+	unsigned mask = queue_logical_block_size(q) - 1;
+
+	/* aligned to logical block size */
+	sectors &= ~(mask >> 9);
+
+	return sectors;
+}
+
+static struct bio *blk_bio_segment_split(struct request_queue *q,
+					 struct bio *bio,
+					 struct bio_set *bs,
+					 unsigned *segs)
+{
+	struct bio_vec bv, bvprv, *bvprvp = NULL;
+	struct bvec_iter iter;
+	unsigned seg_size = 0, nsegs = 0, sectors = 0;
+	unsigned front_seg_size = bio->bi_seg_front_size;
+	bool do_split = true;
+	struct bio *new = NULL;
+	const unsigned max_sectors = get_max_io_size(q, bio);
+
+	bio_for_each_segment(bv, bio, iter) {
+	    if (bio->bi_disk && (bio->bi_disk->major == 8)) { // sda, sda*
+	    	pr_debug(" *X3* sectors=0x%08x, max_sectors=0x%08x, (bv.bv_len >> 9)=0x%08x", sectors, max_sectors, bv.bv_len >> 9);
+	    }
+		/*
+		 * If the queue doesn't support SG gaps and adding this
+		 * offset would create a gap, disallow it.
+		 */
+		if (bvprvp && bvec_gap_to_prev(q, bvprvp, bv.bv_offset)) {
+		    if (bio->bi_disk && (bio->bi_disk->major == 8)) { // sda, sda* // never
+		    	pr_debug(" *X4* ");
+		    }
+			goto split;
+		}
+		if (sectors + (bv.bv_len >> 9) > max_sectors) {
+		    if (bio->bi_disk && (bio->bi_disk->major == 8)) { // sda, sda*  // never
+		    	pr_debug(" *X5* ");
+		    }
+			/*
+			 * Consider this a new segment if we're splitting in
+			 * the middle of this vector.
+			 */
+			if (nsegs < queue_max_segments(q) &&
+			    sectors < max_sectors) {
+				nsegs++;
+				sectors = max_sectors;
+			}
+			goto split;
+		}
+
+		if (bvprvp && blk_queue_cluster(q)) { // first - skip
+		    if (bio->bi_disk && (bio->bi_disk->major == 8)) { // sda, sda*
+		    	pr_debug(" *X6* ");
+		    }
+			if (seg_size + bv.bv_len > queue_max_segment_size(q)) {
+			    if (bio->bi_disk && (bio->bi_disk->major == 8)) { // sda, sda*  // here!
+			    	pr_debug(" *X61* ");
+			    }
+				goto new_segment;
+			}
+			if (!BIOVEC_PHYS_MERGEABLE(bvprvp, &bv)) {
+			    if (bio->bi_disk && (bio->bi_disk->major == 8)) { // sda, sda*
+			    	pr_debug(" *X62* ");
+			    }
+				goto new_segment;
+			}
+			if (!BIOVEC_SEG_BOUNDARY(q, bvprvp, &bv)) {
+			    if (bio->bi_disk && (bio->bi_disk->major == 8)) { // sda, sda*
+			    	pr_debug(" *X63* ");
+			    }
+				goto new_segment;
+			}
+		    if (bio->bi_disk && (bio->bi_disk->major == 8)) { // sda, sda*
+		    	pr_debug(" *X7* seg_size=0x%08x, bv.bv_len=0x%08x, blk_queue_cluster(q)=0x%x", seg_size, bv.bv_len, blk_queue_cluster(q));
+		    }
+			seg_size += bv.bv_len;
+			bvprv = bv;
+			bvprvp = &bvprv;
+			sectors += bv.bv_len >> 9;
+
+			continue;
+		}
+new_segment:
+		if (bio->bi_disk && (bio->bi_disk->major == 8)) { // sda, sda*
+			pr_debug(" *X8* nsegs=0x%08x", nsegs);
+		}
+		if (nsegs == queue_max_segments(q))
+			goto split;
+
+		if (nsegs == 1 && seg_size > front_seg_size)
+			front_seg_size = seg_size;
+
+		nsegs++;
+		bvprv = bv;
+		bvprvp = &bvprv;
+		seg_size = bv.bv_len;
+		sectors += bv.bv_len >> 9;
+
+	}
+
+	do_split = false;
+split:
+	if (bio->bi_disk && (bio->bi_disk->major == 8)) { // sda, sda*
+		pr_debug(" *X9* nsegs=0x%08x, do_split=%d, sectors=0x%08x", nsegs, do_split, sectors);
+	}
+	*segs = nsegs;
+
+	if (do_split) {
+		new = bio_split(bio, sectors, GFP_NOIO, bs);
+		if (new)
+			bio = new;
+	}
+
+	if (nsegs == 1 && seg_size > front_seg_size)
+		front_seg_size = seg_size;
+	bio->bi_seg_front_size = front_seg_size;
+	if (seg_size > bio->bi_seg_back_size)
+		bio->bi_seg_back_size = seg_size;
+
+	return do_split ? new : NULL;
+}
+
+void blk_queue_split(struct request_queue *q, struct bio **bio)
+{
+	struct bio *split, *res;
+	unsigned nsegs;
+    if ((*bio)->bi_disk && ((*bio)->bi_disk->major == 8)) { // sda, sda*
+    	pr_debug(" *X* (*bio)->bi_flags=0x%04x, (*bio)->bi_phys_segments=0x%08x, bio_op(*bio)=%d, get_max_io_size(q, *bio)=%d", \
+    			(*bio)->bi_flags, (*bio)->bi_phys_segments, bio_op(*bio), get_max_io_size(q, *bio)); // bio_op(*bio) = 1
+
+    	pr_debug(" *X1* mask =0x%08x, blk_max_size_offset(...) = 0x%08x", \
+    			queue_logical_block_size(q) - 1, blk_max_size_offset(q, (*bio)->bi_iter.bi_sector));
+    	pr_debug(" *X2* q->limits.chunk_sectors =0x%08x, q->limits.max_sectors = 0x%08x,  bio->bi_seg_front_size=0x%08x,  bio->bi_seg_back_size=0x%08x",\
+    			q->limits.chunk_sectors, q->limits.max_sectors,  (*bio)->bi_seg_front_size,  (*bio)->bi_seg_back_size);
+    }
+//const unsigned max_sectors = get_max_io_size(q, bio);
+	switch (bio_op(*bio)) {
+	case REQ_OP_DISCARD:
+	case REQ_OP_SECURE_ERASE:
+		split = blk_bio_discard_split(q, *bio, &q->bio_split, &nsegs);
+		break;
+	case REQ_OP_WRITE_ZEROES:
+		split = blk_bio_write_zeroes_split(q, *bio, &q->bio_split, &nsegs);
+		break;
+	case REQ_OP_WRITE_SAME:
+		split = blk_bio_write_same_split(q, *bio, &q->bio_split, &nsegs);
+		break;
+	default:
+		split = blk_bio_segment_split(q, *bio, &q->bio_split, &nsegs);
+		break;
+	}
+
+	/* physical segments can be figured out during splitting */
+	res = split ? split : *bio;
+	res->bi_phys_segments = nsegs;
+	bio_set_flag(res, BIO_SEG_VALID);
+
+    if ((*bio)->bi_disk && ((*bio)->bi_disk->major == 8)) { // sda, sda*
+    	pr_debug(" *Y* split = 0x%p, res->bi_phys_segments=%d", split, res->bi_phys_segments); // null
+    }
+	if (split) {
+		/* there isn't chance to merge the splitted bio */
+		split->bi_opf |= REQ_NOMERGE;
+
+		/*
+		 * Since we're recursing into make_request here, ensure
+		 * that we mark this bio as already having entered the queue.
+		 * If not, and the queue is going away, we can get stuck
+		 * forever on waiting for the queue reference to drop. But
+		 * that will never happen, as we're already holding a
+		 * reference to it.
+		 */
+		bio_set_flag(*bio, BIO_QUEUE_ENTERED);
+
+		bio_chain(split, *bio);
+		trace_block_split(q, split, (*bio)->bi_iter.bi_sector);
+		generic_make_request(*bio);
+		*bio = split;
+	}
+}
+EXPORT_SYMBOL(blk_queue_split);
+
+static unsigned int __blk_recalc_rq_segments(struct request_queue *q,
+					     struct bio *bio,
+					     bool no_sg_merge)
+{
+	struct bio_vec bv, bvprv = { NULL };
+	int cluster, prev = 0;
+	unsigned int seg_size, nr_phys_segs;
+	struct bio *fbio, *bbio;
+	struct bvec_iter iter;
+
+	if (!bio)
+		return 0;
+
+	switch (bio_op(bio)) {
+	case REQ_OP_DISCARD:
+	case REQ_OP_SECURE_ERASE:
+	case REQ_OP_WRITE_ZEROES:
+		return 0;
+	case REQ_OP_WRITE_SAME:
+		return 1;
+	}
+
+	fbio = bio;
+	cluster = blk_queue_cluster(q);
+	seg_size = 0;
+	nr_phys_segs = 0;
+	for_each_bio(bio) {
+		bio_for_each_segment(bv, bio, iter) {
+			/*
+			 * If SG merging is disabled, each bio vector is
+			 * a segment
+			 */
+			if (no_sg_merge)
+				goto new_segment;
+
+			if (prev && cluster) {
+				if (seg_size + bv.bv_len
+				    > queue_max_segment_size(q))
+					goto new_segment;
+				if (!BIOVEC_PHYS_MERGEABLE(&bvprv, &bv))
+					goto new_segment;
+				if (!BIOVEC_SEG_BOUNDARY(q, &bvprv, &bv))
+					goto new_segment;
+
+				seg_size += bv.bv_len;
+				bvprv = bv;
+				continue;
+			}
+new_segment:
+			if (nr_phys_segs == 1 && seg_size >
+			    fbio->bi_seg_front_size)
+				fbio->bi_seg_front_size = seg_size;
+
+			nr_phys_segs++;
+			bvprv = bv;
+			prev = 1;
+			seg_size = bv.bv_len;
+		}
+		bbio = bio;
+	}
+
+	if (nr_phys_segs == 1 && seg_size > fbio->bi_seg_front_size)
+		fbio->bi_seg_front_size = seg_size;
+	if (seg_size > bbio->bi_seg_back_size)
+		bbio->bi_seg_back_size = seg_size;
+
+	return nr_phys_segs;
+}
+
+void blk_recalc_rq_segments(struct request *rq)
+{
+	bool no_sg_merge = !!test_bit(QUEUE_FLAG_NO_SG_MERGE,
+			&rq->q->queue_flags);
+
+	rq->nr_phys_segments = __blk_recalc_rq_segments(rq->q, rq->bio,
+			no_sg_merge);
+}
+
+void blk_recount_segments(struct request_queue *q, struct bio *bio)
+{
+	unsigned short seg_cnt;
+
+	/* estimate segment number by bi_vcnt for non-cloned bio */
+	if (bio_flagged(bio, BIO_CLONED))
+		seg_cnt = bio_segments(bio);
+	else
+		seg_cnt = bio->bi_vcnt;
+
+	if (test_bit(QUEUE_FLAG_NO_SG_MERGE, &q->queue_flags) &&
+			(seg_cnt < queue_max_segments(q)))
+		bio->bi_phys_segments = seg_cnt;
+	else {
+		struct bio *nxt = bio->bi_next;
+
+		bio->bi_next = NULL;
+		bio->bi_phys_segments = __blk_recalc_rq_segments(q, bio, false);
+		bio->bi_next = nxt;
+	}
+
+	bio_set_flag(bio, BIO_SEG_VALID);
+}
+EXPORT_SYMBOL(blk_recount_segments);
+
+static int blk_phys_contig_segment(struct request_queue *q, struct bio *bio,
+				   struct bio *nxt)
+{
+	struct bio_vec end_bv = { NULL }, nxt_bv;
+
+	if (!blk_queue_cluster(q))
+		return 0;
+
+	if (bio->bi_seg_back_size + nxt->bi_seg_front_size >
+	    queue_max_segment_size(q))
+		return 0;
+
+	if (!bio_has_data(bio))
+		return 1;
+
+	bio_get_last_bvec(bio, &end_bv);
+	bio_get_first_bvec(nxt, &nxt_bv);
+
+	if (!BIOVEC_PHYS_MERGEABLE(&end_bv, &nxt_bv))
+		return 0;
+
+	/*
+	 * bio and nxt are contiguous in memory; check if the queue allows
+	 * these two to be merged into one
+	 */
+	if (BIOVEC_SEG_BOUNDARY(q, &end_bv, &nxt_bv))
+		return 1;
+
+	return 0;
+}
+
+static inline void
+__blk_segment_map_sg(struct request_queue *q, struct bio_vec *bvec,
+		     struct scatterlist *sglist, struct bio_vec *bvprv,
+		     struct scatterlist **sg, int *nsegs, int *cluster)
+{
+
+	int nbytes = bvec->bv_len;
+
+	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;
+
+		(*sg)->length += nbytes;
+	} else {
+new_segment:
+		if (!*sg)
+			*sg = sglist;
+		else {
+			/*
+			 * If the driver previously mapped a shorter
+			 * list, we could see a termination bit
+			 * prematurely unless it fully inits the sg
+			 * table on each mapping. We KNOW that there
+			 * must be more entries here or the driver
+			 * would be buggy, so force clear the
+			 * termination bit to avoid doing a full
+			 * sg_init_table() in drivers for each command.
+			 */
+			sg_unmark_end(*sg);
+			*sg = sg_next(*sg);
+		}
+
+		sg_set_page(*sg, bvec->bv_page, nbytes, bvec->bv_offset);
+		(*nsegs)++;
+	}
+	*bvprv = *bvec;
+}
+
+static inline int __blk_bvec_map_sg(struct request_queue *q, struct bio_vec bv,
+		struct scatterlist *sglist, struct scatterlist **sg)
+{
+	*sg = sglist;
+	sg_set_page(*sg, bv.bv_page, bv.bv_len, bv.bv_offset);
+	return 1;
+}
+
+static int __blk_bios_map_sg(struct request_queue *q, struct bio *bio,
+			     struct scatterlist *sglist,
+			     struct scatterlist **sg)
+{
+	struct bio_vec bvec, bvprv = { NULL };
+	struct bvec_iter iter;
+	int cluster = blk_queue_cluster(q), nsegs = 0;
+
+	for_each_bio(bio)
+		bio_for_each_segment(bvec, bio, iter)
+			__blk_segment_map_sg(q, &bvec, sglist, &bvprv, sg,
+					     &nsegs, &cluster);
+
+	return nsegs;
+}
+
+/*
+ * map a request to scatterlist, return number of sg entries setup. Caller
+ * must make sure sg can hold rq->nr_phys_segments entries
+ */
+int blk_rq_map_sg(struct request_queue *q, struct request *rq,
+		  struct scatterlist *sglist)
+{
+	struct scatterlist *sg = NULL;
+	int nsegs = 0;
+
+	if (rq->rq_flags & RQF_SPECIAL_PAYLOAD)
+		nsegs = __blk_bvec_map_sg(q, rq->special_vec, sglist, &sg);
+	else if (rq->bio && bio_op(rq->bio) == REQ_OP_WRITE_SAME)
+		nsegs = __blk_bvec_map_sg(q, bio_iovec(rq->bio), sglist, &sg);
+	else if (rq->bio)
+		nsegs = __blk_bios_map_sg(q, rq->bio, sglist, &sg);
+
+	if (unlikely(rq->rq_flags & RQF_COPY_USER) &&
+	    (blk_rq_bytes(rq) & q->dma_pad_mask)) {
+		unsigned int pad_len =
+			(q->dma_pad_mask & ~blk_rq_bytes(rq)) + 1;
+
+		sg->length += pad_len;
+		rq->extra_len += pad_len;
+	}
+
+	if (q->dma_drain_size && q->dma_drain_needed(rq)) {
+		if (op_is_write(req_op(rq)))
+			memset(q->dma_drain_buffer, 0, q->dma_drain_size);
+
+		sg_unmark_end(sg);
+		sg = sg_next(sg);
+		sg_set_page(sg, virt_to_page(q->dma_drain_buffer),
+			    q->dma_drain_size,
+			    ((unsigned long)q->dma_drain_buffer) &
+			    (PAGE_SIZE - 1));
+		nsegs++;
+		rq->extra_len += q->dma_drain_size;
+	}
+
+	if (sg)
+		sg_mark_end(sg);
+
+	/*
+	 * Something must have been wrong if the figured number of
+	 * segment is bigger than number of req's physical segments
+	 */
+	WARN_ON(nsegs > blk_rq_nr_phys_segments(rq));
+
+	return nsegs;
+}
+EXPORT_SYMBOL(blk_rq_map_sg);
+
+static inline int ll_new_hw_segment(struct request_queue *q,
+				    struct request *req,
+				    struct bio *bio)
+{
+	int nr_phys_segs = bio_phys_segments(q, bio);
+
+	if (req->nr_phys_segments + nr_phys_segs > queue_max_segments(q))
+		goto no_merge;
+
+	if (blk_integrity_merge_bio(q, req, bio) == false)
+		goto no_merge;
+
+	/*
+	 * This will form the start of a new hw segment.  Bump both
+	 * counters.
+	 */
+	req->nr_phys_segments += nr_phys_segs;
+	return 1;
+
+no_merge:
+	req_set_nomerge(q, req);
+	return 0;
+}
+
+int ll_back_merge_fn(struct request_queue *q, struct request *req,
+		     struct bio *bio)
+{
+	if (req_gap_back_merge(req, bio))
+		return 0;
+	if (blk_integrity_rq(req) &&
+	    integrity_req_gap_back_merge(req, bio))
+		return 0;
+	if (blk_rq_sectors(req) + bio_sectors(bio) >
+	    blk_rq_get_max_sectors(req, blk_rq_pos(req))) {
+		req_set_nomerge(q, req);
+		return 0;
+	}
+	if (!bio_flagged(req->biotail, BIO_SEG_VALID))
+		blk_recount_segments(q, req->biotail);
+	if (!bio_flagged(bio, BIO_SEG_VALID))
+		blk_recount_segments(q, bio);
+
+	return ll_new_hw_segment(q, req, bio);
+}
+
+int ll_front_merge_fn(struct request_queue *q, struct request *req,
+		      struct bio *bio)
+{
+
+	if (req_gap_front_merge(req, bio))
+		return 0;
+	if (blk_integrity_rq(req) &&
+	    integrity_req_gap_front_merge(req, bio))
+		return 0;
+	if (blk_rq_sectors(req) + bio_sectors(bio) >
+	    blk_rq_get_max_sectors(req, bio->bi_iter.bi_sector)) {
+		req_set_nomerge(q, req);
+		return 0;
+	}
+	if (!bio_flagged(bio, BIO_SEG_VALID))
+		blk_recount_segments(q, bio);
+	if (!bio_flagged(req->bio, BIO_SEG_VALID))
+		blk_recount_segments(q, req->bio);
+
+	return ll_new_hw_segment(q, req, bio);
+}
+
+/*
+ * blk-mq uses req->special to carry normal driver per-request payload, it
+ * does not indicate a prepared command that we cannot merge with.
+ */
+static bool req_no_special_merge(struct request *req)
+{
+	struct request_queue *q = req->q;
+
+	return !q->mq_ops && req->special;
+}
+
+static bool req_attempt_discard_merge(struct request_queue *q, struct request *req,
+		struct request *next)
+{
+	unsigned short segments = blk_rq_nr_discard_segments(req);
+
+	if (segments >= queue_max_discard_segments(q))
+		goto no_merge;
+	if (blk_rq_sectors(req) + bio_sectors(next->bio) >
+	    blk_rq_get_max_sectors(req, blk_rq_pos(req)))
+		goto no_merge;
+
+	req->nr_phys_segments = segments + blk_rq_nr_discard_segments(next);
+	return true;
+no_merge:
+	req_set_nomerge(q, req);
+	return false;
+}
+
+static int ll_merge_requests_fn(struct request_queue *q, struct request *req,
+				struct request *next)
+{
+	int total_phys_segments;
+	unsigned int seg_size =
+		req->biotail->bi_seg_back_size + next->bio->bi_seg_front_size;
+
+	/*
+	 * First check if the either of the requests are re-queued
+	 * requests.  Can't merge them if they are.
+	 */
+	if (req_no_special_merge(req) || req_no_special_merge(next))
+		return 0;
+
+	if (req_gap_back_merge(req, next->bio))
+		return 0;
+
+	/*
+	 * Will it become too large?
+	 */
+	if ((blk_rq_sectors(req) + blk_rq_sectors(next)) >
+	    blk_rq_get_max_sectors(req, blk_rq_pos(req)))
+		return 0;
+
+	total_phys_segments = req->nr_phys_segments + next->nr_phys_segments;
+	if (blk_phys_contig_segment(q, req->biotail, next->bio)) {
+		if (req->nr_phys_segments == 1)
+			req->bio->bi_seg_front_size = seg_size;
+		if (next->nr_phys_segments == 1)
+			next->biotail->bi_seg_back_size = seg_size;
+		total_phys_segments--;
+	}
+
+	if (total_phys_segments > queue_max_segments(q))
+		return 0;
+
+	if (blk_integrity_merge_rq(q, req, next) == false)
+		return 0;
+
+	/* Merge is OK... */
+	req->nr_phys_segments = total_phys_segments;
+	return 1;
+}
+
+/**
+ * blk_rq_set_mixed_merge - mark a request as mixed merge
+ * @rq: request to mark as mixed merge
+ *
+ * Description:
+ *     @rq is about to be mixed merged.  Make sure the attributes
+ *     which can be mixed are set in each bio and mark @rq as mixed
+ *     merged.
+ */
+void blk_rq_set_mixed_merge(struct request *rq)
+{
+	unsigned int ff = rq->cmd_flags & REQ_FAILFAST_MASK;
+	struct bio *bio;
+
+	if (rq->rq_flags & RQF_MIXED_MERGE)
+		return;
+
+	/*
+	 * @rq will no longer represent mixable attributes for all the
+	 * contained bios.  It will just track those of the first one.
+	 * Distributes the attributs to each bio.
+	 */
+	for (bio = rq->bio; bio; bio = bio->bi_next) {
+		WARN_ON_ONCE((bio->bi_opf & REQ_FAILFAST_MASK) &&
+			     (bio->bi_opf & REQ_FAILFAST_MASK) != ff);
+		bio->bi_opf |= ff;
+	}
+	rq->rq_flags |= RQF_MIXED_MERGE;
+}
+
+static void blk_account_io_merge(struct request *req)
+{
+	if (blk_do_io_stat(req)) {
+		struct hd_struct *part;
+		int cpu;
+
+		cpu = part_stat_lock();
+		part = req->part;
+
+		part_round_stats(req->q, cpu, part);
+		part_dec_in_flight(req->q, part, rq_data_dir(req));
+
+		hd_struct_put(part);
+		part_stat_unlock();
+	}
+}
+
+/*
+ * For non-mq, this has to be called with the request spinlock acquired.
+ * For mq with scheduling, the appropriate queue wide lock should be held.
+ */
+static struct request *attempt_merge(struct request_queue *q,
+				     struct request *req, struct request *next)
+{
+	if (!q->mq_ops)
+		lockdep_assert_held(q->queue_lock);
+
+	if (!rq_mergeable(req) || !rq_mergeable(next))
+		return NULL;
+
+	if (req_op(req) != req_op(next))
+		return NULL;
+
+	/*
+	 * not contiguous
+	 */
+	if (blk_rq_pos(req) + blk_rq_sectors(req) != blk_rq_pos(next))
+		return NULL;
+
+	if (rq_data_dir(req) != rq_data_dir(next)
+	    || req->rq_disk != next->rq_disk
+	    || req_no_special_merge(next))
+		return NULL;
+
+	if (req_op(req) == REQ_OP_WRITE_SAME &&
+	    !blk_write_same_mergeable(req->bio, next->bio))
+		return NULL;
+
+	/*
+	 * Don't allow merge of different write hints, or for a hint with
+	 * non-hint IO.
+	 */
+	if (req->write_hint != next->write_hint)
+		return NULL;
+
+	/*
+	 * If we are allowed to merge, then append bio list
+	 * from next to rq and release next. merge_requests_fn
+	 * will have updated segment counts, update sector
+	 * counts here. Handle DISCARDs separately, as they
+	 * have separate settings.
+	 */
+	if (req_op(req) == REQ_OP_DISCARD) {
+		if (!req_attempt_discard_merge(q, req, next))
+			return NULL;
+	} else if (!ll_merge_requests_fn(q, req, next))
+		return NULL;
+
+	/*
+	 * If failfast settings disagree or any of the two is already
+	 * a mixed merge, mark both as mixed before proceeding.  This
+	 * makes sure that all involved bios have mixable attributes
+	 * set properly.
+	 */
+	if (((req->rq_flags | next->rq_flags) & RQF_MIXED_MERGE) ||
+	    (req->cmd_flags & REQ_FAILFAST_MASK) !=
+	    (next->cmd_flags & REQ_FAILFAST_MASK)) {
+		blk_rq_set_mixed_merge(req);
+		blk_rq_set_mixed_merge(next);
+	}
+
+	/*
+	 * At this point we have either done a back merge or front merge. We
+	 * need the smaller start_time_ns of the merged requests to be the
+	 * current request for accounting purposes.
+	 */
+	if (next->start_time_ns < req->start_time_ns)
+		req->start_time_ns = next->start_time_ns;
+
+	req->biotail->bi_next = next->bio;
+	req->biotail = next->biotail;
+
+	req->__data_len += blk_rq_bytes(next);
+
+	if (req_op(req) != REQ_OP_DISCARD)
+		elv_merge_requests(q, req, next);
+
+	/*
+	 * 'next' is going away, so update stats accordingly
+	 */
+	blk_account_io_merge(next);
+
+	req->ioprio = ioprio_best(req->ioprio, next->ioprio);
+	if (blk_rq_cpu_valid(next))
+		req->cpu = next->cpu;
+
+	/*
+	 * ownership of bio passed from next to req, return 'next' for
+	 * the caller to free
+	 */
+	next->bio = NULL;
+	return next;
+}
+
+struct request *attempt_back_merge(struct request_queue *q, struct request *rq)
+{
+	struct request *next = elv_latter_request(q, rq);
+
+	if (next)
+		return attempt_merge(q, rq, next);
+
+	return NULL;
+}
+
+struct request *attempt_front_merge(struct request_queue *q, struct request *rq)
+{
+	struct request *prev = elv_former_request(q, rq);
+
+	if (prev)
+		return attempt_merge(q, prev, rq);
+
+	return NULL;
+}
+
+int blk_attempt_req_merge(struct request_queue *q, struct request *rq,
+			  struct request *next)
+{
+	struct elevator_queue *e = q->elevator;
+	struct request *free;
+
+	if (!e->uses_mq && e->type->ops.sq.elevator_allow_rq_merge_fn)
+		if (!e->type->ops.sq.elevator_allow_rq_merge_fn(q, rq, next))
+			return 0;
+
+	free = attempt_merge(q, rq, next);
+	if (free) {
+		__blk_put_request(q, free);
+		return 1;
+	}
+
+	return 0;
+}
+
+bool blk_rq_merge_ok(struct request *rq, struct bio *bio)
+{
+	if (!rq_mergeable(rq) || !bio_mergeable(bio))
+		return false;
+
+	if (req_op(rq) != bio_op(bio))
+		return false;
+
+	/* different data direction or already started, don't merge */
+	if (bio_data_dir(bio) != rq_data_dir(rq))
+		return false;
+
+	/* must be same device and not a special request */
+	if (rq->rq_disk != bio->bi_disk || req_no_special_merge(rq))
+		return false;
+
+	/* only merge integrity protected bio into ditto rq */
+	if (blk_integrity_merge_bio(rq->q, rq, bio) == false)
+		return false;
+
+	/* must be using the same buffer */
+	if (req_op(rq) == REQ_OP_WRITE_SAME &&
+	    !blk_write_same_mergeable(rq->bio, bio))
+		return false;
+
+	/*
+	 * Don't allow merge of different write hints, or for a hint with
+	 * non-hint IO.
+	 */
+	if (rq->write_hint != bio->bi_write_hint)
+		return false;
+
+	return true;
+}
+
+enum elv_merge blk_try_merge(struct request *rq, struct bio *bio)
+{
+	if (req_op(rq) == REQ_OP_DISCARD &&
+	    queue_max_discard_segments(rq->q) > 1)
+		return ELEVATOR_DISCARD_MERGE;
+	else if (blk_rq_pos(rq) + blk_rq_sectors(rq) == bio->bi_iter.bi_sector)
+		return ELEVATOR_BACK_MERGE;
+	else if (blk_rq_pos(rq) - bio_sectors(bio) == bio->bi_iter.bi_sector)
+		return ELEVATOR_FRONT_MERGE;
+	return ELEVATOR_NO_MERGE;
+}

src/block/blk-mq.c

@@ -1832,7 +1832,7 @@ static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
     }
 	blk_queue_split(q, &bio);
 
-    if (bio->bi_disk && (bio->bi_disk->major == 8)) { // sda, sda*
+    if (bio->bi_disk && (bio->bi_disk->major == 8)) { // sda, sda* already split - 0xa segments (16 pages each)
     	pr_debug(" *2* bio->bi_flags=0x%04x, bio->bi_phys_segments=0x%08x",bio->bi_flags, bio->bi_phys_segments); // already set
     }