block/blk-merge.c - third_party/linux - Git at Google

 // 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 <linux/blkdev.h>
 #include <linux/blk-cgroup.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,
 			bio_allowed_max_sectors(q));
 	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 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))
 			goto split;

 		if (sectors + (bv.bv_len >> 9) > max_sectors) {
 			/*
 			 * 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)) {
 			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;

 			seg_size += bv.bv_len;
 			bvprv = bv;
 			bvprvp = &bvprv;
 			sectors += bv.bv_len >> 9;

 			continue;
 		}
 new_segment:
 		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:
 	*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;

 	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 (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 = bio_segments(bio);

 	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_cgroup_mergeable(req, bio))
 		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_cgroup_mergeable(req, next->bio))
 		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();
 	}
 }
 /*
  * Two cases of handling DISCARD merge:
  * If max_discard_segments > 1, the driver takes every bio
  * as a range and send them to controller together. The ranges
  * needn't to be contiguous.
  * Otherwise, the bios/requests will be handled as same as
  * others which should be contiguous.
  */
 static inline bool blk_discard_mergable(struct request *req)
 {
 	if (req_op(req) == REQ_OP_DISCARD &&
 	    queue_max_discard_segments(req->q) > 1)
 		return true;
 	return false;
 }

 enum elv_merge blk_try_req_merge(struct request *req, struct request *next)
 {
 	if (blk_discard_mergable(req))
 		return ELEVATOR_DISCARD_MERGE;
 	else if (blk_rq_pos(req) + blk_rq_sectors(req) == blk_rq_pos(next))
 		return ELEVATOR_BACK_MERGE;

 	return ELEVATOR_NO_MERGE;
 }

 /*
  * 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;

 	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.
 	 */

 	switch (blk_try_req_merge(req, next)) {
 	case ELEVATOR_DISCARD_MERGE:
 		if (!req_attempt_discard_merge(q, req, next))
 			return NULL;
 		break;
 	case ELEVATOR_BACK_MERGE:
 		if (!ll_merge_requests_fn(q, req, next))
 			return NULL;
 		break;
 	default:
 		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 (!blk_discard_mergable(req))
 		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;

 	/* don't merge across cgroup boundaries */
 	if (!blk_cgroup_mergeable(rq, bio))
 		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 (blk_discard_mergable(rq))
 		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;
 }
	// 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 <linux/blkdev.h>
	#include <linux/blk-cgroup.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,
	bio_allowed_max_sectors(q));
	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 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))
	goto split;

	if (sectors + (bv.bv_len >> 9) > max_sectors) {
	/*
	* 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)) {
	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;

	seg_size += bv.bv_len;
	bvprv = bv;
	bvprvp = &bvprv;
	sectors += bv.bv_len >> 9;

	continue;
	}
	new_segment:
	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:
	*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;

	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 (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 = bio_segments(bio);

	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_cgroup_mergeable(req, bio))
	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_cgroup_mergeable(req, next->bio))
	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();
	}
	}
	/*
	* Two cases of handling DISCARD merge:
	* If max_discard_segments > 1, the driver takes every bio
	* as a range and send them to controller together. The ranges
	* needn't to be contiguous.
	* Otherwise, the bios/requests will be handled as same as
	* others which should be contiguous.
	*/
	static inline bool blk_discard_mergable(struct request *req)
	{
	if (req_op(req) == REQ_OP_DISCARD &&
	queue_max_discard_segments(req->q) > 1)
	return true;
	return false;
	}

	enum elv_merge blk_try_req_merge(struct request req, struct request next)
	{
	if (blk_discard_mergable(req))
	return ELEVATOR_DISCARD_MERGE;
	else if (blk_rq_pos(req) + blk_rq_sectors(req) == blk_rq_pos(next))
	return ELEVATOR_BACK_MERGE;

	return ELEVATOR_NO_MERGE;
	}

	/*
	* 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;

	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.
	*/

	switch (blk_try_req_merge(req, next)) {
	case ELEVATOR_DISCARD_MERGE:
	if (!req_attempt_discard_merge(q, req, next))
	return NULL;
	break;
	case ELEVATOR_BACK_MERGE:
	if (!ll_merge_requests_fn(q, req, next))
	return NULL;
	break;
	default:
	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 (!blk_discard_mergable(req))
	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;

	/* don't merge across cgroup boundaries */
	if (!blk_cgroup_mergeable(rq, bio))
	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 (blk_discard_mergable(rq))
	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;
	}