/*
   * 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_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);

  	unsigned bvecs = 0;

  	bio_for_each_segment(bv, bio, iter) {

  		/*

  		 * With arbitrary bio size, the incoming bio may be very
  		 * big. We have to split the bio into small bios so that
  		 * each holds at most BIO_MAX_PAGES bvecs because
  		 * bio_clone() can fail to allocate big bvecs.
  		 *
  		 * It should have been better to apply the limit per
  		 * request queue in which bio_clone() is involved,
  		 * instead of globally. The biggest blocker is the
  		 * bio_clone() in bio bounce.
  		 *
  		 * If bio is splitted by this reason, we should have
  		 * allowed to continue bios merging, but don't do
  		 * that now for making the change simple.
  		 *
  		 * TODO: deal with bio bounce's bio_clone() gracefully
  		 * and convert the global limit into per-queue limit.
  		 */
  		if (bvecs++ >= BIO_MAX_PAGES)
  			goto split;
  
  		/*

  		 * 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;

  			}

  			if (sectors)
  				goto split;
  			/* Make this single bvec as the 1st segment */

  		}

  		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;

  
  			if (nsegs == 1 && seg_size > front_seg_size)
  				front_seg_size = seg_size;

  			continue;
  		}
  new_segment:
  		if (nsegs == queue_max_segments(q))
  			goto split;
  
  		nsegs++;
  		bvprv = bv;

  		bvprvp = &bvprv;

  		seg_size = bv.bv_len;

  		sectors += bv.bv_len >> 9;

  
  		if (nsegs == 1 && seg_size > front_seg_size)
  			front_seg_size = seg_size;

  	}

  	do_split = false;

  split:

  	*segs = nsegs;

  
  	if (do_split) {
  		new = bio_split(bio, sectors, GFP_NOIO, bs);
  		if (new)
  			bio = new;
  	}
  
  	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_set *bs)
  {

  	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, bs, &nsegs);

  		break;
  	case REQ_OP_WRITE_SAME:

  		split = blk_bio_write_same_split(q, *bio, bs, &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;

  		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;

  	/*
  	 * This should probably be returning 0, but blk_add_request_payload()
  	 * (Christoph!!!!)
  	 */

  	if (bio_op(bio) == REQ_OP_DISCARD || bio_op(bio) == REQ_OP_SECURE_ERASE)

  		return 1;

  	if (bio_op(bio) == 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 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 nsegs, cluster;
  
  	nsegs = 0;

  	cluster = blk_queue_cluster(q);

  	switch (bio_op(bio)) {
  	case REQ_OP_DISCARD:
  	case REQ_OP_SECURE_ERASE:

  		/*
  		 * This is a hack - drivers should be neither modifying the
  		 * biovec, nor relying on bi_vcnt - but because of
  		 * blk_add_request_payload(), a discard bio may or may not have
  		 * a payload we need to set up here (thank you Christoph) and
  		 * bi_vcnt is really the only way of telling if we need to.
  		 */

  		if (!bio->bi_vcnt)
  			return 0;
  		/* Fall through */
  	case REQ_OP_WRITE_SAME:

  		*sg = sglist;
  		bvec = bio_iovec(bio);
  		sg_set_page(*sg, bvec.bv_page, bvec.bv_len, bvec.bv_offset);
  		return 1;

  	default:
  		break;

  	}
  
  	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->bio)
  		nsegs = __blk_bios_map_sg(q, rq->bio, sglist, &sg);

  
  	if (unlikely(rq->cmd_flags & REQ_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 > rq->nr_phys_segments);

  	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->cmd_flags |= REQ_NOMERGE;
  	if (req == q->last_merge)
  		q->last_merge = NULL;
  	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->cmd_flags |= REQ_NOMERGE;
  		if (req == q->last_merge)
  			q->last_merge = NULL;
  		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->cmd_flags |= REQ_NOMERGE;
  		if (req == q->last_merge)
  			q->last_merge = NULL;
  		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 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->cmd_flags & REQ_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->cmd_flags |= REQ_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(cpu, part);

  		part_dec_in_flight(part, rq_data_dir(req));

  		hd_struct_put(part);

  		part_stat_unlock();
  	}
  }

  /*
   * Has to be called with the request spinlock acquired
   */
  static int attempt_merge(struct request_queue *q, struct request *req,
  			  struct request *next)
  {
  	if (!rq_mergeable(req) || !rq_mergeable(next))
  		return 0;

  	if (req_op(req) != req_op(next))

  		return 0;

  	/*
  	 * not contiguous
  	 */

  	if (blk_rq_pos(req) + blk_rq_sectors(req) != blk_rq_pos(next))

  		return 0;
  
  	if (rq_data_dir(req) != rq_data_dir(next)
  	    || req->rq_disk != next->rq_disk

  	    || req_no_special_merge(next))

  		return 0;

  	if (req_op(req) == REQ_OP_WRITE_SAME &&

  	    !blk_write_same_mergeable(req->bio, next->bio))
  		return 0;

  	/*
  	 * 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.
  	 */
  	if (!ll_merge_requests_fn(q, req, next))
  		return 0;
  
  	/*

  	 * 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->cmd_flags | next->cmd_flags) & REQ_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 of
  	 * the merged requests to be the current request
  	 * for accounting purposes.
  	 */
  	if (time_after(req->start_time, next->start_time))
  		req->start_time = next->start_time;
  
  	req->biotail->bi_next = next->bio;
  	req->biotail = next->biotail;

  	req->__data_len += blk_rq_bytes(next);

  
  	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;

  	/* owner-ship of bio passed from next to req */
  	next->bio = NULL;

  	__blk_put_request(q, next);
  	return 1;
  }
  
  int 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 0;
  }
  
  int 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 0;
  }

  
  int blk_attempt_req_merge(struct request_queue *q, struct request *rq,
  			  struct request *next)
  {

  	struct elevator_queue *e = q->elevator;
  
  	if (e->type->ops.elevator_allow_rq_merge_fn)
  		if (!e->type->ops.elevator_allow_rq_merge_fn(q, rq, next))
  			return 0;

  	return attempt_merge(q, rq, next);
  }

  
  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_bdev->bd_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;

  	return true;
  }
  
  int blk_try_merge(struct request *rq, struct bio *bio)
  {

  	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;
  }