// 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 inline bool bio_will_gap(struct request_queue *q,
  		struct request *prev_rq, struct bio *prev, struct bio *next)
  {
  	struct bio_vec pb, nb;
  
  	if (!bio_has_data(prev) || !queue_virt_boundary(q))
  		return false;
  
  	/*
  	 * Don't merge if the 1st bio starts with non-zero offset, otherwise it
  	 * is quite difficult to respect the sg gap limit.  We work hard to
  	 * merge a huge number of small single bios in case of mkfs.
  	 */
  	if (prev_rq)
  		bio_get_first_bvec(prev_rq->bio, &pb);
  	else
  		bio_get_first_bvec(prev, &pb);

  	if (pb.bv_offset & queue_virt_boundary(q))

  		return true;
  
  	/*
  	 * We don't need to worry about the situation that the merged segment
  	 * ends in unaligned virt boundary:
  	 *
  	 * - if 'pb' ends aligned, the merged segment ends aligned
  	 * - if 'pb' ends unaligned, the next bio must include
  	 *   one single bvec of 'nb', otherwise the 'nb' can't
  	 *   merge with 'pb'
  	 */
  	bio_get_last_bvec(prev, &pb);
  	bio_get_first_bvec(next, &nb);

  	if (biovec_phys_mergeable(q, &pb, &nb))

  		return false;
  	return __bvec_gap_to_prev(q, &pb, nb.bv_offset);
  }
  
  static inline bool req_gap_back_merge(struct request *req, struct bio *bio)
  {
  	return bio_will_gap(req->q, req, req->biotail, bio);
  }
  
  static inline bool req_gap_front_merge(struct request *req, struct bio *bio)
  {
  	return bio_will_gap(req->q, NULL, bio, req->bio);
  }

  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 = 0;

  
  	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);
  }

  /*
   * Return the maximum number of sectors from the start of a bio that may be
   * submitted as a single request to a block device. If enough sectors remain,
   * align the end to the physical block size. Otherwise align the end to the
   * logical block size. This approach minimizes the number of non-aligned
   * requests that are submitted to a block device if the start of a bio is not
   * aligned to a physical block boundary.
   */

  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 max_sectors = sectors;
  	unsigned pbs = queue_physical_block_size(q) >> SECTOR_SHIFT;
  	unsigned lbs = queue_logical_block_size(q) >> SECTOR_SHIFT;
  	unsigned start_offset = bio->bi_iter.bi_sector & (pbs - 1);

  	max_sectors += start_offset;
  	max_sectors &= ~(pbs - 1);
  	if (max_sectors > start_offset)
  		return max_sectors - start_offset;

  	return sectors & (lbs - 1);

  }

  static unsigned get_max_segment_size(const struct request_queue *q,

  				     unsigned offset)
  {
  	unsigned long mask = queue_segment_boundary(q);
  
  	/* default segment boundary mask means no boundary limit */
  	if (mask == BLK_SEG_BOUNDARY_MASK)
  		return queue_max_segment_size(q);
  
  	return min_t(unsigned long, mask - (mask & offset) + 1,
  		     queue_max_segment_size(q));
  }

  /**
   * bvec_split_segs - verify whether or not a bvec should be split in the middle
   * @q:        [in] request queue associated with the bio associated with @bv
   * @bv:       [in] bvec to examine
   * @nsegs:    [in,out] Number of segments in the bio being built. Incremented
   *            by the number of segments from @bv that may be appended to that
   *            bio without exceeding @max_segs
   * @sectors:  [in,out] Number of sectors in the bio being built. Incremented
   *            by the number of sectors from @bv that may be appended to that
   *            bio without exceeding @max_sectors
   * @max_segs: [in] upper bound for *@nsegs
   * @max_sectors: [in] upper bound for *@sectors
   *
   * When splitting a bio, it can happen that a bvec is encountered that is too
   * big to fit in a single segment and hence that it has to be split in the
   * middle. This function verifies whether or not that should happen. The value
   * %true is returned if and only if appending the entire @bv to a bio with
   * *@nsegs segments and *@sectors sectors would make that bio unacceptable for
   * the block driver.

   */

  static bool bvec_split_segs(const struct request_queue *q,
  			    const struct bio_vec *bv, unsigned *nsegs,

  			    unsigned *sectors, unsigned max_segs,
  			    unsigned max_sectors)

  {

  	unsigned max_len = (min(max_sectors, UINT_MAX >> 9) - *sectors) << 9;
  	unsigned len = min(bv->bv_len, max_len);

  	unsigned total_len = 0;

  	unsigned seg_size = 0;

  	while (len && *nsegs < max_segs) {

  		seg_size = get_max_segment_size(q, bv->bv_offset + total_len);
  		seg_size = min(seg_size, len);

  		(*nsegs)++;

  		total_len += seg_size;
  		len -= seg_size;
  
  		if ((bv->bv_offset + total_len) & queue_virt_boundary(q))
  			break;
  	}

  	*sectors += total_len >> 9;

  	/* tell the caller to split the bvec if it is too big to fit */
  	return len > 0 || bv->bv_len > max_len;

  }

  /**
   * blk_bio_segment_split - split a bio in two bios
   * @q:    [in] request queue pointer
   * @bio:  [in] bio to be split
   * @bs:	  [in] bio set to allocate the clone from
   * @segs: [out] number of segments in the bio with the first half of the sectors
   *
   * Clone @bio, update the bi_iter of the clone to represent the first sectors
   * of @bio and update @bio->bi_iter to represent the remaining sectors. The
   * following is guaranteed for the cloned bio:
   * - That it has at most get_max_io_size(@q, @bio) sectors.
   * - That it has at most queue_max_segments(@q) segments.
   *
   * Except for discard requests the cloned bio will point at the bi_io_vec of
   * the original bio. It is the responsibility of the caller to ensure that the
   * original bio is not freed before the cloned bio. The caller is also
   * responsible for ensuring that @bs is only destroyed after processing of the
   * split bio has finished.
   */

  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 nsegs = 0, sectors = 0;

  	const unsigned max_sectors = get_max_io_size(q, bio);

  	const unsigned max_segs = queue_max_segments(q);

  	bio_for_each_bvec(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 (nsegs < max_segs &&
  		    sectors + (bv.bv_len >> 9) <= max_sectors &&
  		    bv.bv_offset + bv.bv_len <= PAGE_SIZE) {
  			nsegs++;
  			sectors += bv.bv_len >> 9;
  		} else if (bvec_split_segs(q, &bv, &nsegs, &sectors, max_segs,
  					 max_sectors)) {

  			goto split;

  		}

  		bvprv = bv;

  		bvprvp = &bvprv;

  	}

  	*segs = nsegs;
  	return NULL;

  split:

  	*segs = nsegs;

  	return bio_split(bio, sectors, GFP_NOIO, bs);

  }

  /**
   * __blk_queue_split - split a bio and submit the second half
   * @q:       [in] request queue pointer
   * @bio:     [in, out] bio to be split
   * @nr_segs: [out] number of segments in the first bio
   *
   * Split a bio into two bios, chain the two bios, submit the second half and
   * store a pointer to the first half in *@bio. If the second bio is still too
   * big it will be split by a recursive call to this function. Since this
   * function may allocate a new bio from @q->bio_split, it is the responsibility
   * of the caller to ensure that @q is only released after processing of the
   * split bio has finished.
   */

  void __blk_queue_split(struct request_queue *q, struct bio **bio,
  		unsigned int *nr_segs)

  {

  	struct bio *split;

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

  		split = blk_bio_discard_split(q, *bio, &q->bio_split, nr_segs);

  		break;

  	case REQ_OP_WRITE_ZEROES:

  		split = blk_bio_write_zeroes_split(q, *bio, &q->bio_split,
  				nr_segs);

  		break;

  	case REQ_OP_WRITE_SAME:

  		split = blk_bio_write_same_split(q, *bio, &q->bio_split,
  				nr_segs);

  		break;
  	default:

  		split = blk_bio_segment_split(q, *bio, &q->bio_split, nr_segs);

  		break;
  	}

  	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;
  	}
  }

  /**
   * blk_queue_split - split a bio and submit the second half
   * @q:   [in] request queue pointer
   * @bio: [in, out] bio to be split
   *
   * Split a bio into two bios, chains the two bios, submit the second half and
   * store a pointer to the first half in *@bio. Since this function may allocate
   * a new bio from @q->bio_split, it is the responsibility of the caller to
   * ensure that @q is only released after processing of the split bio has
   * finished.
   */

  void blk_queue_split(struct request_queue *q, struct bio **bio)
  {
  	unsigned int nr_segs;
  
  	__blk_queue_split(q, bio, &nr_segs);
  }

  EXPORT_SYMBOL(blk_queue_split);

  unsigned int blk_recalc_rq_segments(struct request *rq)

  {

  	unsigned int nr_phys_segs = 0;

  	unsigned int nr_sectors = 0;

  	struct req_iterator iter;

  	struct bio_vec bv;

  	if (!rq->bio)

  		return 0;

  	switch (bio_op(rq->bio)) {

  	case REQ_OP_DISCARD:
  	case REQ_OP_SECURE_ERASE:

  	case REQ_OP_WRITE_ZEROES:

  		return 0;
  	case REQ_OP_WRITE_SAME:

  		return 1;

  	}

  	rq_for_each_bvec(bv, rq, iter)

  		bvec_split_segs(rq->q, &bv, &nr_phys_segs, &nr_sectors,

  				UINT_MAX, UINT_MAX);

  	return nr_phys_segs;
  }

  static inline struct scatterlist *blk_next_sg(struct scatterlist **sg,

  		struct scatterlist *sglist)
  {
  	if (!*sg)
  		return sglist;
  
  	/*
  	 * 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);
  	return sg_next(*sg);
  }
  
  static unsigned blk_bvec_map_sg(struct request_queue *q,
  		struct bio_vec *bvec, struct scatterlist *sglist,
  		struct scatterlist **sg)
  {
  	unsigned nbytes = bvec->bv_len;

  	unsigned nsegs = 0, total = 0;

  
  	while (nbytes > 0) {

  		unsigned offset = bvec->bv_offset + total;
  		unsigned len = min(get_max_segment_size(q, offset), nbytes);

  		struct page *page = bvec->bv_page;
  
  		/*
  		 * Unfortunately a fair number of drivers barf on scatterlists
  		 * that have an offset larger than PAGE_SIZE, despite other
  		 * subsystems dealing with that invariant just fine.  For now
  		 * stick to the legacy format where we never present those from
  		 * the block layer, but the code below should be removed once
  		 * these offenders (mostly MMC/SD drivers) are fixed.
  		 */
  		page += (offset >> PAGE_SHIFT);
  		offset &= ~PAGE_MASK;

  
  		*sg = blk_next_sg(sg, sglist);

  		sg_set_page(*sg, page, len, offset);

  		total += len;
  		nbytes -= len;

  		nsegs++;
  	}
  
  	return nsegs;
  }

  static inline int __blk_bvec_map_sg(struct bio_vec bv,
  		struct scatterlist *sglist, struct scatterlist **sg)
  {
  	*sg = blk_next_sg(sg, sglist);
  	sg_set_page(*sg, bv.bv_page, bv.bv_len, bv.bv_offset);
  	return 1;
  }

  /* only try to merge bvecs into one sg if they are from two bios */
  static inline bool
  __blk_segment_map_sg_merge(struct request_queue *q, struct bio_vec *bvec,
  			   struct bio_vec *bvprv, struct scatterlist **sg)

  {
  
  	int nbytes = bvec->bv_len;

  	if (!*sg)
  		return false;

  	if ((*sg)->length + nbytes > queue_max_segment_size(q))
  		return false;
  
  	if (!biovec_phys_mergeable(q, bvprv, bvec))
  		return false;
  
  	(*sg)->length += nbytes;
  
  	return true;

  }

  static int __blk_bios_map_sg(struct request_queue *q, struct bio *bio,
  			     struct scatterlist *sglist,
  			     struct scatterlist **sg)

  {

  	struct bio_vec uninitialized_var(bvec), bvprv = { NULL };

  	struct bvec_iter iter;

  	int nsegs = 0;

  	bool new_bio = false;

  	for_each_bio(bio) {
  		bio_for_each_bvec(bvec, bio, iter) {
  			/*
  			 * Only try to merge bvecs from two bios given we
  			 * have done bio internal merge when adding pages
  			 * to bio
  			 */
  			if (new_bio &&
  			    __blk_segment_map_sg_merge(q, &bvec, &bvprv, sg))
  				goto next_bvec;
  
  			if (bvec.bv_offset + bvec.bv_len <= PAGE_SIZE)
  				nsegs += __blk_bvec_map_sg(bvec, sglist, sg);
  			else
  				nsegs += blk_bvec_map_sg(q, &bvec, sglist, sg);
   next_bvec:
  			new_bio = false;
  		}

  		if (likely(bio->bi_iter.bi_size)) {
  			bvprv = bvec;
  			new_bio = true;
  		}

  	}

  	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(rq->special_vec, sglist, &sg);

  	else if (rq->bio && bio_op(rq->bio) == REQ_OP_WRITE_SAME)

  		nsegs = __blk_bvec_map_sg(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 *req, struct bio *bio,
  		unsigned int nr_phys_segs)

  {

  	if (req->nr_phys_segments + nr_phys_segs > queue_max_segments(req->q))

  		goto no_merge;

  	if (blk_integrity_merge_bio(req->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(req->q, req);

  	return 0;

  }

  int ll_back_merge_fn(struct request *req, struct bio *bio, unsigned int nr_segs)

  {

  	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(req->q, req);

  		return 0;
  	}

  	return ll_new_hw_segment(req, bio, nr_segs);

  }

  int ll_front_merge_fn(struct request *req, struct bio *bio, unsigned int nr_segs)

  {

  	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(req->q, req);

  		return 0;
  	}

  	return ll_new_hw_segment(req, bio, nr_segs);

  }

  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;

  	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 (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;

  		part_stat_lock();

  		part = req->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;
  }

  static 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 (!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)

  		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 (req->ioprio != next->ioprio)
  		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);

  	/*
  	 * 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 request *free;

  	free = attempt_merge(q, rq, next);
  	if (free) {

  		blk_put_request(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 */
  	if (rq->rq_disk != bio->bi_disk)

  		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;

  	if (rq->ioprio != bio_prio(bio))
  		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;
  }