OpenCloudOS-Kernel/block/blk-merge.c

// SPDX-License-Identifier: GPL-2.0
/*
 * Functions related to segment and merge handling
 */
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/bio.h>
#include <linux/blkdev.h>
#include <linux/scatterlist.h>
#include <linux/blk-cgroup.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 inline unsigned get_max_segment_size(const struct request_queue *q,
					    struct page *start_page,
					    unsigned long offset)
{
	unsigned long mask = queue_segment_boundary(q);

	offset = mask & (page_to_phys(start_page) + offset);

	/*
	 * overflow may be triggered in case of zero page physical address
	 * on 32bit arch, use queue's max segment size when that happens.
	 */
	return min_not_zero(mask - offset + 1,
			(unsigned long)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_page,
						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:
		if (queue_max_discard_segments(rq->q) > 1) {
			struct bio *bio = rq->bio;

			for_each_bio(bio)
				nr_phys_segs++;
			return nr_phys_segs;
		}
		return 1;
	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, bvec->bv_page,
					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 unsigned int blk_rq_get_max_segments(struct request *rq)
{
	if (req_op(rq) == REQ_OP_DISCARD)
		return queue_max_discard_segments(rq->q);
	return queue_max_segments(rq->q);
}

static inline int ll_new_hw_segment(struct request *req, struct bio *bio,
		unsigned int nr_phys_segs)
{
	if (!blk_cgroup_mergeable(req, bio))
		goto no_merge;

	if (blk_integrity_merge_bio(req->q, req, bio) == false)
		goto no_merge;

	/* discard request merge won't add new segment */
	if (req_op(req) == REQ_OP_DISCARD)
		return 1;

	if (req->nr_phys_segments + nr_phys_segs > blk_rq_get_max_segments(req))
		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 > blk_rq_get_max_segments(req))
		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;

		part_stat_lock();
		part = req->part;

		part_dec_in_flight(req->q, part, rq_data_dir(req));

		hd_struct_put(part);
		part_stat_unlock();
	}
}

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;

	/* 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;

	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;
}