// 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);
}

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 unsigned get_max_segment_size(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));
}

/*
 * Split the bvec @bv into segments, and update all kinds of
 * variables.
 */
static bool bvec_split_segs(struct request_queue *q, struct bio_vec *bv,
                unsigned *nsegs, unsigned *sectors, unsigned max_segs)
{
        unsigned len = bv->bv_len;
        unsigned total_len = 0;
        unsigned new_nsegs = 0, seg_size = 0;

        /*
         * Multi-page bvec may be too big to hold in one segment, so the
         * current bvec has to be splitted as multiple segments.
         */
        while (len && new_nsegs + *nsegs < max_segs) {
                seg_size = get_max_segment_size(q, bv->bv_offset + total_len);
                seg_size = min(seg_size, len);

                new_nsegs++;
                total_len += seg_size;
                len -= seg_size;

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

        if (new_nsegs) {
                *nsegs += new_nsegs;
                if (sectors)
                        *sectors += total_len >> 9;
        }

        /* split in the middle of the bvec if len != 0 */
        return !!len;
}

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 (sectors + (bv.bv_len >> 9) > max_sectors) {
                        /*
                         * Consider this a new segment if we're splitting in
                         * the middle of this vector.
                         */
                        if (nsegs < max_segs &&
                            sectors < max_sectors) {
                                /* split in the middle of bvec */
                                bv.bv_len = (max_sectors - sectors) << 9;
                                bvec_split_segs(q, &bv, &nsegs,
                                                &sectors, max_segs);
                        }
                        goto split;
                }

                if (nsegs == max_segs)
                        goto split;

                bvprv = bv;
                bvprvp = &bvprv;

                if (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)) {
                        goto split;
                }
        }

        *segs = nsegs;
        return NULL;
split:
        *segs = nsegs;
        return bio_split(bio, sectors, GFP_NOIO, bs);
}

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;
        }
}

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;
        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, NULL, 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;
}