Linux 4.8.3

[linux/fpc-iii.git] / block / blk-merge.c

/*
 * 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;
}