1
2
3
4
5#include <linux/kernel.h>
6#include <linux/module.h>
7#include <linux/bio.h>
8#include <linux/blkdev.h>
9#include <linux/scatterlist.h>
10
11#include <trace/events/block.h>
12
13#include "blk.h"
14
15static struct bio *blk_bio_discard_split(struct request_queue *q,
16 struct bio *bio,
17 struct bio_set *bs,
18 unsigned *nsegs)
19{
20 unsigned int max_discard_sectors, granularity;
21 int alignment;
22 sector_t tmp;
23 unsigned split_sectors;
24
25 *nsegs = 1;
26
27
28 granularity = max(q->limits.discard_granularity >> 9, 1U);
29
30 max_discard_sectors = min(q->limits.max_discard_sectors, UINT_MAX >> 9);
31 max_discard_sectors -= max_discard_sectors % granularity;
32
33 if (unlikely(!max_discard_sectors)) {
34
35 return NULL;
36 }
37
38 if (bio_sectors(bio) <= max_discard_sectors)
39 return NULL;
40
41 split_sectors = max_discard_sectors;
42
43
44
45
46
47 alignment = (q->limits.discard_alignment >> 9) % granularity;
48
49 tmp = bio->bi_iter.bi_sector + split_sectors - alignment;
50 tmp = sector_div(tmp, granularity);
51
52 if (split_sectors > tmp)
53 split_sectors -= tmp;
54
55 return bio_split(bio, split_sectors, GFP_NOIO, bs);
56}
57
58static struct bio *blk_bio_write_zeroes_split(struct request_queue *q,
59 struct bio *bio, struct bio_set *bs, unsigned *nsegs)
60{
61 *nsegs = 1;
62
63 if (!q->limits.max_write_zeroes_sectors)
64 return NULL;
65
66 if (bio_sectors(bio) <= q->limits.max_write_zeroes_sectors)
67 return NULL;
68
69 return bio_split(bio, q->limits.max_write_zeroes_sectors, GFP_NOIO, bs);
70}
71
72static struct bio *blk_bio_write_same_split(struct request_queue *q,
73 struct bio *bio,
74 struct bio_set *bs,
75 unsigned *nsegs)
76{
77 *nsegs = 1;
78
79 if (!q->limits.max_write_same_sectors)
80 return NULL;
81
82 if (bio_sectors(bio) <= q->limits.max_write_same_sectors)
83 return NULL;
84
85 return bio_split(bio, q->limits.max_write_same_sectors, GFP_NOIO, bs);
86}
87
88static inline unsigned get_max_io_size(struct request_queue *q,
89 struct bio *bio)
90{
91 unsigned sectors = blk_max_size_offset(q, bio->bi_iter.bi_sector);
92 unsigned mask = queue_logical_block_size(q) - 1;
93
94
95 sectors &= ~(mask >> 9);
96
97 return sectors;
98}
99
100static struct bio *blk_bio_segment_split(struct request_queue *q,
101 struct bio *bio,
102 struct bio_set *bs,
103 unsigned *segs)
104{
105 struct bio_vec bv, bvprv, *bvprvp = NULL;
106 struct bvec_iter iter;
107 unsigned seg_size = 0, nsegs = 0, sectors = 0;
108 unsigned front_seg_size = bio->bi_seg_front_size;
109 bool do_split = true;
110 struct bio *new = NULL;
111 const unsigned max_sectors = get_max_io_size(q, bio);
112
113 bio_for_each_segment(bv, bio, iter) {
114
115
116
117
118 if (bvprvp && bvec_gap_to_prev(q, bvprvp, bv.bv_offset))
119 goto split;
120
121 if (sectors + (bv.bv_len >> 9) > max_sectors) {
122
123
124
125
126 if (nsegs < queue_max_segments(q) &&
127 sectors < max_sectors) {
128 nsegs++;
129 sectors = max_sectors;
130 }
131 goto split;
132 }
133
134 if (bvprvp && blk_queue_cluster(q)) {
135 if (seg_size + bv.bv_len > queue_max_segment_size(q))
136 goto new_segment;
137 if (!BIOVEC_PHYS_MERGEABLE(bvprvp, &bv))
138 goto new_segment;
139 if (!BIOVEC_SEG_BOUNDARY(q, bvprvp, &bv))
140 goto new_segment;
141
142 seg_size += bv.bv_len;
143 bvprv = bv;
144 bvprvp = &bvprv;
145 sectors += bv.bv_len >> 9;
146
147 continue;
148 }
149new_segment:
150 if (nsegs == queue_max_segments(q))
151 goto split;
152
153 if (nsegs == 1 && seg_size > front_seg_size)
154 front_seg_size = seg_size;
155
156 nsegs++;
157 bvprv = bv;
158 bvprvp = &bvprv;
159 seg_size = bv.bv_len;
160 sectors += bv.bv_len >> 9;
161
162 }
163
164 do_split = false;
165split:
166 *segs = nsegs;
167
168 if (do_split) {
169 new = bio_split(bio, sectors, GFP_NOIO, bs);
170 if (new)
171 bio = new;
172 }
173
174 if (nsegs == 1 && seg_size > front_seg_size)
175 front_seg_size = seg_size;
176 bio->bi_seg_front_size = front_seg_size;
177 if (seg_size > bio->bi_seg_back_size)
178 bio->bi_seg_back_size = seg_size;
179
180 return do_split ? new : NULL;
181}
182
183void blk_queue_split(struct request_queue *q, struct bio **bio)
184{
185 struct bio *split, *res;
186 unsigned nsegs;
187
188 switch (bio_op(*bio)) {
189 case REQ_OP_DISCARD:
190 case REQ_OP_SECURE_ERASE:
191 split = blk_bio_discard_split(q, *bio, &q->bio_split, &nsegs);
192 break;
193 case REQ_OP_WRITE_ZEROES:
194 split = blk_bio_write_zeroes_split(q, *bio, &q->bio_split, &nsegs);
195 break;
196 case REQ_OP_WRITE_SAME:
197 split = blk_bio_write_same_split(q, *bio, &q->bio_split, &nsegs);
198 break;
199 default:
200 split = blk_bio_segment_split(q, *bio, &q->bio_split, &nsegs);
201 break;
202 }
203
204
205 res = split ? split : *bio;
206 res->bi_phys_segments = nsegs;
207 bio_set_flag(res, BIO_SEG_VALID);
208
209 if (split) {
210
211 split->bi_opf |= REQ_NOMERGE;
212
213
214
215
216
217
218
219
220
221 bio_set_flag(*bio, BIO_QUEUE_ENTERED);
222
223 bio_chain(split, *bio);
224 trace_block_split(q, split, (*bio)->bi_iter.bi_sector);
225 generic_make_request(*bio);
226 *bio = split;
227 }
228}
229EXPORT_SYMBOL(blk_queue_split);
230
231static unsigned int __blk_recalc_rq_segments(struct request_queue *q,
232 struct bio *bio,
233 bool no_sg_merge)
234{
235 struct bio_vec bv, bvprv = { NULL };
236 int cluster, prev = 0;
237 unsigned int seg_size, nr_phys_segs;
238 struct bio *fbio, *bbio;
239 struct bvec_iter iter;
240
241 if (!bio)
242 return 0;
243
244 switch (bio_op(bio)) {
245 case REQ_OP_DISCARD:
246 case REQ_OP_SECURE_ERASE:
247 case REQ_OP_WRITE_ZEROES:
248 return 0;
249 case REQ_OP_WRITE_SAME:
250 return 1;
251 }
252
253 fbio = bio;
254 cluster = blk_queue_cluster(q);
255 seg_size = 0;
256 nr_phys_segs = 0;
257 for_each_bio(bio) {
258 bio_for_each_segment(bv, bio, iter) {
259
260
261
262
263 if (no_sg_merge)
264 goto new_segment;
265
266 if (prev && cluster) {
267 if (seg_size + bv.bv_len
268 > queue_max_segment_size(q))
269 goto new_segment;
270 if (!BIOVEC_PHYS_MERGEABLE(&bvprv, &bv))
271 goto new_segment;
272 if (!BIOVEC_SEG_BOUNDARY(q, &bvprv, &bv))
273 goto new_segment;
274
275 seg_size += bv.bv_len;
276 bvprv = bv;
277 continue;
278 }
279new_segment:
280 if (nr_phys_segs == 1 && seg_size >
281 fbio->bi_seg_front_size)
282 fbio->bi_seg_front_size = seg_size;
283
284 nr_phys_segs++;
285 bvprv = bv;
286 prev = 1;
287 seg_size = bv.bv_len;
288 }
289 bbio = bio;
290 }
291
292 if (nr_phys_segs == 1 && seg_size > fbio->bi_seg_front_size)
293 fbio->bi_seg_front_size = seg_size;
294 if (seg_size > bbio->bi_seg_back_size)
295 bbio->bi_seg_back_size = seg_size;
296
297 return nr_phys_segs;
298}
299
300void blk_recalc_rq_segments(struct request *rq)
301{
302 bool no_sg_merge = !!test_bit(QUEUE_FLAG_NO_SG_MERGE,
303 &rq->q->queue_flags);
304
305 rq->nr_phys_segments = __blk_recalc_rq_segments(rq->q, rq->bio,
306 no_sg_merge);
307}
308
309void blk_recount_segments(struct request_queue *q, struct bio *bio)
310{
311 unsigned short seg_cnt;
312
313
314 if (bio_flagged(bio, BIO_CLONED))
315 seg_cnt = bio_segments(bio);
316 else
317 seg_cnt = bio->bi_vcnt;
318
319 if (test_bit(QUEUE_FLAG_NO_SG_MERGE, &q->queue_flags) &&
320 (seg_cnt < queue_max_segments(q)))
321 bio->bi_phys_segments = seg_cnt;
322 else {
323 struct bio *nxt = bio->bi_next;
324
325 bio->bi_next = NULL;
326 bio->bi_phys_segments = __blk_recalc_rq_segments(q, bio, false);
327 bio->bi_next = nxt;
328 }
329
330 bio_set_flag(bio, BIO_SEG_VALID);
331}
332EXPORT_SYMBOL(blk_recount_segments);
333
334static int blk_phys_contig_segment(struct request_queue *q, struct bio *bio,
335 struct bio *nxt)
336{
337 struct bio_vec end_bv = { NULL }, nxt_bv;
338
339 if (!blk_queue_cluster(q))
340 return 0;
341
342 if (bio->bi_seg_back_size + nxt->bi_seg_front_size >
343 queue_max_segment_size(q))
344 return 0;
345
346 if (!bio_has_data(bio))
347 return 1;
348
349 bio_get_last_bvec(bio, &end_bv);
350 bio_get_first_bvec(nxt, &nxt_bv);
351
352 if (!BIOVEC_PHYS_MERGEABLE(&end_bv, &nxt_bv))
353 return 0;
354
355
356
357
358
359 if (BIOVEC_SEG_BOUNDARY(q, &end_bv, &nxt_bv))
360 return 1;
361
362 return 0;
363}
364
365static inline void
366__blk_segment_map_sg(struct request_queue *q, struct bio_vec *bvec,
367 struct scatterlist *sglist, struct bio_vec *bvprv,
368 struct scatterlist **sg, int *nsegs, int *cluster)
369{
370
371 int nbytes = bvec->bv_len;
372
373 if (*sg && *cluster) {
374 if ((*sg)->length + nbytes > queue_max_segment_size(q))
375 goto new_segment;
376
377 if (!BIOVEC_PHYS_MERGEABLE(bvprv, bvec))
378 goto new_segment;
379 if (!BIOVEC_SEG_BOUNDARY(q, bvprv, bvec))
380 goto new_segment;
381
382 (*sg)->length += nbytes;
383 } else {
384new_segment:
385 if (!*sg)
386 *sg = sglist;
387 else {
388
389
390
391
392
393
394
395
396
397
398 sg_unmark_end(*sg);
399 *sg = sg_next(*sg);
400 }
401
402 sg_set_page(*sg, bvec->bv_page, nbytes, bvec->bv_offset);
403 (*nsegs)++;
404 }
405 *bvprv = *bvec;
406}
407
408static inline int __blk_bvec_map_sg(struct request_queue *q, struct bio_vec bv,
409 struct scatterlist *sglist, struct scatterlist **sg)
410{
411 *sg = sglist;
412 sg_set_page(*sg, bv.bv_page, bv.bv_len, bv.bv_offset);
413 return 1;
414}
415
416static int __blk_bios_map_sg(struct request_queue *q, struct bio *bio,
417 struct scatterlist *sglist,
418 struct scatterlist **sg)
419{
420 struct bio_vec bvec, bvprv = { NULL };
421 struct bvec_iter iter;
422 int cluster = blk_queue_cluster(q), nsegs = 0;
423
424 for_each_bio(bio)
425 bio_for_each_segment(bvec, bio, iter)
426 __blk_segment_map_sg(q, &bvec, sglist, &bvprv, sg,
427 &nsegs, &cluster);
428
429 return nsegs;
430}
431
432
433
434
435
436int blk_rq_map_sg(struct request_queue *q, struct request *rq,
437 struct scatterlist *sglist)
438{
439 struct scatterlist *sg = NULL;
440 int nsegs = 0;
441
442 if (rq->rq_flags & RQF_SPECIAL_PAYLOAD)
443 nsegs = __blk_bvec_map_sg(q, rq->special_vec, sglist, &sg);
444 else if (rq->bio && bio_op(rq->bio) == REQ_OP_WRITE_SAME)
445 nsegs = __blk_bvec_map_sg(q, bio_iovec(rq->bio), sglist, &sg);
446 else if (rq->bio)
447 nsegs = __blk_bios_map_sg(q, rq->bio, sglist, &sg);
448
449 if (unlikely(rq->rq_flags & RQF_COPY_USER) &&
450 (blk_rq_bytes(rq) & q->dma_pad_mask)) {
451 unsigned int pad_len =
452 (q->dma_pad_mask & ~blk_rq_bytes(rq)) + 1;
453
454 sg->length += pad_len;
455 rq->extra_len += pad_len;
456 }
457
458 if (q->dma_drain_size && q->dma_drain_needed(rq)) {
459 if (op_is_write(req_op(rq)))
460 memset(q->dma_drain_buffer, 0, q->dma_drain_size);
461
462 sg_unmark_end(sg);
463 sg = sg_next(sg);
464 sg_set_page(sg, virt_to_page(q->dma_drain_buffer),
465 q->dma_drain_size,
466 ((unsigned long)q->dma_drain_buffer) &
467 (PAGE_SIZE - 1));
468 nsegs++;
469 rq->extra_len += q->dma_drain_size;
470 }
471
472 if (sg)
473 sg_mark_end(sg);
474
475
476
477
478
479 WARN_ON(nsegs > blk_rq_nr_phys_segments(rq));
480
481 return nsegs;
482}
483EXPORT_SYMBOL(blk_rq_map_sg);
484
485static inline int ll_new_hw_segment(struct request_queue *q,
486 struct request *req,
487 struct bio *bio)
488{
489 int nr_phys_segs = bio_phys_segments(q, bio);
490
491 if (req->nr_phys_segments + nr_phys_segs > queue_max_segments(q))
492 goto no_merge;
493
494 if (blk_integrity_merge_bio(q, req, bio) == false)
495 goto no_merge;
496
497
498
499
500
501 req->nr_phys_segments += nr_phys_segs;
502 return 1;
503
504no_merge:
505 req_set_nomerge(q, req);
506 return 0;
507}
508
509int ll_back_merge_fn(struct request_queue *q, struct request *req,
510 struct bio *bio)
511{
512 if (req_gap_back_merge(req, bio))
513 return 0;
514 if (blk_integrity_rq(req) &&
515 integrity_req_gap_back_merge(req, bio))
516 return 0;
517 if (blk_rq_sectors(req) + bio_sectors(bio) >
518 blk_rq_get_max_sectors(req, blk_rq_pos(req))) {
519 req_set_nomerge(q, req);
520 return 0;
521 }
522 if (!bio_flagged(req->biotail, BIO_SEG_VALID))
523 blk_recount_segments(q, req->biotail);
524 if (!bio_flagged(bio, BIO_SEG_VALID))
525 blk_recount_segments(q, bio);
526
527 return ll_new_hw_segment(q, req, bio);
528}
529
530int ll_front_merge_fn(struct request_queue *q, struct request *req,
531 struct bio *bio)
532{
533
534 if (req_gap_front_merge(req, bio))
535 return 0;
536 if (blk_integrity_rq(req) &&
537 integrity_req_gap_front_merge(req, bio))
538 return 0;
539 if (blk_rq_sectors(req) + bio_sectors(bio) >
540 blk_rq_get_max_sectors(req, bio->bi_iter.bi_sector)) {
541 req_set_nomerge(q, req);
542 return 0;
543 }
544 if (!bio_flagged(bio, BIO_SEG_VALID))
545 blk_recount_segments(q, bio);
546 if (!bio_flagged(req->bio, BIO_SEG_VALID))
547 blk_recount_segments(q, req->bio);
548
549 return ll_new_hw_segment(q, req, bio);
550}
551
552
553
554
555
556static bool req_no_special_merge(struct request *req)
557{
558 struct request_queue *q = req->q;
559
560 return !q->mq_ops && req->special;
561}
562
563static bool req_attempt_discard_merge(struct request_queue *q, struct request *req,
564 struct request *next)
565{
566 unsigned short segments = blk_rq_nr_discard_segments(req);
567
568 if (segments >= queue_max_discard_segments(q))
569 goto no_merge;
570 if (blk_rq_sectors(req) + bio_sectors(next->bio) >
571 blk_rq_get_max_sectors(req, blk_rq_pos(req)))
572 goto no_merge;
573
574 req->nr_phys_segments = segments + blk_rq_nr_discard_segments(next);
575 return true;
576no_merge:
577 req_set_nomerge(q, req);
578 return false;
579}
580
581static int ll_merge_requests_fn(struct request_queue *q, struct request *req,
582 struct request *next)
583{
584 int total_phys_segments;
585 unsigned int seg_size =
586 req->biotail->bi_seg_back_size + next->bio->bi_seg_front_size;
587
588
589
590
591
592 if (req_no_special_merge(req) || req_no_special_merge(next))
593 return 0;
594
595 if (req_gap_back_merge(req, next->bio))
596 return 0;
597
598
599
600
601 if ((blk_rq_sectors(req) + blk_rq_sectors(next)) >
602 blk_rq_get_max_sectors(req, blk_rq_pos(req)))
603 return 0;
604
605 total_phys_segments = req->nr_phys_segments + next->nr_phys_segments;
606 if (blk_phys_contig_segment(q, req->biotail, next->bio)) {
607 if (req->nr_phys_segments == 1)
608 req->bio->bi_seg_front_size = seg_size;
609 if (next->nr_phys_segments == 1)
610 next->biotail->bi_seg_back_size = seg_size;
611 total_phys_segments--;
612 }
613
614 if (total_phys_segments > queue_max_segments(q))
615 return 0;
616
617 if (blk_integrity_merge_rq(q, req, next) == false)
618 return 0;
619
620
621 req->nr_phys_segments = total_phys_segments;
622 return 1;
623}
624
625
626
627
628
629
630
631
632
633
634void blk_rq_set_mixed_merge(struct request *rq)
635{
636 unsigned int ff = rq->cmd_flags & REQ_FAILFAST_MASK;
637 struct bio *bio;
638
639 if (rq->rq_flags & RQF_MIXED_MERGE)
640 return;
641
642
643
644
645
646
647 for (bio = rq->bio; bio; bio = bio->bi_next) {
648 WARN_ON_ONCE((bio->bi_opf & REQ_FAILFAST_MASK) &&
649 (bio->bi_opf & REQ_FAILFAST_MASK) != ff);
650 bio->bi_opf |= ff;
651 }
652 rq->rq_flags |= RQF_MIXED_MERGE;
653}
654
655static void blk_account_io_merge(struct request *req)
656{
657 if (blk_do_io_stat(req)) {
658 struct hd_struct *part;
659 int cpu;
660
661 cpu = part_stat_lock();
662 part = req->part;
663
664 part_round_stats(req->q, cpu, part);
665 part_dec_in_flight(req->q, part, rq_data_dir(req));
666
667 hd_struct_put(part);
668 part_stat_unlock();
669 }
670}
671
672
673
674
675
676static struct request *attempt_merge(struct request_queue *q,
677 struct request *req, struct request *next)
678{
679 if (!q->mq_ops)
680 lockdep_assert_held(q->queue_lock);
681
682 if (!rq_mergeable(req) || !rq_mergeable(next))
683 return NULL;
684
685 if (req_op(req) != req_op(next))
686 return NULL;
687
688
689
690
691 if (blk_rq_pos(req) + blk_rq_sectors(req) != blk_rq_pos(next))
692 return NULL;
693
694 if (rq_data_dir(req) != rq_data_dir(next)
695 || req->rq_disk != next->rq_disk
696 || req_no_special_merge(next))
697 return NULL;
698
699 if (req_op(req) == REQ_OP_WRITE_SAME &&
700 !blk_write_same_mergeable(req->bio, next->bio))
701 return NULL;
702
703
704
705
706
707 if (req->write_hint != next->write_hint)
708 return NULL;
709
710
711
712
713
714
715
716
717 if (req_op(req) == REQ_OP_DISCARD) {
718 if (!req_attempt_discard_merge(q, req, next))
719 return NULL;
720 } else if (!ll_merge_requests_fn(q, req, next))
721 return NULL;
722
723
724
725
726
727
728
729 if (((req->rq_flags | next->rq_flags) & RQF_MIXED_MERGE) ||
730 (req->cmd_flags & REQ_FAILFAST_MASK) !=
731 (next->cmd_flags & REQ_FAILFAST_MASK)) {
732 blk_rq_set_mixed_merge(req);
733 blk_rq_set_mixed_merge(next);
734 }
735
736
737
738
739
740
741 if (next->start_time_ns < req->start_time_ns)
742 req->start_time_ns = next->start_time_ns;
743
744 req->biotail->bi_next = next->bio;
745 req->biotail = next->biotail;
746
747 req->__data_len += blk_rq_bytes(next);
748
749 if (req_op(req) != REQ_OP_DISCARD)
750 elv_merge_requests(q, req, next);
751
752
753
754
755 blk_account_io_merge(next);
756
757 req->ioprio = ioprio_best(req->ioprio, next->ioprio);
758 if (blk_rq_cpu_valid(next))
759 req->cpu = next->cpu;
760
761
762
763
764
765 next->bio = NULL;
766 return next;
767}
768
769struct request *attempt_back_merge(struct request_queue *q, struct request *rq)
770{
771 struct request *next = elv_latter_request(q, rq);
772
773 if (next)
774 return attempt_merge(q, rq, next);
775
776 return NULL;
777}
778
779struct request *attempt_front_merge(struct request_queue *q, struct request *rq)
780{
781 struct request *prev = elv_former_request(q, rq);
782
783 if (prev)
784 return attempt_merge(q, prev, rq);
785
786 return NULL;
787}
788
789int blk_attempt_req_merge(struct request_queue *q, struct request *rq,
790 struct request *next)
791{
792 struct elevator_queue *e = q->elevator;
793 struct request *free;
794
795 if (!e->uses_mq && e->type->ops.sq.elevator_allow_rq_merge_fn)
796 if (!e->type->ops.sq.elevator_allow_rq_merge_fn(q, rq, next))
797 return 0;
798
799 free = attempt_merge(q, rq, next);
800 if (free) {
801 __blk_put_request(q, free);
802 return 1;
803 }
804
805 return 0;
806}
807
808bool blk_rq_merge_ok(struct request *rq, struct bio *bio)
809{
810 if (!rq_mergeable(rq) || !bio_mergeable(bio))
811 return false;
812
813 if (req_op(rq) != bio_op(bio))
814 return false;
815
816
817 if (bio_data_dir(bio) != rq_data_dir(rq))
818 return false;
819
820
821 if (rq->rq_disk != bio->bi_disk || req_no_special_merge(rq))
822 return false;
823
824
825 if (blk_integrity_merge_bio(rq->q, rq, bio) == false)
826 return false;
827
828
829 if (req_op(rq) == REQ_OP_WRITE_SAME &&
830 !blk_write_same_mergeable(rq->bio, bio))
831 return false;
832
833
834
835
836
837 if (rq->write_hint != bio->bi_write_hint)
838 return false;
839
840 return true;
841}
842
843enum elv_merge blk_try_merge(struct request *rq, struct bio *bio)
844{
845 if (req_op(rq) == REQ_OP_DISCARD &&
846 queue_max_discard_segments(rq->q) > 1)
847 return ELEVATOR_DISCARD_MERGE;
848 else if (blk_rq_pos(rq) + blk_rq_sectors(rq) == bio->bi_iter.bi_sector)
849 return ELEVATOR_BACK_MERGE;
850 else if (blk_rq_pos(rq) - bio_sectors(bio) == bio->bi_iter.bi_sector)
851 return ELEVATOR_FRONT_MERGE;
852 return ELEVATOR_NO_MERGE;
853}
854