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1/*
2 * Functions related to segment and merge handling
3 */
4#include <linux/kernel.h>
5#include <linux/module.h>
6#include <linux/bio.h>
7#include <linux/blkdev.h>
8#include <linux/scatterlist.h>
9
10#include "blk.h"
11
12static unsigned int __blk_recalc_rq_segments(struct request_queue *q,
13 struct bio *bio)
14{
15 struct bio_vec bv, bvprv = { NULL };
16 int cluster, high, highprv = 1;
17 unsigned int seg_size, nr_phys_segs;
18 struct bio *fbio, *bbio;
19 struct bvec_iter iter;
20
21 if (!bio)
22 return 0;
23
24 /*
25 * This should probably be returning 0, but blk_add_request_payload()
26 * (Christoph!!!!)
27 */
28 if (bio->bi_rw & REQ_DISCARD)
29 return 1;
30
31 if (bio->bi_rw & REQ_WRITE_SAME)
32 return 1;
33
34 fbio = bio;
35 cluster = blk_queue_cluster(q);
36 seg_size = 0;
37 nr_phys_segs = 0;
38 for_each_bio(bio) {
39 bio_for_each_segment(bv, bio, iter) {
40 /*
41 * the trick here is making sure that a high page is
42 * never considered part of another segment, since that
43 * might change with the bounce page.
44 */
45 high = page_to_pfn(bv.bv_page) > queue_bounce_pfn(q);
46 if (!high && !highprv && cluster) {
47 if (seg_size + bv.bv_len
48 > queue_max_segment_size(q))
49 goto new_segment;
50 if (!BIOVEC_PHYS_MERGEABLE(&bvprv, &bv))
51 goto new_segment;
52 if (!BIOVEC_SEG_BOUNDARY(q, &bvprv, &bv))
53 goto new_segment;
54
55 seg_size += bv.bv_len;
56 bvprv = bv;
57 continue;
58 }
59new_segment:
60 if (nr_phys_segs == 1 && seg_size >
61 fbio->bi_seg_front_size)
62 fbio->bi_seg_front_size = seg_size;
63
64 nr_phys_segs++;
65 bvprv = bv;
66 seg_size = bv.bv_len;
67 highprv = high;
68 }
69 bbio = bio;
70 }
71
72 if (nr_phys_segs == 1 && seg_size > fbio->bi_seg_front_size)
73 fbio->bi_seg_front_size = seg_size;
74 if (seg_size > bbio->bi_seg_back_size)
75 bbio->bi_seg_back_size = seg_size;
76
77 return nr_phys_segs;
78}
79
80void blk_recalc_rq_segments(struct request *rq)
81{
82 rq->nr_phys_segments = __blk_recalc_rq_segments(rq->q, rq->bio);
83}
84
85void blk_recount_segments(struct request_queue *q, struct bio *bio)
86{
87 struct bio *nxt = bio->bi_next;
88
89 bio->bi_next = NULL;
90 bio->bi_phys_segments = __blk_recalc_rq_segments(q, bio);
91 bio->bi_next = nxt;
92 bio->bi_flags |= (1 << BIO_SEG_VALID);
93}
94EXPORT_SYMBOL(blk_recount_segments);
95
96static int blk_phys_contig_segment(struct request_queue *q, struct bio *bio,
97 struct bio *nxt)
98{
99 struct bio_vec end_bv = { NULL }, nxt_bv;
100 struct bvec_iter iter;
101
102 if (!blk_queue_cluster(q))
103 return 0;
104
105 if (bio->bi_seg_back_size + nxt->bi_seg_front_size >
106 queue_max_segment_size(q))
107 return 0;
108
109 if (!bio_has_data(bio))
110 return 1;
111
112 bio_for_each_segment(end_bv, bio, iter)
113 if (end_bv.bv_len == iter.bi_size)
114 break;
115
116 nxt_bv = bio_iovec(nxt);
117
118 if (!BIOVEC_PHYS_MERGEABLE(&end_bv, &nxt_bv))
119 return 0;
120
121 /*
122 * bio and nxt are contiguous in memory; check if the queue allows
123 * these two to be merged into one
124 */
125 if (BIOVEC_SEG_BOUNDARY(q, &end_bv, &nxt_bv))
126 return 1;
127
128 return 0;
129}
130
131static inline void
132__blk_segment_map_sg(struct request_queue *q, struct bio_vec *bvec,
133 struct scatterlist *sglist, struct bio_vec *bvprv,
134 struct scatterlist **sg, int *nsegs, int *cluster)
135{
136
137 int nbytes = bvec->bv_len;
138
139 if (*sg && *cluster) {
140 if ((*sg)->length + nbytes > queue_max_segment_size(q))
141 goto new_segment;
142
143 if (!BIOVEC_PHYS_MERGEABLE(bvprv, bvec))
144 goto new_segment;
145 if (!BIOVEC_SEG_BOUNDARY(q, bvprv, bvec))
146 goto new_segment;
147
148 (*sg)->length += nbytes;
149 } else {
150new_segment:
151 if (!*sg)
152 *sg = sglist;
153 else {
154 /*
155 * If the driver previously mapped a shorter
156 * list, we could see a termination bit
157 * prematurely unless it fully inits the sg
158 * table on each mapping. We KNOW that there
159 * must be more entries here or the driver
160 * would be buggy, so force clear the
161 * termination bit to avoid doing a full
162 * sg_init_table() in drivers for each command.
163 */
164 sg_unmark_end(*sg);
165 *sg = sg_next(*sg);
166 }
167
168 sg_set_page(*sg, bvec->bv_page, nbytes, bvec->bv_offset);
169 (*nsegs)++;
170 }
171 *bvprv = *bvec;
172}
173
174static int __blk_bios_map_sg(struct request_queue *q, struct bio *bio,
175 struct scatterlist *sglist,
176 struct scatterlist **sg)
177{
178 struct bio_vec bvec, bvprv = { NULL };
179 struct bvec_iter iter;
180 int nsegs, cluster;
181
182 nsegs = 0;
183 cluster = blk_queue_cluster(q);
184
185 if (bio->bi_rw & REQ_DISCARD) {
186 /*
187 * This is a hack - drivers should be neither modifying the
188 * biovec, nor relying on bi_vcnt - but because of
189 * blk_add_request_payload(), a discard bio may or may not have
190 * a payload we need to set up here (thank you Christoph) and
191 * bi_vcnt is really the only way of telling if we need to.
192 */
193
194 if (bio->bi_vcnt)
195 goto single_segment;
196
197 return 0;
198 }
199
200 if (bio->bi_rw & REQ_WRITE_SAME) {
201single_segment:
202 *sg = sglist;
203 bvec = bio_iovec(bio);
204 sg_set_page(*sg, bvec.bv_page, bvec.bv_len, bvec.bv_offset);
205 return 1;
206 }
207
208 for_each_bio(bio)
209 bio_for_each_segment(bvec, bio, iter)
210 __blk_segment_map_sg(q, &bvec, sglist, &bvprv, sg,
211 &nsegs, &cluster);
212
213 return nsegs;
214}
215
216/*
217 * map a request to scatterlist, return number of sg entries setup. Caller
218 * must make sure sg can hold rq->nr_phys_segments entries
219 */
220int blk_rq_map_sg(struct request_queue *q, struct request *rq,
221 struct scatterlist *sglist)
222{
223 struct scatterlist *sg = NULL;
224 int nsegs = 0;
225
226 if (rq->bio)
227 nsegs = __blk_bios_map_sg(q, rq->bio, sglist, &sg);
228
229 if (unlikely(rq->cmd_flags & REQ_COPY_USER) &&
230 (blk_rq_bytes(rq) & q->dma_pad_mask)) {
231 unsigned int pad_len =
232 (q->dma_pad_mask & ~blk_rq_bytes(rq)) + 1;
233
234 sg->length += pad_len;
235 rq->extra_len += pad_len;
236 }
237
238 if (q->dma_drain_size && q->dma_drain_needed(rq)) {
239 if (rq->cmd_flags & REQ_WRITE)
240 memset(q->dma_drain_buffer, 0, q->dma_drain_size);
241
242 sg->page_link &= ~0x02;
243 sg = sg_next(sg);
244 sg_set_page(sg, virt_to_page(q->dma_drain_buffer),
245 q->dma_drain_size,
246 ((unsigned long)q->dma_drain_buffer) &
247 (PAGE_SIZE - 1));
248 nsegs++;
249 rq->extra_len += q->dma_drain_size;
250 }
251
252 if (sg)
253 sg_mark_end(sg);
254
255 return nsegs;
256}
257EXPORT_SYMBOL(blk_rq_map_sg);
258
259/**
260 * blk_bio_map_sg - map a bio to a scatterlist
261 * @q: request_queue in question
262 * @bio: bio being mapped
263 * @sglist: scatterlist being mapped
264 *
265 * Note:
266 * Caller must make sure sg can hold bio->bi_phys_segments entries
267 *
268 * Will return the number of sg entries setup
269 */
270int blk_bio_map_sg(struct request_queue *q, struct bio *bio,
271 struct scatterlist *sglist)
272{
273 struct scatterlist *sg = NULL;
274 int nsegs;
275 struct bio *next = bio->bi_next;
276 bio->bi_next = NULL;
277
278 nsegs = __blk_bios_map_sg(q, bio, sglist, &sg);
279 bio->bi_next = next;
280 if (sg)
281 sg_mark_end(sg);
282
283 BUG_ON(bio->bi_phys_segments && nsegs > bio->bi_phys_segments);
284 return nsegs;
285}
286EXPORT_SYMBOL(blk_bio_map_sg);
287
288static inline int ll_new_hw_segment(struct request_queue *q,
289 struct request *req,
290 struct bio *bio)
291{
292 int nr_phys_segs = bio_phys_segments(q, bio);
293
294 if (req->nr_phys_segments + nr_phys_segs > queue_max_segments(q))
295 goto no_merge;
296
297 if (bio_integrity(bio) && blk_integrity_merge_bio(q, req, bio))
298 goto no_merge;
299
300 /*
301 * This will form the start of a new hw segment. Bump both
302 * counters.
303 */
304 req->nr_phys_segments += nr_phys_segs;
305 return 1;
306
307no_merge:
308 req->cmd_flags |= REQ_NOMERGE;
309 if (req == q->last_merge)
310 q->last_merge = NULL;
311 return 0;
312}
313
314int ll_back_merge_fn(struct request_queue *q, struct request *req,
315 struct bio *bio)
316{
317 if (blk_rq_sectors(req) + bio_sectors(bio) >
318 blk_rq_get_max_sectors(req)) {
319 req->cmd_flags |= REQ_NOMERGE;
320 if (req == q->last_merge)
321 q->last_merge = NULL;
322 return 0;
323 }
324 if (!bio_flagged(req->biotail, BIO_SEG_VALID))
325 blk_recount_segments(q, req->biotail);
326 if (!bio_flagged(bio, BIO_SEG_VALID))
327 blk_recount_segments(q, bio);
328
329 return ll_new_hw_segment(q, req, bio);
330}
331
332int ll_front_merge_fn(struct request_queue *q, struct request *req,
333 struct bio *bio)
334{
335 if (blk_rq_sectors(req) + bio_sectors(bio) >
336 blk_rq_get_max_sectors(req)) {
337 req->cmd_flags |= REQ_NOMERGE;
338 if (req == q->last_merge)
339 q->last_merge = NULL;
340 return 0;
341 }
342 if (!bio_flagged(bio, BIO_SEG_VALID))
343 blk_recount_segments(q, bio);
344 if (!bio_flagged(req->bio, BIO_SEG_VALID))
345 blk_recount_segments(q, req->bio);
346
347 return ll_new_hw_segment(q, req, bio);
348}
349
350/*
351 * blk-mq uses req->special to carry normal driver per-request payload, it
352 * does not indicate a prepared command that we cannot merge with.
353 */
354static bool req_no_special_merge(struct request *req)
355{
356 struct request_queue *q = req->q;
357
358 return !q->mq_ops && req->special;
359}
360
361static int ll_merge_requests_fn(struct request_queue *q, struct request *req,
362 struct request *next)
363{
364 int total_phys_segments;
365 unsigned int seg_size =
366 req->biotail->bi_seg_back_size + next->bio->bi_seg_front_size;
367
368 /*
369 * First check if the either of the requests are re-queued
370 * requests. Can't merge them if they are.
371 */
372 if (req_no_special_merge(req) || req_no_special_merge(next))
373 return 0;
374
375 /*
376 * Will it become too large?
377 */
378 if ((blk_rq_sectors(req) + blk_rq_sectors(next)) >
379 blk_rq_get_max_sectors(req))
380 return 0;
381
382 total_phys_segments = req->nr_phys_segments + next->nr_phys_segments;
383 if (blk_phys_contig_segment(q, req->biotail, next->bio)) {
384 if (req->nr_phys_segments == 1)
385 req->bio->bi_seg_front_size = seg_size;
386 if (next->nr_phys_segments == 1)
387 next->biotail->bi_seg_back_size = seg_size;
388 total_phys_segments--;
389 }
390
391 if (total_phys_segments > queue_max_segments(q))
392 return 0;
393
394 if (blk_integrity_rq(req) && blk_integrity_merge_rq(q, req, next))
395 return 0;
396
397 /* Merge is OK... */
398 req->nr_phys_segments = total_phys_segments;
399 return 1;
400}
401
402/**
403 * blk_rq_set_mixed_merge - mark a request as mixed merge
404 * @rq: request to mark as mixed merge
405 *
406 * Description:
407 * @rq is about to be mixed merged. Make sure the attributes
408 * which can be mixed are set in each bio and mark @rq as mixed
409 * merged.
410 */
411void blk_rq_set_mixed_merge(struct request *rq)
412{
413 unsigned int ff = rq->cmd_flags & REQ_FAILFAST_MASK;
414 struct bio *bio;
415
416 if (rq->cmd_flags & REQ_MIXED_MERGE)
417 return;
418
419 /*
420 * @rq will no longer represent mixable attributes for all the
421 * contained bios. It will just track those of the first one.
422 * Distributes the attributs to each bio.
423 */
424 for (bio = rq->bio; bio; bio = bio->bi_next) {
425 WARN_ON_ONCE((bio->bi_rw & REQ_FAILFAST_MASK) &&
426 (bio->bi_rw & REQ_FAILFAST_MASK) != ff);
427 bio->bi_rw |= ff;
428 }
429 rq->cmd_flags |= REQ_MIXED_MERGE;
430}
431
432static void blk_account_io_merge(struct request *req)
433{
434 if (blk_do_io_stat(req)) {
435 struct hd_struct *part;
436 int cpu;
437
438 cpu = part_stat_lock();
439 part = req->part;
440
441 part_round_stats(cpu, part);
442 part_dec_in_flight(part, rq_data_dir(req));
443
444 hd_struct_put(part);
445 part_stat_unlock();
446 }
447}
448
449/*
450 * Has to be called with the request spinlock acquired
451 */
452static int attempt_merge(struct request_queue *q, struct request *req,
453 struct request *next)
454{
455 if (!rq_mergeable(req) || !rq_mergeable(next))
456 return 0;
457
458 if (!blk_check_merge_flags(req->cmd_flags, next->cmd_flags))
459 return 0;
460
461 /*
462 * not contiguous
463 */
464 if (blk_rq_pos(req) + blk_rq_sectors(req) != blk_rq_pos(next))
465 return 0;
466
467 if (rq_data_dir(req) != rq_data_dir(next)
468 || req->rq_disk != next->rq_disk
469 || req_no_special_merge(next))
470 return 0;
471
472 if (req->cmd_flags & REQ_WRITE_SAME &&
473 !blk_write_same_mergeable(req->bio, next->bio))
474 return 0;
475
476 /*
477 * If we are allowed to merge, then append bio list
478 * from next to rq and release next. merge_requests_fn
479 * will have updated segment counts, update sector
480 * counts here.
481 */
482 if (!ll_merge_requests_fn(q, req, next))
483 return 0;
484
485 /*
486 * If failfast settings disagree or any of the two is already
487 * a mixed merge, mark both as mixed before proceeding. This
488 * makes sure that all involved bios have mixable attributes
489 * set properly.
490 */
491 if ((req->cmd_flags | next->cmd_flags) & REQ_MIXED_MERGE ||
492 (req->cmd_flags & REQ_FAILFAST_MASK) !=
493 (next->cmd_flags & REQ_FAILFAST_MASK)) {
494 blk_rq_set_mixed_merge(req);
495 blk_rq_set_mixed_merge(next);
496 }
497
498 /*
499 * At this point we have either done a back merge
500 * or front merge. We need the smaller start_time of
501 * the merged requests to be the current request
502 * for accounting purposes.
503 */
504 if (time_after(req->start_time, next->start_time))
505 req->start_time = next->start_time;
506
507 req->biotail->bi_next = next->bio;
508 req->biotail = next->biotail;
509
510 req->__data_len += blk_rq_bytes(next);
511
512 elv_merge_requests(q, req, next);
513
514 /*
515 * 'next' is going away, so update stats accordingly
516 */
517 blk_account_io_merge(next);
518
519 req->ioprio = ioprio_best(req->ioprio, next->ioprio);
520 if (blk_rq_cpu_valid(next))
521 req->cpu = next->cpu;
522
523 /* owner-ship of bio passed from next to req */
524 next->bio = NULL;
525 __blk_put_request(q, next);
526 return 1;
527}
528
529int attempt_back_merge(struct request_queue *q, struct request *rq)
530{
531 struct request *next = elv_latter_request(q, rq);
532
533 if (next)
534 return attempt_merge(q, rq, next);
535
536 return 0;
537}
538
539int attempt_front_merge(struct request_queue *q, struct request *rq)
540{
541 struct request *prev = elv_former_request(q, rq);
542
543 if (prev)
544 return attempt_merge(q, prev, rq);
545
546 return 0;
547}
548
549int blk_attempt_req_merge(struct request_queue *q, struct request *rq,
550 struct request *next)
551{
552 return attempt_merge(q, rq, next);
553}
554
555bool blk_rq_merge_ok(struct request *rq, struct bio *bio)
556{
557 if (!rq_mergeable(rq) || !bio_mergeable(bio))
558 return false;
559
560 if (!blk_check_merge_flags(rq->cmd_flags, bio->bi_rw))
561 return false;
562
563 /* different data direction or already started, don't merge */
564 if (bio_data_dir(bio) != rq_data_dir(rq))
565 return false;
566
567 /* must be same device and not a special request */
568 if (rq->rq_disk != bio->bi_bdev->bd_disk || req_no_special_merge(rq))
569 return false;
570
571 /* only merge integrity protected bio into ditto rq */
572 if (bio_integrity(bio) != blk_integrity_rq(rq))
573 return false;
574
575 /* must be using the same buffer */
576 if (rq->cmd_flags & REQ_WRITE_SAME &&
577 !blk_write_same_mergeable(rq->bio, bio))
578 return false;
579
580 return true;
581}
582
583int blk_try_merge(struct request *rq, struct bio *bio)
584{
585 if (blk_rq_pos(rq) + blk_rq_sectors(rq) == bio->bi_iter.bi_sector)
586 return ELEVATOR_BACK_MERGE;
587 else if (blk_rq_pos(rq) - bio_sectors(bio) == bio->bi_iter.bi_sector)
588 return ELEVATOR_FRONT_MERGE;
589 return ELEVATOR_NO_MERGE;
590}
1// SPDX-License-Identifier: GPL-2.0
2/*
3 * Functions related to segment and merge handling
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 inline bool bio_will_gap(struct request_queue *q,
16 struct request *prev_rq, struct bio *prev, struct bio *next)
17{
18 struct bio_vec pb, nb;
19
20 if (!bio_has_data(prev) || !queue_virt_boundary(q))
21 return false;
22
23 /*
24 * Don't merge if the 1st bio starts with non-zero offset, otherwise it
25 * is quite difficult to respect the sg gap limit. We work hard to
26 * merge a huge number of small single bios in case of mkfs.
27 */
28 if (prev_rq)
29 bio_get_first_bvec(prev_rq->bio, &pb);
30 else
31 bio_get_first_bvec(prev, &pb);
32 if (pb.bv_offset & queue_virt_boundary(q))
33 return true;
34
35 /*
36 * We don't need to worry about the situation that the merged segment
37 * ends in unaligned virt boundary:
38 *
39 * - if 'pb' ends aligned, the merged segment ends aligned
40 * - if 'pb' ends unaligned, the next bio must include
41 * one single bvec of 'nb', otherwise the 'nb' can't
42 * merge with 'pb'
43 */
44 bio_get_last_bvec(prev, &pb);
45 bio_get_first_bvec(next, &nb);
46 if (biovec_phys_mergeable(q, &pb, &nb))
47 return false;
48 return __bvec_gap_to_prev(q, &pb, nb.bv_offset);
49}
50
51static inline bool req_gap_back_merge(struct request *req, struct bio *bio)
52{
53 return bio_will_gap(req->q, req, req->biotail, bio);
54}
55
56static inline bool req_gap_front_merge(struct request *req, struct bio *bio)
57{
58 return bio_will_gap(req->q, NULL, bio, req->bio);
59}
60
61static struct bio *blk_bio_discard_split(struct request_queue *q,
62 struct bio *bio,
63 struct bio_set *bs,
64 unsigned *nsegs)
65{
66 unsigned int max_discard_sectors, granularity;
67 int alignment;
68 sector_t tmp;
69 unsigned split_sectors;
70
71 *nsegs = 1;
72
73 /* Zero-sector (unknown) and one-sector granularities are the same. */
74 granularity = max(q->limits.discard_granularity >> 9, 1U);
75
76 max_discard_sectors = min(q->limits.max_discard_sectors,
77 bio_allowed_max_sectors(q));
78 max_discard_sectors -= max_discard_sectors % granularity;
79
80 if (unlikely(!max_discard_sectors)) {
81 /* XXX: warn */
82 return NULL;
83 }
84
85 if (bio_sectors(bio) <= max_discard_sectors)
86 return NULL;
87
88 split_sectors = max_discard_sectors;
89
90 /*
91 * If the next starting sector would be misaligned, stop the discard at
92 * the previous aligned sector.
93 */
94 alignment = (q->limits.discard_alignment >> 9) % granularity;
95
96 tmp = bio->bi_iter.bi_sector + split_sectors - alignment;
97 tmp = sector_div(tmp, granularity);
98
99 if (split_sectors > tmp)
100 split_sectors -= tmp;
101
102 return bio_split(bio, split_sectors, GFP_NOIO, bs);
103}
104
105static struct bio *blk_bio_write_zeroes_split(struct request_queue *q,
106 struct bio *bio, struct bio_set *bs, unsigned *nsegs)
107{
108 *nsegs = 0;
109
110 if (!q->limits.max_write_zeroes_sectors)
111 return NULL;
112
113 if (bio_sectors(bio) <= q->limits.max_write_zeroes_sectors)
114 return NULL;
115
116 return bio_split(bio, q->limits.max_write_zeroes_sectors, GFP_NOIO, bs);
117}
118
119static struct bio *blk_bio_write_same_split(struct request_queue *q,
120 struct bio *bio,
121 struct bio_set *bs,
122 unsigned *nsegs)
123{
124 *nsegs = 1;
125
126 if (!q->limits.max_write_same_sectors)
127 return NULL;
128
129 if (bio_sectors(bio) <= q->limits.max_write_same_sectors)
130 return NULL;
131
132 return bio_split(bio, q->limits.max_write_same_sectors, GFP_NOIO, bs);
133}
134
135/*
136 * Return the maximum number of sectors from the start of a bio that may be
137 * submitted as a single request to a block device. If enough sectors remain,
138 * align the end to the physical block size. Otherwise align the end to the
139 * logical block size. This approach minimizes the number of non-aligned
140 * requests that are submitted to a block device if the start of a bio is not
141 * aligned to a physical block boundary.
142 */
143static inline unsigned get_max_io_size(struct request_queue *q,
144 struct bio *bio)
145{
146 unsigned sectors = blk_max_size_offset(q, bio->bi_iter.bi_sector);
147 unsigned max_sectors = sectors;
148 unsigned pbs = queue_physical_block_size(q) >> SECTOR_SHIFT;
149 unsigned lbs = queue_logical_block_size(q) >> SECTOR_SHIFT;
150 unsigned start_offset = bio->bi_iter.bi_sector & (pbs - 1);
151
152 max_sectors += start_offset;
153 max_sectors &= ~(pbs - 1);
154 if (max_sectors > start_offset)
155 return max_sectors - start_offset;
156
157 return sectors & (lbs - 1);
158}
159
160static unsigned get_max_segment_size(const struct request_queue *q,
161 unsigned offset)
162{
163 unsigned long mask = queue_segment_boundary(q);
164
165 /* default segment boundary mask means no boundary limit */
166 if (mask == BLK_SEG_BOUNDARY_MASK)
167 return queue_max_segment_size(q);
168
169 return min_t(unsigned long, mask - (mask & offset) + 1,
170 queue_max_segment_size(q));
171}
172
173/**
174 * bvec_split_segs - verify whether or not a bvec should be split in the middle
175 * @q: [in] request queue associated with the bio associated with @bv
176 * @bv: [in] bvec to examine
177 * @nsegs: [in,out] Number of segments in the bio being built. Incremented
178 * by the number of segments from @bv that may be appended to that
179 * bio without exceeding @max_segs
180 * @sectors: [in,out] Number of sectors in the bio being built. Incremented
181 * by the number of sectors from @bv that may be appended to that
182 * bio without exceeding @max_sectors
183 * @max_segs: [in] upper bound for *@nsegs
184 * @max_sectors: [in] upper bound for *@sectors
185 *
186 * When splitting a bio, it can happen that a bvec is encountered that is too
187 * big to fit in a single segment and hence that it has to be split in the
188 * middle. This function verifies whether or not that should happen. The value
189 * %true is returned if and only if appending the entire @bv to a bio with
190 * *@nsegs segments and *@sectors sectors would make that bio unacceptable for
191 * the block driver.
192 */
193static bool bvec_split_segs(const struct request_queue *q,
194 const struct bio_vec *bv, unsigned *nsegs,
195 unsigned *sectors, unsigned max_segs,
196 unsigned max_sectors)
197{
198 unsigned max_len = (min(max_sectors, UINT_MAX >> 9) - *sectors) << 9;
199 unsigned len = min(bv->bv_len, max_len);
200 unsigned total_len = 0;
201 unsigned seg_size = 0;
202
203 while (len && *nsegs < max_segs) {
204 seg_size = get_max_segment_size(q, bv->bv_offset + total_len);
205 seg_size = min(seg_size, len);
206
207 (*nsegs)++;
208 total_len += seg_size;
209 len -= seg_size;
210
211 if ((bv->bv_offset + total_len) & queue_virt_boundary(q))
212 break;
213 }
214
215 *sectors += total_len >> 9;
216
217 /* tell the caller to split the bvec if it is too big to fit */
218 return len > 0 || bv->bv_len > max_len;
219}
220
221/**
222 * blk_bio_segment_split - split a bio in two bios
223 * @q: [in] request queue pointer
224 * @bio: [in] bio to be split
225 * @bs: [in] bio set to allocate the clone from
226 * @segs: [out] number of segments in the bio with the first half of the sectors
227 *
228 * Clone @bio, update the bi_iter of the clone to represent the first sectors
229 * of @bio and update @bio->bi_iter to represent the remaining sectors. The
230 * following is guaranteed for the cloned bio:
231 * - That it has at most get_max_io_size(@q, @bio) sectors.
232 * - That it has at most queue_max_segments(@q) segments.
233 *
234 * Except for discard requests the cloned bio will point at the bi_io_vec of
235 * the original bio. It is the responsibility of the caller to ensure that the
236 * original bio is not freed before the cloned bio. The caller is also
237 * responsible for ensuring that @bs is only destroyed after processing of the
238 * split bio has finished.
239 */
240static struct bio *blk_bio_segment_split(struct request_queue *q,
241 struct bio *bio,
242 struct bio_set *bs,
243 unsigned *segs)
244{
245 struct bio_vec bv, bvprv, *bvprvp = NULL;
246 struct bvec_iter iter;
247 unsigned nsegs = 0, sectors = 0;
248 const unsigned max_sectors = get_max_io_size(q, bio);
249 const unsigned max_segs = queue_max_segments(q);
250
251 bio_for_each_bvec(bv, bio, iter) {
252 /*
253 * If the queue doesn't support SG gaps and adding this
254 * offset would create a gap, disallow it.
255 */
256 if (bvprvp && bvec_gap_to_prev(q, bvprvp, bv.bv_offset))
257 goto split;
258
259 if (nsegs < max_segs &&
260 sectors + (bv.bv_len >> 9) <= max_sectors &&
261 bv.bv_offset + bv.bv_len <= PAGE_SIZE) {
262 nsegs++;
263 sectors += bv.bv_len >> 9;
264 } else if (bvec_split_segs(q, &bv, &nsegs, §ors, max_segs,
265 max_sectors)) {
266 goto split;
267 }
268
269 bvprv = bv;
270 bvprvp = &bvprv;
271 }
272
273 *segs = nsegs;
274 return NULL;
275split:
276 *segs = nsegs;
277 return bio_split(bio, sectors, GFP_NOIO, bs);
278}
279
280/**
281 * __blk_queue_split - split a bio and submit the second half
282 * @q: [in] request queue pointer
283 * @bio: [in, out] bio to be split
284 * @nr_segs: [out] number of segments in the first bio
285 *
286 * Split a bio into two bios, chain the two bios, submit the second half and
287 * store a pointer to the first half in *@bio. If the second bio is still too
288 * big it will be split by a recursive call to this function. Since this
289 * function may allocate a new bio from @q->bio_split, it is the responsibility
290 * of the caller to ensure that @q is only released after processing of the
291 * split bio has finished.
292 */
293void __blk_queue_split(struct request_queue *q, struct bio **bio,
294 unsigned int *nr_segs)
295{
296 struct bio *split;
297
298 switch (bio_op(*bio)) {
299 case REQ_OP_DISCARD:
300 case REQ_OP_SECURE_ERASE:
301 split = blk_bio_discard_split(q, *bio, &q->bio_split, nr_segs);
302 break;
303 case REQ_OP_WRITE_ZEROES:
304 split = blk_bio_write_zeroes_split(q, *bio, &q->bio_split,
305 nr_segs);
306 break;
307 case REQ_OP_WRITE_SAME:
308 split = blk_bio_write_same_split(q, *bio, &q->bio_split,
309 nr_segs);
310 break;
311 default:
312 split = blk_bio_segment_split(q, *bio, &q->bio_split, nr_segs);
313 break;
314 }
315
316 if (split) {
317 /* there isn't chance to merge the splitted bio */
318 split->bi_opf |= REQ_NOMERGE;
319
320 /*
321 * Since we're recursing into make_request here, ensure
322 * that we mark this bio as already having entered the queue.
323 * If not, and the queue is going away, we can get stuck
324 * forever on waiting for the queue reference to drop. But
325 * that will never happen, as we're already holding a
326 * reference to it.
327 */
328 bio_set_flag(*bio, BIO_QUEUE_ENTERED);
329
330 bio_chain(split, *bio);
331 trace_block_split(q, split, (*bio)->bi_iter.bi_sector);
332 generic_make_request(*bio);
333 *bio = split;
334 }
335}
336
337/**
338 * blk_queue_split - split a bio and submit the second half
339 * @q: [in] request queue pointer
340 * @bio: [in, out] bio to be split
341 *
342 * Split a bio into two bios, chains the two bios, submit the second half and
343 * store a pointer to the first half in *@bio. Since this function may allocate
344 * a new bio from @q->bio_split, it is the responsibility of the caller to
345 * ensure that @q is only released after processing of the split bio has
346 * finished.
347 */
348void blk_queue_split(struct request_queue *q, struct bio **bio)
349{
350 unsigned int nr_segs;
351
352 __blk_queue_split(q, bio, &nr_segs);
353}
354EXPORT_SYMBOL(blk_queue_split);
355
356unsigned int blk_recalc_rq_segments(struct request *rq)
357{
358 unsigned int nr_phys_segs = 0;
359 unsigned int nr_sectors = 0;
360 struct req_iterator iter;
361 struct bio_vec bv;
362
363 if (!rq->bio)
364 return 0;
365
366 switch (bio_op(rq->bio)) {
367 case REQ_OP_DISCARD:
368 case REQ_OP_SECURE_ERASE:
369 case REQ_OP_WRITE_ZEROES:
370 return 0;
371 case REQ_OP_WRITE_SAME:
372 return 1;
373 }
374
375 rq_for_each_bvec(bv, rq, iter)
376 bvec_split_segs(rq->q, &bv, &nr_phys_segs, &nr_sectors,
377 UINT_MAX, UINT_MAX);
378 return nr_phys_segs;
379}
380
381static inline struct scatterlist *blk_next_sg(struct scatterlist **sg,
382 struct scatterlist *sglist)
383{
384 if (!*sg)
385 return sglist;
386
387 /*
388 * If the driver previously mapped a shorter list, we could see a
389 * termination bit prematurely unless it fully inits the sg table
390 * on each mapping. We KNOW that there must be more entries here
391 * or the driver would be buggy, so force clear the termination bit
392 * to avoid doing a full sg_init_table() in drivers for each command.
393 */
394 sg_unmark_end(*sg);
395 return sg_next(*sg);
396}
397
398static unsigned blk_bvec_map_sg(struct request_queue *q,
399 struct bio_vec *bvec, struct scatterlist *sglist,
400 struct scatterlist **sg)
401{
402 unsigned nbytes = bvec->bv_len;
403 unsigned nsegs = 0, total = 0;
404
405 while (nbytes > 0) {
406 unsigned offset = bvec->bv_offset + total;
407 unsigned len = min(get_max_segment_size(q, offset), nbytes);
408 struct page *page = bvec->bv_page;
409
410 /*
411 * Unfortunately a fair number of drivers barf on scatterlists
412 * that have an offset larger than PAGE_SIZE, despite other
413 * subsystems dealing with that invariant just fine. For now
414 * stick to the legacy format where we never present those from
415 * the block layer, but the code below should be removed once
416 * these offenders (mostly MMC/SD drivers) are fixed.
417 */
418 page += (offset >> PAGE_SHIFT);
419 offset &= ~PAGE_MASK;
420
421 *sg = blk_next_sg(sg, sglist);
422 sg_set_page(*sg, page, len, offset);
423
424 total += len;
425 nbytes -= len;
426 nsegs++;
427 }
428
429 return nsegs;
430}
431
432static inline int __blk_bvec_map_sg(struct bio_vec bv,
433 struct scatterlist *sglist, struct scatterlist **sg)
434{
435 *sg = blk_next_sg(sg, sglist);
436 sg_set_page(*sg, bv.bv_page, bv.bv_len, bv.bv_offset);
437 return 1;
438}
439
440/* only try to merge bvecs into one sg if they are from two bios */
441static inline bool
442__blk_segment_map_sg_merge(struct request_queue *q, struct bio_vec *bvec,
443 struct bio_vec *bvprv, struct scatterlist **sg)
444{
445
446 int nbytes = bvec->bv_len;
447
448 if (!*sg)
449 return false;
450
451 if ((*sg)->length + nbytes > queue_max_segment_size(q))
452 return false;
453
454 if (!biovec_phys_mergeable(q, bvprv, bvec))
455 return false;
456
457 (*sg)->length += nbytes;
458
459 return true;
460}
461
462static int __blk_bios_map_sg(struct request_queue *q, struct bio *bio,
463 struct scatterlist *sglist,
464 struct scatterlist **sg)
465{
466 struct bio_vec uninitialized_var(bvec), bvprv = { NULL };
467 struct bvec_iter iter;
468 int nsegs = 0;
469 bool new_bio = false;
470
471 for_each_bio(bio) {
472 bio_for_each_bvec(bvec, bio, iter) {
473 /*
474 * Only try to merge bvecs from two bios given we
475 * have done bio internal merge when adding pages
476 * to bio
477 */
478 if (new_bio &&
479 __blk_segment_map_sg_merge(q, &bvec, &bvprv, sg))
480 goto next_bvec;
481
482 if (bvec.bv_offset + bvec.bv_len <= PAGE_SIZE)
483 nsegs += __blk_bvec_map_sg(bvec, sglist, sg);
484 else
485 nsegs += blk_bvec_map_sg(q, &bvec, sglist, sg);
486 next_bvec:
487 new_bio = false;
488 }
489 if (likely(bio->bi_iter.bi_size)) {
490 bvprv = bvec;
491 new_bio = true;
492 }
493 }
494
495 return nsegs;
496}
497
498/*
499 * map a request to scatterlist, return number of sg entries setup. Caller
500 * must make sure sg can hold rq->nr_phys_segments entries
501 */
502int blk_rq_map_sg(struct request_queue *q, struct request *rq,
503 struct scatterlist *sglist)
504{
505 struct scatterlist *sg = NULL;
506 int nsegs = 0;
507
508 if (rq->rq_flags & RQF_SPECIAL_PAYLOAD)
509 nsegs = __blk_bvec_map_sg(rq->special_vec, sglist, &sg);
510 else if (rq->bio && bio_op(rq->bio) == REQ_OP_WRITE_SAME)
511 nsegs = __blk_bvec_map_sg(bio_iovec(rq->bio), sglist, &sg);
512 else if (rq->bio)
513 nsegs = __blk_bios_map_sg(q, rq->bio, sglist, &sg);
514
515 if (unlikely(rq->rq_flags & RQF_COPY_USER) &&
516 (blk_rq_bytes(rq) & q->dma_pad_mask)) {
517 unsigned int pad_len =
518 (q->dma_pad_mask & ~blk_rq_bytes(rq)) + 1;
519
520 sg->length += pad_len;
521 rq->extra_len += pad_len;
522 }
523
524 if (q->dma_drain_size && q->dma_drain_needed(rq)) {
525 if (op_is_write(req_op(rq)))
526 memset(q->dma_drain_buffer, 0, q->dma_drain_size);
527
528 sg_unmark_end(sg);
529 sg = sg_next(sg);
530 sg_set_page(sg, virt_to_page(q->dma_drain_buffer),
531 q->dma_drain_size,
532 ((unsigned long)q->dma_drain_buffer) &
533 (PAGE_SIZE - 1));
534 nsegs++;
535 rq->extra_len += q->dma_drain_size;
536 }
537
538 if (sg)
539 sg_mark_end(sg);
540
541 /*
542 * Something must have been wrong if the figured number of
543 * segment is bigger than number of req's physical segments
544 */
545 WARN_ON(nsegs > blk_rq_nr_phys_segments(rq));
546
547 return nsegs;
548}
549EXPORT_SYMBOL(blk_rq_map_sg);
550
551static inline int ll_new_hw_segment(struct request *req, struct bio *bio,
552 unsigned int nr_phys_segs)
553{
554 if (req->nr_phys_segments + nr_phys_segs > queue_max_segments(req->q))
555 goto no_merge;
556
557 if (blk_integrity_merge_bio(req->q, req, bio) == false)
558 goto no_merge;
559
560 /*
561 * This will form the start of a new hw segment. Bump both
562 * counters.
563 */
564 req->nr_phys_segments += nr_phys_segs;
565 return 1;
566
567no_merge:
568 req_set_nomerge(req->q, req);
569 return 0;
570}
571
572int ll_back_merge_fn(struct request *req, struct bio *bio, unsigned int nr_segs)
573{
574 if (req_gap_back_merge(req, bio))
575 return 0;
576 if (blk_integrity_rq(req) &&
577 integrity_req_gap_back_merge(req, bio))
578 return 0;
579 if (blk_rq_sectors(req) + bio_sectors(bio) >
580 blk_rq_get_max_sectors(req, blk_rq_pos(req))) {
581 req_set_nomerge(req->q, req);
582 return 0;
583 }
584
585 return ll_new_hw_segment(req, bio, nr_segs);
586}
587
588int ll_front_merge_fn(struct request *req, struct bio *bio, unsigned int nr_segs)
589{
590 if (req_gap_front_merge(req, bio))
591 return 0;
592 if (blk_integrity_rq(req) &&
593 integrity_req_gap_front_merge(req, bio))
594 return 0;
595 if (blk_rq_sectors(req) + bio_sectors(bio) >
596 blk_rq_get_max_sectors(req, bio->bi_iter.bi_sector)) {
597 req_set_nomerge(req->q, req);
598 return 0;
599 }
600
601 return ll_new_hw_segment(req, bio, nr_segs);
602}
603
604static bool req_attempt_discard_merge(struct request_queue *q, struct request *req,
605 struct request *next)
606{
607 unsigned short segments = blk_rq_nr_discard_segments(req);
608
609 if (segments >= queue_max_discard_segments(q))
610 goto no_merge;
611 if (blk_rq_sectors(req) + bio_sectors(next->bio) >
612 blk_rq_get_max_sectors(req, blk_rq_pos(req)))
613 goto no_merge;
614
615 req->nr_phys_segments = segments + blk_rq_nr_discard_segments(next);
616 return true;
617no_merge:
618 req_set_nomerge(q, req);
619 return false;
620}
621
622static int ll_merge_requests_fn(struct request_queue *q, struct request *req,
623 struct request *next)
624{
625 int total_phys_segments;
626
627 if (req_gap_back_merge(req, next->bio))
628 return 0;
629
630 /*
631 * Will it become too large?
632 */
633 if ((blk_rq_sectors(req) + blk_rq_sectors(next)) >
634 blk_rq_get_max_sectors(req, blk_rq_pos(req)))
635 return 0;
636
637 total_phys_segments = req->nr_phys_segments + next->nr_phys_segments;
638 if (total_phys_segments > queue_max_segments(q))
639 return 0;
640
641 if (blk_integrity_merge_rq(q, req, next) == false)
642 return 0;
643
644 /* Merge is OK... */
645 req->nr_phys_segments = total_phys_segments;
646 return 1;
647}
648
649/**
650 * blk_rq_set_mixed_merge - mark a request as mixed merge
651 * @rq: request to mark as mixed merge
652 *
653 * Description:
654 * @rq is about to be mixed merged. Make sure the attributes
655 * which can be mixed are set in each bio and mark @rq as mixed
656 * merged.
657 */
658void blk_rq_set_mixed_merge(struct request *rq)
659{
660 unsigned int ff = rq->cmd_flags & REQ_FAILFAST_MASK;
661 struct bio *bio;
662
663 if (rq->rq_flags & RQF_MIXED_MERGE)
664 return;
665
666 /*
667 * @rq will no longer represent mixable attributes for all the
668 * contained bios. It will just track those of the first one.
669 * Distributes the attributs to each bio.
670 */
671 for (bio = rq->bio; bio; bio = bio->bi_next) {
672 WARN_ON_ONCE((bio->bi_opf & REQ_FAILFAST_MASK) &&
673 (bio->bi_opf & REQ_FAILFAST_MASK) != ff);
674 bio->bi_opf |= ff;
675 }
676 rq->rq_flags |= RQF_MIXED_MERGE;
677}
678
679static void blk_account_io_merge(struct request *req)
680{
681 if (blk_do_io_stat(req)) {
682 struct hd_struct *part;
683
684 part_stat_lock();
685 part = req->part;
686
687 part_dec_in_flight(req->q, part, rq_data_dir(req));
688
689 hd_struct_put(part);
690 part_stat_unlock();
691 }
692}
693/*
694 * Two cases of handling DISCARD merge:
695 * If max_discard_segments > 1, the driver takes every bio
696 * as a range and send them to controller together. The ranges
697 * needn't to be contiguous.
698 * Otherwise, the bios/requests will be handled as same as
699 * others which should be contiguous.
700 */
701static inline bool blk_discard_mergable(struct request *req)
702{
703 if (req_op(req) == REQ_OP_DISCARD &&
704 queue_max_discard_segments(req->q) > 1)
705 return true;
706 return false;
707}
708
709static enum elv_merge blk_try_req_merge(struct request *req,
710 struct request *next)
711{
712 if (blk_discard_mergable(req))
713 return ELEVATOR_DISCARD_MERGE;
714 else if (blk_rq_pos(req) + blk_rq_sectors(req) == blk_rq_pos(next))
715 return ELEVATOR_BACK_MERGE;
716
717 return ELEVATOR_NO_MERGE;
718}
719
720/*
721 * For non-mq, this has to be called with the request spinlock acquired.
722 * For mq with scheduling, the appropriate queue wide lock should be held.
723 */
724static struct request *attempt_merge(struct request_queue *q,
725 struct request *req, struct request *next)
726{
727 if (!rq_mergeable(req) || !rq_mergeable(next))
728 return NULL;
729
730 if (req_op(req) != req_op(next))
731 return NULL;
732
733 if (rq_data_dir(req) != rq_data_dir(next)
734 || req->rq_disk != next->rq_disk)
735 return NULL;
736
737 if (req_op(req) == REQ_OP_WRITE_SAME &&
738 !blk_write_same_mergeable(req->bio, next->bio))
739 return NULL;
740
741 /*
742 * Don't allow merge of different write hints, or for a hint with
743 * non-hint IO.
744 */
745 if (req->write_hint != next->write_hint)
746 return NULL;
747
748 if (req->ioprio != next->ioprio)
749 return NULL;
750
751 /*
752 * If we are allowed to merge, then append bio list
753 * from next to rq and release next. merge_requests_fn
754 * will have updated segment counts, update sector
755 * counts here. Handle DISCARDs separately, as they
756 * have separate settings.
757 */
758
759 switch (blk_try_req_merge(req, next)) {
760 case ELEVATOR_DISCARD_MERGE:
761 if (!req_attempt_discard_merge(q, req, next))
762 return NULL;
763 break;
764 case ELEVATOR_BACK_MERGE:
765 if (!ll_merge_requests_fn(q, req, next))
766 return NULL;
767 break;
768 default:
769 return NULL;
770 }
771
772 /*
773 * If failfast settings disagree or any of the two is already
774 * a mixed merge, mark both as mixed before proceeding. This
775 * makes sure that all involved bios have mixable attributes
776 * set properly.
777 */
778 if (((req->rq_flags | next->rq_flags) & RQF_MIXED_MERGE) ||
779 (req->cmd_flags & REQ_FAILFAST_MASK) !=
780 (next->cmd_flags & REQ_FAILFAST_MASK)) {
781 blk_rq_set_mixed_merge(req);
782 blk_rq_set_mixed_merge(next);
783 }
784
785 /*
786 * At this point we have either done a back merge or front merge. We
787 * need the smaller start_time_ns of the merged requests to be the
788 * current request for accounting purposes.
789 */
790 if (next->start_time_ns < req->start_time_ns)
791 req->start_time_ns = next->start_time_ns;
792
793 req->biotail->bi_next = next->bio;
794 req->biotail = next->biotail;
795
796 req->__data_len += blk_rq_bytes(next);
797
798 if (!blk_discard_mergable(req))
799 elv_merge_requests(q, req, next);
800
801 /*
802 * 'next' is going away, so update stats accordingly
803 */
804 blk_account_io_merge(next);
805
806 /*
807 * ownership of bio passed from next to req, return 'next' for
808 * the caller to free
809 */
810 next->bio = NULL;
811 return next;
812}
813
814struct request *attempt_back_merge(struct request_queue *q, struct request *rq)
815{
816 struct request *next = elv_latter_request(q, rq);
817
818 if (next)
819 return attempt_merge(q, rq, next);
820
821 return NULL;
822}
823
824struct request *attempt_front_merge(struct request_queue *q, struct request *rq)
825{
826 struct request *prev = elv_former_request(q, rq);
827
828 if (prev)
829 return attempt_merge(q, prev, rq);
830
831 return NULL;
832}
833
834int blk_attempt_req_merge(struct request_queue *q, struct request *rq,
835 struct request *next)
836{
837 struct request *free;
838
839 free = attempt_merge(q, rq, next);
840 if (free) {
841 blk_put_request(free);
842 return 1;
843 }
844
845 return 0;
846}
847
848bool blk_rq_merge_ok(struct request *rq, struct bio *bio)
849{
850 if (!rq_mergeable(rq) || !bio_mergeable(bio))
851 return false;
852
853 if (req_op(rq) != bio_op(bio))
854 return false;
855
856 /* different data direction or already started, don't merge */
857 if (bio_data_dir(bio) != rq_data_dir(rq))
858 return false;
859
860 /* must be same device */
861 if (rq->rq_disk != bio->bi_disk)
862 return false;
863
864 /* only merge integrity protected bio into ditto rq */
865 if (blk_integrity_merge_bio(rq->q, rq, bio) == false)
866 return false;
867
868 /* must be using the same buffer */
869 if (req_op(rq) == REQ_OP_WRITE_SAME &&
870 !blk_write_same_mergeable(rq->bio, bio))
871 return false;
872
873 /*
874 * Don't allow merge of different write hints, or for a hint with
875 * non-hint IO.
876 */
877 if (rq->write_hint != bio->bi_write_hint)
878 return false;
879
880 if (rq->ioprio != bio_prio(bio))
881 return false;
882
883 return true;
884}
885
886enum elv_merge blk_try_merge(struct request *rq, struct bio *bio)
887{
888 if (blk_discard_mergable(rq))
889 return ELEVATOR_DISCARD_MERGE;
890 else if (blk_rq_pos(rq) + blk_rq_sectors(rq) == bio->bi_iter.bi_sector)
891 return ELEVATOR_BACK_MERGE;
892 else if (blk_rq_pos(rq) - bio_sectors(bio) == bio->bi_iter.bi_sector)
893 return ELEVATOR_FRONT_MERGE;
894 return ELEVATOR_NO_MERGE;
895}