block/blk-merge.c - kernel/msm-4.9 - Gitiles

 /*
  * 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 "blk.h"

 void blk_recalc_rq_sectors(struct request *rq, int nsect)
 {
 	if (blk_fs_request(rq)) {
 		rq->hard_sector += nsect;
 		rq->hard_nr_sectors -= nsect;

 		/*
 		 * Move the I/O submission pointers ahead if required.
 		 */
 		if ((rq->nr_sectors >= rq->hard_nr_sectors) &&
 		    (rq->sector <= rq->hard_sector)) {
 			rq->sector = rq->hard_sector;
 			rq->nr_sectors = rq->hard_nr_sectors;
 			rq->hard_cur_sectors = bio_cur_sectors(rq->bio);
 			rq->current_nr_sectors = rq->hard_cur_sectors;
 			rq->buffer = bio_data(rq->bio);
 		}

 		/*
 		 * if total number of sectors is less than the first segment
 		 * size, something has gone terribly wrong
 		 */
 		if (rq->nr_sectors < rq->current_nr_sectors) {
 			printk(KERN_ERR "blk: request botched\n");
 			rq->nr_sectors = rq->current_nr_sectors;
 		}
 	}
 }

 void blk_recalc_rq_segments(struct request *rq)
 {
 	int nr_phys_segs;
 	int nr_hw_segs;
 	unsigned int phys_size;
 	unsigned int hw_size;
 	struct bio_vec *bv, *bvprv = NULL;
 	int seg_size;
 	int hw_seg_size;
 	int cluster;
 	struct req_iterator iter;
 	int high, highprv = 1;
 	struct request_queue *q = rq->q;

 	if (!rq->bio)
 		return;

 	cluster = test_bit(QUEUE_FLAG_CLUSTER, &q->queue_flags);
 	hw_seg_size = seg_size = 0;
 	phys_size = hw_size = nr_phys_segs = nr_hw_segs = 0;
 	rq_for_each_segment(bv, rq, iter) {
 		/*
 		 * the trick here is making sure that a high page is never
 		 * considered part of another segment, since that might
 		 * change with the bounce page.
 		 */
 		high = page_to_pfn(bv->bv_page) > q->bounce_pfn;
 		if (high || highprv)
 			goto new_hw_segment;
 		if (cluster) {
 			if (seg_size + bv->bv_len > q->max_segment_size)
 				goto new_segment;
 			if (!BIOVEC_PHYS_MERGEABLE(bvprv, bv))
 				goto new_segment;
 			if (!BIOVEC_SEG_BOUNDARY(q, bvprv, bv))
 				goto new_segment;
 			if (BIOVEC_VIRT_OVERSIZE(hw_seg_size + bv->bv_len))
 				goto new_hw_segment;

 			seg_size += bv->bv_len;
 			hw_seg_size += bv->bv_len;
 			bvprv = bv;
 			continue;
 		}
 new_segment:
 		if (BIOVEC_VIRT_MERGEABLE(bvprv, bv) &&
 		    !BIOVEC_VIRT_OVERSIZE(hw_seg_size + bv->bv_len))
 			hw_seg_size += bv->bv_len;
 		else {
 new_hw_segment:
 			if (nr_hw_segs == 1 &&
 			    hw_seg_size > rq->bio->bi_hw_front_size)
 				rq->bio->bi_hw_front_size = hw_seg_size;
 			hw_seg_size = BIOVEC_VIRT_START_SIZE(bv) + bv->bv_len;
 			nr_hw_segs++;
 		}

 		nr_phys_segs++;
 		bvprv = bv;
 		seg_size = bv->bv_len;
 		highprv = high;
 	}

 	if (nr_hw_segs == 1 &&
 	    hw_seg_size > rq->bio->bi_hw_front_size)
 		rq->bio->bi_hw_front_size = hw_seg_size;
 	if (hw_seg_size > rq->biotail->bi_hw_back_size)
 		rq->biotail->bi_hw_back_size = hw_seg_size;
 	rq->nr_phys_segments = nr_phys_segs;
 	rq->nr_hw_segments = nr_hw_segs;
 }

 void blk_recount_segments(struct request_queue *q, struct bio *bio)
 {
 	struct request rq;
 	struct bio *nxt = bio->bi_next;
 	rq.q = q;
 	rq.bio = rq.biotail = bio;
 	bio->bi_next = NULL;
 	blk_recalc_rq_segments(&rq);
 	bio->bi_next = nxt;
 	bio->bi_phys_segments = rq.nr_phys_segments;
 	bio->bi_hw_segments = rq.nr_hw_segments;
 	bio->bi_flags |= (1 << BIO_SEG_VALID);
 }
 EXPORT_SYMBOL(blk_recount_segments);

 static int blk_phys_contig_segment(struct request_queue *q, struct bio *bio,
 				   struct bio *nxt)
 {
 	if (!test_bit(QUEUE_FLAG_CLUSTER, &q->queue_flags))
 		return 0;

 	if (!BIOVEC_PHYS_MERGEABLE(__BVEC_END(bio), __BVEC_START(nxt)))
 		return 0;
 	if (bio->bi_size + nxt->bi_size > q->max_segment_size)
 		return 0;

 	/*
 	 * bio and nxt are contigous in memory, check if the queue allows
 	 * these two to be merged into one
 	 */
 	if (BIO_SEG_BOUNDARY(q, bio, nxt))
 		return 1;

 	return 0;
 }

 static int blk_hw_contig_segment(struct request_queue *q, struct bio *bio,
 				 struct bio *nxt)
 {
 	if (unlikely(!bio_flagged(bio, BIO_SEG_VALID)))
 		blk_recount_segments(q, bio);
 	if (unlikely(!bio_flagged(nxt, BIO_SEG_VALID)))
 		blk_recount_segments(q, nxt);
 	if (!BIOVEC_VIRT_MERGEABLE(__BVEC_END(bio), __BVEC_START(nxt)) ||
 	    BIOVEC_VIRT_OVERSIZE(bio->bi_hw_back_size + nxt->bi_hw_front_size))
 		return 0;
 	if (bio->bi_hw_back_size + nxt->bi_hw_front_size > q->max_segment_size)
 		return 0;

 	return 1;
 }

 /*
  * 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 bio_vec *bvec, *bvprv;
 	struct req_iterator iter;
 	struct scatterlist *sg;
 	int nsegs, cluster;

 	nsegs = 0;
 	cluster = test_bit(QUEUE_FLAG_CLUSTER, &q->queue_flags);

 	/*
 	 * for each bio in rq
 	 */
 	bvprv = NULL;
 	sg = NULL;
 	rq_for_each_segment(bvec, rq, iter) {
 		int nbytes = bvec->bv_len;

 		if (bvprv && cluster) {
 			if (sg->length + nbytes > q->max_segment_size)
 				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->page_link &= ~0x02;
 				sg = sg_next(sg);
 			}

 			sg_set_page(sg, bvec->bv_page, nbytes, bvec->bv_offset);
 			nsegs++;
 		}
 		bvprv = bvec;
 	} /* segments in rq */


 	if (unlikely(rq->cmd_flags & REQ_COPY_USER) &&
 	    (rq->data_len & q->dma_pad_mask)) {
 		unsigned int pad_len = (q->dma_pad_mask & ~rq->data_len) + 1;

 		sg->length += pad_len;
 		rq->extra_len += pad_len;
 	}

 	if (q->dma_drain_size && q->dma_drain_needed(rq)) {
 		if (rq->cmd_flags & REQ_RW)
 			memset(q->dma_drain_buffer, 0, q->dma_drain_size);

 		sg->page_link &= ~0x02;
 		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);

 	return nsegs;
 }
 EXPORT_SYMBOL(blk_rq_map_sg);

 static inline int ll_new_mergeable(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 > q->max_phys_segments) {
 		req->cmd_flags |= REQ_NOMERGE;
 		if (req == q->last_merge)
 			q->last_merge = NULL;
 		return 0;
 	}

 	/*
 	 * A hw segment is just getting larger, bump just the phys
 	 * counter.
 	 */
 	req->nr_phys_segments += nr_phys_segs;
 	return 1;
 }

 static inline int ll_new_hw_segment(struct request_queue *q,
 				    struct request *req,
 				    struct bio *bio)
 {
 	int nr_hw_segs = bio_hw_segments(q, bio);
 	int nr_phys_segs = bio_phys_segments(q, bio);

 	if (req->nr_hw_segments + nr_hw_segs > q->max_hw_segments
 	    || req->nr_phys_segments + nr_phys_segs > q->max_phys_segments) {
 		req->cmd_flags |= REQ_NOMERGE;
 		if (req == q->last_merge)
 			q->last_merge = NULL;
 		return 0;
 	}

 	/*
 	 * This will form the start of a new hw segment.  Bump both
 	 * counters.
 	 */
 	req->nr_hw_segments += nr_hw_segs;
 	req->nr_phys_segments += nr_phys_segs;
 	return 1;
 }

 int ll_back_merge_fn(struct request_queue *q, struct request *req,
 		     struct bio *bio)
 {
 	unsigned short max_sectors;
 	int len;

 	if (unlikely(blk_pc_request(req)))
 		max_sectors = q->max_hw_sectors;
 	else
 		max_sectors = q->max_sectors;

 	if (req->nr_sectors + bio_sectors(bio) > max_sectors) {
 		req->cmd_flags |= REQ_NOMERGE;
 		if (req == q->last_merge)
 			q->last_merge = NULL;
 		return 0;
 	}
 	if (unlikely(!bio_flagged(req->biotail, BIO_SEG_VALID)))
 		blk_recount_segments(q, req->biotail);
 	if (unlikely(!bio_flagged(bio, BIO_SEG_VALID)))
 		blk_recount_segments(q, bio);
 	len = req->biotail->bi_hw_back_size + bio->bi_hw_front_size;
 	if (BIOVEC_VIRT_MERGEABLE(__BVEC_END(req->biotail), __BVEC_START(bio))
 	    && !BIOVEC_VIRT_OVERSIZE(len)) {
 		int mergeable =  ll_new_mergeable(q, req, bio);

 		if (mergeable) {
 			if (req->nr_hw_segments == 1)
 				req->bio->bi_hw_front_size = len;
 			if (bio->bi_hw_segments == 1)
 				bio->bi_hw_back_size = len;
 		}
 		return mergeable;
 	}

 	return ll_new_hw_segment(q, req, bio);
 }

 int ll_front_merge_fn(struct request_queue *q, struct request *req,
 		      struct bio *bio)
 {
 	unsigned short max_sectors;
 	int len;

 	if (unlikely(blk_pc_request(req)))
 		max_sectors = q->max_hw_sectors;
 	else
 		max_sectors = q->max_sectors;


 	if (req->nr_sectors + bio_sectors(bio) > max_sectors) {
 		req->cmd_flags |= REQ_NOMERGE;
 		if (req == q->last_merge)
 			q->last_merge = NULL;
 		return 0;
 	}
 	len = bio->bi_hw_back_size + req->bio->bi_hw_front_size;
 	if (unlikely(!bio_flagged(bio, BIO_SEG_VALID)))
 		blk_recount_segments(q, bio);
 	if (unlikely(!bio_flagged(req->bio, BIO_SEG_VALID)))
 		blk_recount_segments(q, req->bio);
 	if (BIOVEC_VIRT_MERGEABLE(__BVEC_END(bio), __BVEC_START(req->bio)) &&
 	    !BIOVEC_VIRT_OVERSIZE(len)) {
 		int mergeable =  ll_new_mergeable(q, req, bio);

 		if (mergeable) {
 			if (bio->bi_hw_segments == 1)
 				bio->bi_hw_front_size = len;
 			if (req->nr_hw_segments == 1)
 				req->biotail->bi_hw_back_size = len;
 		}
 		return mergeable;
 	}

 	return ll_new_hw_segment(q, req, bio);
 }

 static int ll_merge_requests_fn(struct request_queue *q, struct request *req,
 				struct request *next)
 {
 	int total_phys_segments;
 	int total_hw_segments;

 	/*
 	 * First check if the either of the requests are re-queued
 	 * requests.  Can't merge them if they are.
 	 */
 	if (req->special || next->special)
 		return 0;

 	/*
 	 * Will it become too large?
 	 */
 	if ((req->nr_sectors + next->nr_sectors) > q->max_sectors)
 		return 0;

 	total_phys_segments = req->nr_phys_segments + next->nr_phys_segments;
 	if (blk_phys_contig_segment(q, req->biotail, next->bio))
 		total_phys_segments--;

 	if (total_phys_segments > q->max_phys_segments)
 		return 0;

 	total_hw_segments = req->nr_hw_segments + next->nr_hw_segments;
 	if (blk_hw_contig_segment(q, req->biotail, next->bio)) {
 		int len = req->biotail->bi_hw_back_size +
 				next->bio->bi_hw_front_size;
 		/*
 		 * propagate the combined length to the end of the requests
 		 */
 		if (req->nr_hw_segments == 1)
 			req->bio->bi_hw_front_size = len;
 		if (next->nr_hw_segments == 1)
 			next->biotail->bi_hw_back_size = len;
 		total_hw_segments--;
 	}

 	if (total_hw_segments > q->max_hw_segments)
 		return 0;

 	/* Merge is OK... */
 	req->nr_phys_segments = total_phys_segments;
 	req->nr_hw_segments = total_hw_segments;
 	return 1;
 }

 /*
  * 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;

 	/*
 	 * not contiguous
 	 */
 	if (req->sector + req->nr_sectors != next->sector)
 		return 0;

 	if (rq_data_dir(req) != rq_data_dir(next)
 	    || req->rq_disk != next->rq_disk
 	    || next->special)
 		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;

 	/*
 	 * 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->nr_sectors = req->hard_nr_sectors += next->hard_nr_sectors;

 	elv_merge_requests(q, req, next);

 	if (req->rq_disk) {
 		struct hd_struct *part
 			= get_part(req->rq_disk, req->sector);
 		disk_round_stats(req->rq_disk);
 		req->rq_disk->in_flight--;
 		if (part) {
 			part_round_stats(part);
 			part->in_flight--;
 		}
 	}

 	req->ioprio = ioprio_best(req->ioprio, next->ioprio);

 	__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;
 }
	/*
	* 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 "blk.h"

	void blk_recalc_rq_sectors(struct request *rq, int nsect)
	{
	if (blk_fs_request(rq)) {
	rq->hard_sector += nsect;
	rq->hard_nr_sectors -= nsect;

	/*
	* Move the I/O submission pointers ahead if required.
	*/
	if ((rq->nr_sectors >= rq->hard_nr_sectors) &&
	(rq->sector <= rq->hard_sector)) {
	rq->sector = rq->hard_sector;
	rq->nr_sectors = rq->hard_nr_sectors;
	rq->hard_cur_sectors = bio_cur_sectors(rq->bio);
	rq->current_nr_sectors = rq->hard_cur_sectors;
	rq->buffer = bio_data(rq->bio);
	}

	/*
	* if total number of sectors is less than the first segment
	* size, something has gone terribly wrong
	*/
	if (rq->nr_sectors < rq->current_nr_sectors) {
	printk(KERN_ERR "blk: request botched\n");
	rq->nr_sectors = rq->current_nr_sectors;
	}
	}
	}

	void blk_recalc_rq_segments(struct request *rq)
	{
	int nr_phys_segs;
	int nr_hw_segs;
	unsigned int phys_size;
	unsigned int hw_size;
	struct bio_vec bv, bvprv = NULL;
	int seg_size;
	int hw_seg_size;
	int cluster;
	struct req_iterator iter;
	int high, highprv = 1;
	struct request_queue *q = rq->q;

	if (!rq->bio)
	return;

	cluster = test_bit(QUEUE_FLAG_CLUSTER, &q->queue_flags);
	hw_seg_size = seg_size = 0;
	phys_size = hw_size = nr_phys_segs = nr_hw_segs = 0;
	rq_for_each_segment(bv, rq, iter) {
	/*
	* the trick here is making sure that a high page is never
	* considered part of another segment, since that might
	* change with the bounce page.
	*/
	high = page_to_pfn(bv->bv_page) > q->bounce_pfn;
	if (high \|\| highprv)
	goto new_hw_segment;
	if (cluster) {
	if (seg_size + bv->bv_len > q->max_segment_size)
	goto new_segment;
	if (!BIOVEC_PHYS_MERGEABLE(bvprv, bv))
	goto new_segment;
	if (!BIOVEC_SEG_BOUNDARY(q, bvprv, bv))
	goto new_segment;
	if (BIOVEC_VIRT_OVERSIZE(hw_seg_size + bv->bv_len))
	goto new_hw_segment;

	seg_size += bv->bv_len;
	hw_seg_size += bv->bv_len;
	bvprv = bv;
	continue;
	}
	new_segment:
	if (BIOVEC_VIRT_MERGEABLE(bvprv, bv) &&
	!BIOVEC_VIRT_OVERSIZE(hw_seg_size + bv->bv_len))
	hw_seg_size += bv->bv_len;
	else {
	new_hw_segment:
	if (nr_hw_segs == 1 &&
	hw_seg_size > rq->bio->bi_hw_front_size)
	rq->bio->bi_hw_front_size = hw_seg_size;
	hw_seg_size = BIOVEC_VIRT_START_SIZE(bv) + bv->bv_len;
	nr_hw_segs++;
	}

	nr_phys_segs++;
	bvprv = bv;
	seg_size = bv->bv_len;
	highprv = high;
	}

	if (nr_hw_segs == 1 &&
	hw_seg_size > rq->bio->bi_hw_front_size)
	rq->bio->bi_hw_front_size = hw_seg_size;
	if (hw_seg_size > rq->biotail->bi_hw_back_size)
	rq->biotail->bi_hw_back_size = hw_seg_size;
	rq->nr_phys_segments = nr_phys_segs;
	rq->nr_hw_segments = nr_hw_segs;
	}

	void blk_recount_segments(struct request_queue q, struct bio bio)
	{
	struct request rq;
	struct bio *nxt = bio->bi_next;
	rq.q = q;
	rq.bio = rq.biotail = bio;
	bio->bi_next = NULL;
	blk_recalc_rq_segments(&rq);
	bio->bi_next = nxt;
	bio->bi_phys_segments = rq.nr_phys_segments;
	bio->bi_hw_segments = rq.nr_hw_segments;
	bio->bi_flags \|= (1 << BIO_SEG_VALID);
	}
	EXPORT_SYMBOL(blk_recount_segments);

	static int blk_phys_contig_segment(struct request_queue q, struct bio bio,
	struct bio *nxt)
	{
	if (!test_bit(QUEUE_FLAG_CLUSTER, &q->queue_flags))
	return 0;

	if (!BIOVEC_PHYS_MERGEABLE(__BVEC_END(bio), __BVEC_START(nxt)))
	return 0;
	if (bio->bi_size + nxt->bi_size > q->max_segment_size)
	return 0;

	/*
	* bio and nxt are contigous in memory, check if the queue allows
	* these two to be merged into one
	*/
	if (BIO_SEG_BOUNDARY(q, bio, nxt))
	return 1;

	return 0;
	}

	static int blk_hw_contig_segment(struct request_queue q, struct bio bio,
	struct bio *nxt)
	{
	if (unlikely(!bio_flagged(bio, BIO_SEG_VALID)))
	blk_recount_segments(q, bio);
	if (unlikely(!bio_flagged(nxt, BIO_SEG_VALID)))
	blk_recount_segments(q, nxt);
	if (!BIOVEC_VIRT_MERGEABLE(__BVEC_END(bio), __BVEC_START(nxt)) \|\|
	BIOVEC_VIRT_OVERSIZE(bio->bi_hw_back_size + nxt->bi_hw_front_size))
	return 0;
	if (bio->bi_hw_back_size + nxt->bi_hw_front_size > q->max_segment_size)
	return 0;

	return 1;
	}

	/*
	* 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 bio_vec bvec, bvprv;
	struct req_iterator iter;
	struct scatterlist *sg;
	int nsegs, cluster;

	nsegs = 0;
	cluster = test_bit(QUEUE_FLAG_CLUSTER, &q->queue_flags);

	/*
	* for each bio in rq
	*/
	bvprv = NULL;
	sg = NULL;
	rq_for_each_segment(bvec, rq, iter) {
	int nbytes = bvec->bv_len;

	if (bvprv && cluster) {
	if (sg->length + nbytes > q->max_segment_size)
	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->page_link &= ~0x02;
	sg = sg_next(sg);
	}

	sg_set_page(sg, bvec->bv_page, nbytes, bvec->bv_offset);
	nsegs++;
	}
	bvprv = bvec;
	} /* segments in rq */


	if (unlikely(rq->cmd_flags & REQ_COPY_USER) &&
	(rq->data_len & q->dma_pad_mask)) {
	unsigned int pad_len = (q->dma_pad_mask & ~rq->data_len) + 1;

	sg->length += pad_len;
	rq->extra_len += pad_len;
	}

	if (q->dma_drain_size && q->dma_drain_needed(rq)) {
	if (rq->cmd_flags & REQ_RW)
	memset(q->dma_drain_buffer, 0, q->dma_drain_size);

	sg->page_link &= ~0x02;
	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);

	return nsegs;
	}
	EXPORT_SYMBOL(blk_rq_map_sg);

	static inline int ll_new_mergeable(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 > q->max_phys_segments) {
	req->cmd_flags \|= REQ_NOMERGE;
	if (req == q->last_merge)
	q->last_merge = NULL;
	return 0;
	}

	/*
	* A hw segment is just getting larger, bump just the phys
	* counter.
	*/
	req->nr_phys_segments += nr_phys_segs;
	return 1;
	}

	static inline int ll_new_hw_segment(struct request_queue *q,
	struct request *req,
	struct bio *bio)
	{
	int nr_hw_segs = bio_hw_segments(q, bio);
	int nr_phys_segs = bio_phys_segments(q, bio);

	if (req->nr_hw_segments + nr_hw_segs > q->max_hw_segments
	\|\| req->nr_phys_segments + nr_phys_segs > q->max_phys_segments) {
	req->cmd_flags \|= REQ_NOMERGE;
	if (req == q->last_merge)
	q->last_merge = NULL;
	return 0;
	}

	/*
	* This will form the start of a new hw segment. Bump both
	* counters.
	*/
	req->nr_hw_segments += nr_hw_segs;
	req->nr_phys_segments += nr_phys_segs;
	return 1;
	}

	int ll_back_merge_fn(struct request_queue q, struct request req,
	struct bio *bio)
	{
	unsigned short max_sectors;
	int len;

	if (unlikely(blk_pc_request(req)))
	max_sectors = q->max_hw_sectors;
	else
	max_sectors = q->max_sectors;

	if (req->nr_sectors + bio_sectors(bio) > max_sectors) {
	req->cmd_flags \|= REQ_NOMERGE;
	if (req == q->last_merge)
	q->last_merge = NULL;
	return 0;
	}
	if (unlikely(!bio_flagged(req->biotail, BIO_SEG_VALID)))
	blk_recount_segments(q, req->biotail);
	if (unlikely(!bio_flagged(bio, BIO_SEG_VALID)))
	blk_recount_segments(q, bio);
	len = req->biotail->bi_hw_back_size + bio->bi_hw_front_size;
	if (BIOVEC_VIRT_MERGEABLE(__BVEC_END(req->biotail), __BVEC_START(bio))
	&& !BIOVEC_VIRT_OVERSIZE(len)) {
	int mergeable = ll_new_mergeable(q, req, bio);

	if (mergeable) {
	if (req->nr_hw_segments == 1)
	req->bio->bi_hw_front_size = len;
	if (bio->bi_hw_segments == 1)
	bio->bi_hw_back_size = len;
	}
	return mergeable;
	}

	return ll_new_hw_segment(q, req, bio);
	}

	int ll_front_merge_fn(struct request_queue q, struct request req,
	struct bio *bio)
	{
	unsigned short max_sectors;
	int len;

	if (unlikely(blk_pc_request(req)))
	max_sectors = q->max_hw_sectors;
	else
	max_sectors = q->max_sectors;


	if (req->nr_sectors + bio_sectors(bio) > max_sectors) {
	req->cmd_flags \|= REQ_NOMERGE;
	if (req == q->last_merge)
	q->last_merge = NULL;
	return 0;
	}
	len = bio->bi_hw_back_size + req->bio->bi_hw_front_size;
	if (unlikely(!bio_flagged(bio, BIO_SEG_VALID)))
	blk_recount_segments(q, bio);
	if (unlikely(!bio_flagged(req->bio, BIO_SEG_VALID)))
	blk_recount_segments(q, req->bio);
	if (BIOVEC_VIRT_MERGEABLE(__BVEC_END(bio), __BVEC_START(req->bio)) &&
	!BIOVEC_VIRT_OVERSIZE(len)) {
	int mergeable = ll_new_mergeable(q, req, bio);

	if (mergeable) {
	if (bio->bi_hw_segments == 1)
	bio->bi_hw_front_size = len;
	if (req->nr_hw_segments == 1)
	req->biotail->bi_hw_back_size = len;
	}
	return mergeable;
	}

	return ll_new_hw_segment(q, req, bio);
	}

	static int ll_merge_requests_fn(struct request_queue q, struct request req,
	struct request *next)
	{
	int total_phys_segments;
	int total_hw_segments;

	/*
	* First check if the either of the requests are re-queued
	* requests. Can't merge them if they are.
	*/
	if (req->special \|\| next->special)
	return 0;

	/*
	* Will it become too large?
	*/
	if ((req->nr_sectors + next->nr_sectors) > q->max_sectors)
	return 0;

	total_phys_segments = req->nr_phys_segments + next->nr_phys_segments;
	if (blk_phys_contig_segment(q, req->biotail, next->bio))
	total_phys_segments--;

	if (total_phys_segments > q->max_phys_segments)
	return 0;

	total_hw_segments = req->nr_hw_segments + next->nr_hw_segments;
	if (blk_hw_contig_segment(q, req->biotail, next->bio)) {
	int len = req->biotail->bi_hw_back_size +
	next->bio->bi_hw_front_size;
	/*
	* propagate the combined length to the end of the requests
	*/
	if (req->nr_hw_segments == 1)
	req->bio->bi_hw_front_size = len;
	if (next->nr_hw_segments == 1)
	next->biotail->bi_hw_back_size = len;
	total_hw_segments--;
	}

	if (total_hw_segments > q->max_hw_segments)
	return 0;

	/* Merge is OK... */
	req->nr_phys_segments = total_phys_segments;
	req->nr_hw_segments = total_hw_segments;
	return 1;
	}

	/*
	* 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;

	/*
	* not contiguous
	*/
	if (req->sector + req->nr_sectors != next->sector)
	return 0;

	if (rq_data_dir(req) != rq_data_dir(next)
	\|\| req->rq_disk != next->rq_disk
	\|\| next->special)
	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;

	/*
	* 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->nr_sectors = req->hard_nr_sectors += next->hard_nr_sectors;

	elv_merge_requests(q, req, next);

	if (req->rq_disk) {
	struct hd_struct *part
	= get_part(req->rq_disk, req->sector);
	disk_round_stats(req->rq_disk);
	req->rq_disk->in_flight--;
	if (part) {
	part_round_stats(part);
	part->in_flight--;
	}
	}

	req->ioprio = ioprio_best(req->ioprio, next->ioprio);

	__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;
	}