/*
 * Copyright (C) 1991, 1992 Linus Torvalds
 * Copyright (C) 1994,      Karl Keyte: Added support for disk statistics
 * Elevator latency, (C) 2000  Andrea Arcangeli <andrea@suse.de> SuSE
 * Queue request tables / lock, selectable elevator, Jens Axboe <axboe@suse.de>
 * kernel-doc documentation started by NeilBrown <neilb@cse.unsw.edu.au>
 *	-  July2000
 * bio rewrite, highmem i/o, etc, Jens Axboe <axboe@suse.de> - may 2001
 */

/*
 * This handles all read/write requests to block devices
 */
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/backing-dev.h>
#include <linux/bio.h>
#include <linux/blkdev.h>
#include <linux/highmem.h>
#include <linux/mm.h>
#include <linux/kernel_stat.h>
#include <linux/string.h>
#include <linux/init.h>
#include <linux/completion.h>
#include <linux/slab.h>
#include <linux/swap.h>
#include <linux/writeback.h>
#include <linux/task_io_accounting_ops.h>
#include <linux/blktrace_api.h>
#include <linux/fault-inject.h>
#include <trace/block.h>

#include "blk.h"

DEFINE_TRACE(block_plug);
DEFINE_TRACE(block_unplug_io);
DEFINE_TRACE(block_unplug_timer);
DEFINE_TRACE(block_getrq);
DEFINE_TRACE(block_sleeprq);
DEFINE_TRACE(block_rq_requeue);
DEFINE_TRACE(block_bio_backmerge);
DEFINE_TRACE(block_bio_frontmerge);
DEFINE_TRACE(block_bio_queue);
DEFINE_TRACE(block_rq_complete);
DEFINE_TRACE(block_remap);	/* Also used in drivers/md/dm.c */
EXPORT_TRACEPOINT_SYMBOL_GPL(block_remap);

static int __make_request(struct request_queue *q, struct bio *bio);

/*
 * For the allocated request tables
 */
static struct kmem_cache *request_cachep;

/*
 * For queue allocation
 */
struct kmem_cache *blk_requestq_cachep;

/*
 * Controlling structure to kblockd
 */
static struct workqueue_struct *kblockd_workqueue;

static void drive_stat_acct(struct request *rq, int new_io)
{
	struct gendisk *disk = rq->rq_disk;
	struct hd_struct *part;
	int rw = rq_data_dir(rq);
	int cpu;

	if (!blk_fs_request(rq) || !disk || !blk_do_io_stat(disk->queue))
		return;

	cpu = part_stat_lock();
	part = disk_map_sector_rcu(rq->rq_disk, rq->sector);

	if (!new_io)
		part_stat_inc(cpu, part, merges[rw]);
	else {
		part_round_stats(cpu, part);
		part_inc_in_flight(part);
	}

	part_stat_unlock();
}

void blk_queue_congestion_threshold(struct request_queue *q)
{
	int nr;

	nr = q->nr_requests - (q->nr_requests / 8) + 1;
	if (nr > q->nr_requests)
		nr = q->nr_requests;
	q->nr_congestion_on = nr;

	nr = q->nr_requests - (q->nr_requests / 8) - (q->nr_requests / 16) - 1;
	if (nr < 1)
		nr = 1;
	q->nr_congestion_off = nr;
}

/**
 * blk_get_backing_dev_info - get the address of a queue's backing_dev_info
 * @bdev:	device
 *
 * Locates the passed device's request queue and returns the address of its
 * backing_dev_info
 *
 * Will return NULL if the request queue cannot be located.
 */
struct backing_dev_info *blk_get_backing_dev_info(struct block_device *bdev)
{
	struct backing_dev_info *ret = NULL;
	struct request_queue *q = bdev_get_queue(bdev);

	if (q)
		ret = &q->backing_dev_info;
	return ret;
}
EXPORT_SYMBOL(blk_get_backing_dev_info);

void blk_rq_init(struct request_queue *q, struct request *rq)
{
	memset(rq, 0, sizeof(*rq));

	INIT_LIST_HEAD(&rq->queuelist);
	INIT_LIST_HEAD(&rq->timeout_list);
	rq->cpu = -1;
	rq->q = q;
	rq->sector = rq->hard_sector = (sector_t) -1;
	INIT_HLIST_NODE(&rq->hash);
	RB_CLEAR_NODE(&rq->rb_node);
	rq->cmd = rq->__cmd;
	rq->cmd_len = BLK_MAX_CDB;
	rq->tag = -1;
	rq->ref_count = 1;
}
EXPORT_SYMBOL(blk_rq_init);

static void req_bio_endio(struct request *rq, struct bio *bio,
			  unsigned int nbytes, int error)
{
	struct request_queue *q = rq->q;

	if (&q->bar_rq != rq) {
		if (error)
			clear_bit(BIO_UPTODATE, &bio->bi_flags);
		else if (!test_bit(BIO_UPTODATE, &bio->bi_flags))
			error = -EIO;

		if (unlikely(nbytes > bio->bi_size)) {
			printk(KERN_ERR "%s: want %u bytes done, %u left\n",
			       __func__, nbytes, bio->bi_size);
			nbytes = bio->bi_size;
		}

		if (unlikely(rq->cmd_flags & REQ_QUIET))
			set_bit(BIO_QUIET, &bio->bi_flags);

		bio->bi_size -= nbytes;
		bio->bi_sector += (nbytes >> 9);

		if (bio_integrity(bio))
			bio_integrity_advance(bio, nbytes);

		if (bio->bi_size == 0)
			bio_endio(bio, error);
	} else {

		/*
		 * Okay, this is the barrier request in progress, just
		 * record the error;
		 */
		if (error && !q->orderr)
			q->orderr = error;
	}
}

void blk_dump_rq_flags(struct request *rq, char *msg)
{
	int bit;

	printk(KERN_INFO "%s: dev %s: type=%x, flags=%x\n", msg,
		rq->rq_disk ? rq->rq_disk->disk_name : "?", rq->cmd_type,
		rq->cmd_flags);

	printk(KERN_INFO "  sector %llu, nr/cnr %lu/%u\n",
						(unsigned long long)rq->sector,
						rq->nr_sectors,
						rq->current_nr_sectors);
	printk(KERN_INFO "  bio %p, biotail %p, buffer %p, data %p, len %u\n",
						rq->bio, rq->biotail,
						rq->buffer, rq->data,
						rq->data_len);

	if (blk_pc_request(rq)) {
		printk(KERN_INFO "  cdb: ");
		for (bit = 0; bit < BLK_MAX_CDB; bit++)
			printk("%02x ", rq->cmd[bit]);
		printk("\n");
	}
}
EXPORT_SYMBOL(blk_dump_rq_flags);

/*
 * "plug" the device if there are no outstanding requests: this will
 * force the transfer to start only after we have put all the requests
 * on the list.
 *
 * This is called with interrupts off and no requests on the queue and
 * with the queue lock held.
 */
void blk_plug_device(struct request_queue *q)
{
	WARN_ON(!irqs_disabled());

	/*
	 * don't plug a stopped queue, it must be paired with blk_start_queue()
	 * which will restart the queueing
	 */
	if (blk_queue_stopped(q))
		return;

	if (!queue_flag_test_and_set(QUEUE_FLAG_PLUGGED, q)) {
		mod_timer(&q->unplug_timer, jiffies + q->unplug_delay);
		trace_block_plug(q);
	}
}
EXPORT_SYMBOL(blk_plug_device);

/**
 * blk_plug_device_unlocked - plug a device without queue lock held
 * @q:    The &struct request_queue to plug
 *
 * Description:
 *   Like @blk_plug_device(), but grabs the queue lock and disables
 *   interrupts.
 **/
void blk_plug_device_unlocked(struct request_queue *q)
{
	unsigned long flags;

	spin_lock_irqsave(q->queue_lock, flags);
	blk_plug_device(q);
	spin_unlock_irqrestore(q->queue_lock, flags);
}
EXPORT_SYMBOL(blk_plug_device_unlocked);

/*
 * remove the queue from the plugged list, if present. called with
 * queue lock held and interrupts disabled.
 */
int blk_remove_plug(struct request_queue *q)
{
	WARN_ON(!irqs_disabled());

	if (!queue_flag_test_and_clear(QUEUE_FLAG_PLUGGED, q))
		return 0;

	del_timer(&q->unplug_timer);
	return 1;
}
EXPORT_SYMBOL(blk_remove_plug);

/*
 * remove the plug and let it rip..
 */
void __generic_unplug_device(struct request_queue *q)
{
	if (unlikely(blk_queue_stopped(q)))
		return;
	if (!blk_remove_plug(q) && !blk_queue_nonrot(q))
		return;

	q->request_fn(q);
}

/**
 * generic_unplug_device - fire a request queue
 * @q:    The &struct request_queue in question
 *
 * Description:
 *   Linux uses plugging to build bigger requests queues before letting
 *   the device have at them. If a queue is plugged, the I/O scheduler
 *   is still adding and merging requests on the queue. Once the queue
 *   gets unplugged, the request_fn defined for the queue is invoked and
 *   transfers started.
 **/
void generic_unplug_device(struct request_queue *q)
{
	if (blk_queue_plugged(q)) {
		spin_lock_irq(q->queue_lock);
		__generic_unplug_device(q);
		spin_unlock_irq(q->queue_lock);
	}
}
EXPORT_SYMBOL(generic_unplug_device);

static void blk_backing_dev_unplug(struct backing_dev_info *bdi,
				   struct page *page)
{
	struct request_queue *q = bdi->unplug_io_data;

	blk_unplug(q);
}

void blk_unplug_work(struct work_struct *work)
{
	struct request_queue *q =
		container_of(work, struct request_queue, unplug_work);

	trace_block_unplug_io(q);
	q->unplug_fn(q);
}

void blk_unplug_timeout(unsigned long data)
{
	struct request_queue *q = (struct request_queue *)data;

	trace_block_unplug_timer(q);
	kblockd_schedule_work(q, &q->unplug_work);
}

void blk_unplug(struct request_queue *q)
{
	/*
	 * devices don't necessarily have an ->unplug_fn defined
	 */
	if (q->unplug_fn) {
		trace_block_unplug_io(q);
		q->unplug_fn(q);
	}
}
EXPORT_SYMBOL(blk_unplug);

static void blk_invoke_request_fn(struct request_queue *q)
{
	if (unlikely(blk_queue_stopped(q)))
		return;

	/*
	 * one level of recursion is ok and is much faster than kicking
	 * the unplug handling
	 */
	if (!queue_flag_test_and_set(QUEUE_FLAG_REENTER, q)) {
		q->request_fn(q);
		queue_flag_clear(QUEUE_FLAG_REENTER, q);
	} else {
		queue_flag_set(QUEUE_FLAG_PLUGGED, q);
		kblockd_schedule_work(q, &q->unplug_work);
	}
}

/**
 * blk_start_queue - restart a previously stopped queue
 * @q:    The &struct request_queue in question
 *
 * Description:
 *   blk_start_queue() will clear the stop flag on the queue, and call
 *   the request_fn for the queue if it was in a stopped state when
 *   entered. Also see blk_stop_queue(). Queue lock must be held.
 **/
void blk_start_queue(struct request_queue *q)
{
	WARN_ON(!irqs_disabled());

	queue_flag_clear(QUEUE_FLAG_STOPPED, q);
	blk_invoke_request_fn(q);
}
EXPORT_SYMBOL(blk_start_queue);

/**
 * blk_stop_queue - stop a queue
 * @q:    The &struct request_queue in question
 *
 * Description:
 *   The Linux block layer assumes that a block driver will consume all
 *   entries on the request queue when the request_fn strategy is called.
 *   Often this will not happen, because of hardware limitations (queue
 *   depth settings). If a device driver gets a 'queue full' response,
 *   or if it simply chooses not to queue more I/O at one point, it can
 *   call this function to prevent the request_fn from being called until
 *   the driver has signalled it's ready to go again. This happens by calling
 *   blk_start_queue() to restart queue operations. Queue lock must be held.
 **/
void blk_stop_queue(struct request_queue *q)
{
	blk_remove_plug(q);
	queue_flag_set(QUEUE_FLAG_STOPPED, q);
}
EXPORT_SYMBOL(blk_stop_queue);

/**
 * blk_sync_queue - cancel any pending callbacks on a queue
 * @q: the queue
 *
 * Description:
 *     The block layer may perform asynchronous callback activity
 *     on a queue, such as calling the unplug function after a timeout.
 *     A block device may call blk_sync_queue to ensure that any
 *     such activity is cancelled, thus allowing it to release resources
 *     that the callbacks might use. The caller must already have made sure
 *     that its ->make_request_fn will not re-add plugging prior to calling
 *     this function.
 *
 */
void blk_sync_queue(struct request_queue *q)
{
	del_timer_sync(&q->unplug_timer);
	del_timer_sync(&q->timeout);
	cancel_work_sync(&q->unplug_work);
}
EXPORT_SYMBOL(blk_sync_queue);

/**
 * __blk_run_queue - run a single device queue
 * @q:	The queue to run
 *
 * Description:
 *    See @blk_run_queue. This variant must be called with the queue lock
 *    held and interrupts disabled.
 *
 */
void __blk_run_queue(struct request_queue *q)
{
	blk_remove_plug(q);

	/*
	 * Only recurse once to avoid overrunning the stack, let the unplug
	 * handling reinvoke the handler shortly if we already got there.
	 */
	if (!elv_queue_empty(q))
		blk_invoke_request_fn(q);
}
EXPORT_SYMBOL(__blk_run_queue);

/**
 * blk_run_queue - run a single device queue
 * @q: The queue to run
 *
 * Description:
 *    Invoke request handling on this queue, if it has pending work to do.
 *    May be used to restart queueing when a request has completed. Also
 *    See @blk_start_queueing.
 *
 */
void blk_run_queue(struct request_queue *q)
{
	unsigned long flags;

	spin_lock_irqsave(q->queue_lock, flags);
	__blk_run_queue(q);
	spin_unlock_irqrestore(q->queue_lock, flags);
}
EXPORT_SYMBOL(blk_run_queue);

void blk_put_queue(struct request_queue *q)
{
	kobject_put(&q->kobj);
}

void blk_cleanup_queue(struct request_queue *q)
{
	/*
	 * We know we have process context here, so we can be a little
	 * cautious and ensure that pending block actions on this device
	 * are done before moving on. Going into this function, we should
	 * not have processes doing IO to this device.
	 */
	blk_sync_queue(q);

	mutex_lock(&q->sysfs_lock);
	queue_flag_set_unlocked(QUEUE_FLAG_DEAD, q);
	mutex_unlock(&q->sysfs_lock);

	if (q->elevator)
		elevator_exit(q->elevator);

	blk_put_queue(q);
}
EXPORT_SYMBOL(blk_cleanup_queue);

static int blk_init_free_list(struct request_queue *q)
{
	struct request_list *rl = &q->rq;

	rl->count[BLK_RW_SYNC] = rl->count[BLK_RW_ASYNC] = 0;
	rl->starved[BLK_RW_SYNC] = rl->starved[BLK_RW_ASYNC] = 0;
	rl->elvpriv = 0;
	init_waitqueue_head(&rl->wait[BLK_RW_SYNC]);
	init_waitqueue_head(&rl->wait[BLK_RW_ASYNC]);

	rl->rq_pool = mempool_create_node(BLKDEV_MIN_RQ, mempool_alloc_slab,
				mempool_free_slab, request_cachep, q->node);

	if (!rl->rq_pool)
		return -ENOMEM;

	return 0;
}

struct request_queue *blk_alloc_queue(gfp_t gfp_mask)
{
	return blk_alloc_queue_node(gfp_mask, -1);
}
EXPORT_SYMBOL(blk_alloc_queue);

struct request_queue *blk_alloc_queue_node(gfp_t gfp_mask, int node_id)
{
	struct request_queue *q;
	int err;

	q = kmem_cache_alloc_node(blk_requestq_cachep,
				gfp_mask | __GFP_ZERO, node_id);
	if (!q)
		return NULL;

	q->backing_dev_info.unplug_io_fn = blk_backing_dev_unplug;
	q->backing_dev_info.unplug_io_data = q;
	err = bdi_init(&q->backing_dev_info);
	if (err) {
		kmem_cache_free(blk_requestq_cachep, q);
		return NULL;
	}

	init_timer(&q->unplug_timer);
	setup_timer(&q->timeout, blk_rq_timed_out_timer, (unsigned long) q);
	INIT_LIST_HEAD(&q->timeout_list);
	INIT_WORK(&q->unplug_work, blk_unplug_work);

	kobject_init(&q->kobj, &blk_queue_ktype);

	mutex_init(&q->sysfs_lock);
	spin_lock_init(&q->__queue_lock);

	return q;
}
EXPORT_SYMBOL(blk_alloc_queue_node);

/**
 * blk_init_queue  - prepare a request queue for use with a block device
 * @rfn:  The function to be called to process requests that have been
 *        placed on the queue.
 * @lock: Request queue spin lock
 *
 * Description:
 *    If a block device wishes to use the standard request handling procedures,
 *    which sorts requests and coalesces adjacent requests, then it must
 *    call blk_init_queue().  The function @rfn will be called when there
 *    are requests on the queue that need to be processed.  If the device
 *    supports plugging, then @rfn may not be called immediately when requests
 *    are available on the queue, but may be called at some time later instead.
 *    Plugged queues are generally unplugged when a buffer belonging to one
 *    of the requests on the queue is needed, or due to memory pressure.
 *
 *    @rfn is not required, or even expected, to remove all requests off the
 *    queue, but only as many as it can handle at a time.  If it does leave
 *    requests on the queue, it is responsible for arranging that the requests
 *    get dealt with eventually.
 *
 *    The queue spin lock must be held while manipulating the requests on the
 *    request queue; this lock will be taken also from interrupt context, so irq
 *    disabling is needed for it.
 *
 *    Function returns a pointer to the initialized request queue, or %NULL if
 *    it didn't succeed.
 *
 * Note:
 *    blk_init_queue() must be paired with a blk_cleanup_queue() call
 *    when the block device is deactivated (such as at module unload).
 **/

struct request_queue *blk_init_queue(request_fn_proc *rfn, spinlock_t *lock)
{
	return blk_init_queue_node(rfn, lock, -1);
}
EXPORT_SYMBOL(blk_init_queue);

struct request_queue *
blk_init_queue_node(request_fn_proc *rfn, spinlock_t *lock, int node_id)
{
	struct request_queue *q = blk_alloc_queue_node(GFP_KERNEL, node_id);

	if (!q)
		return NULL;

	q->node = node_id;
	if (blk_init_free_list(q)) {
		kmem_cache_free(blk_requestq_cachep, q);
		return NULL;
	}

	/*
	 * if caller didn't supply a lock, they get per-queue locking with
	 * our embedded lock
	 */
	if (!lock)
		lock = &q->__queue_lock;

	q->request_fn		= rfn;
	q->prep_rq_fn		= NULL;
	q->unplug_fn		= generic_unplug_device;
	q->queue_flags		= QUEUE_FLAG_DEFAULT;
	q->queue_lock		= lock;

	/*
	 * This also sets hw/phys segments, boundary and size
	 */
	blk_queue_make_request(q, __make_request);

	q->sg_reserved_size = INT_MAX;

	blk_set_cmd_filter_defaults(&q->cmd_filter);

	/*
	 * all done
	 */
	if (!elevator_init(q, NULL)) {
		blk_queue_congestion_threshold(q);
		return q;
	}

	blk_put_queue(q);
	return NULL;
}
EXPORT_SYMBOL(blk_init_queue_node);

int blk_get_queue(struct request_queue *q)
{
	if (likely(!test_bit(QUEUE_FLAG_DEAD, &q->queue_flags))) {
		kobject_get(&q->kobj);
		return 0;
	}

	return 1;
}

static inline void blk_free_request(struct request_queue *q, struct request *rq)
{
	if (rq->cmd_flags & REQ_ELVPRIV)
		elv_put_request(q, rq);
	mempool_free(rq, q->rq.rq_pool);
}

static struct request *
blk_alloc_request(struct request_queue *q, int rw, int priv, gfp_t gfp_mask)
{
	struct request *rq = mempool_alloc(q->rq.rq_pool, gfp_mask);

	if (!rq)
		return NULL;

	blk_rq_init(q, rq);

	rq->cmd_flags = rw | REQ_ALLOCED;

	if (priv) {
		if (unlikely(elv_set_request(q, rq, gfp_mask))) {
			mempool_free(rq, q->rq.rq_pool);
			return NULL;
		}
		rq->cmd_flags |= REQ_ELVPRIV;
	}

	return rq;
}

/*
 * ioc_batching returns true if the ioc is a valid batching request and
 * should be given priority access to a request.
 */
static inline int ioc_batching(struct request_queue *q, struct io_context *ioc)
{
	if (!ioc)
		return 0;

	/*
	 * Make sure the process is able to allocate at least 1 request
	 * even if the batch times out, otherwise we could theoretically
	 * lose wakeups.
	 */
	return ioc->nr_batch_requests == q->nr_batching ||
		(ioc->nr_batch_requests > 0
		&& time_before(jiffies, ioc->last_waited + BLK_BATCH_TIME));
}

/*
 * ioc_set_batching sets ioc to be a new "batcher" if it is not one. This
 * will cause the process to be a "batcher" on all queues in the system. This
 * is the behaviour we want though - once it gets a wakeup it should be given
 * a nice run.
 */
static void ioc_set_batching(struct request_queue *q, struct io_context *ioc)
{
	if (!ioc || ioc_batching(q, ioc))
		return;

	ioc->nr_batch_requests = q->nr_batching;
	ioc->last_waited = jiffies;
}

static void __freed_request(struct request_queue *q, int sync)
{
	struct request_list *rl = &q->rq;

	if (rl->count[sync] < queue_congestion_off_threshold(q))
		blk_clear_queue_congested(q, sync);

	if (rl->count[sync] + 1 <= q->nr_requests) {
		if (waitqueue_active(&rl->wait[sync]))
			wake_up(&rl->wait[sync]);

		blk_clear_queue_full(q, sync);
	}
}

/*
 * A request has just been released.  Account for it, update the full and
 * congestion status, wake up any waiters.   Called under q->queue_lock.
 */
static void freed_request(struct request_queue *q, int sync, int priv)
{
	struct request_list *rl = &q->rq;

	rl->count[sync]--;
	if (priv)
		rl->elvpriv--;

	__freed_request(q, sync);

	if (unlikely(rl->starved[sync ^ 1]))
		__freed_request(q, sync ^ 1);
}

/*
 * Get a free request, queue_lock must be held.
 * Returns NULL on failure, with queue_lock held.
 * Returns !NULL on success, with queue_lock *not held*.
 */
static struct request *get_request(struct request_queue *q, int rw_flags,
				   struct bio *bio, gfp_t gfp_mask)
{
	struct request *rq = NULL;
	struct request_list *rl = &q->rq;
	struct io_context *ioc = NULL;
	const bool is_sync = rw_is_sync(rw_flags) != 0;
	int may_queue, priv;

	may_queue = elv_may_queue(q, rw_flags);
	if (may_queue == ELV_MQUEUE_NO)
		goto rq_starved;

	if (rl->count[is_sync]+1 >= queue_congestion_on_threshold(q)) {
		if (rl->count[is_sync]+1 >= q->nr_requests) {
			ioc = current_io_context(GFP_ATOMIC, q->node);
			/*
			 * The queue will fill after this allocation, so set
			 * it as full, and mark this process as "batching".
			 * This process will be allowed to complete a batch of
			 * requests, others will be blocked.
			 */
			if (!blk_queue_full(q, is_sync)) {
				ioc_set_batching(q, ioc);
				blk_set_queue_full(q, is_sync);
			} else {
				if (may_queue != ELV_MQUEUE_MUST
						&& !ioc_batching(q, ioc)) {
					/*
					 * The queue is full and the allocating
					 * process is not a "batcher", and not
					 * exempted by the IO scheduler
					 */
					goto out;
				}
			}
		}
		blk_set_queue_congested(q, is_sync);
	}

	/*
	 * Only allow batching queuers to allocate up to 50% over the defined
	 * limit of requests, otherwise we could have thousands of requests
	 * allocated with any setting of ->nr_requests
	 */
	if (rl->count[is_sync] >= (3 * q->nr_requests / 2))
		goto out;

	rl->count[is_sync]++;
	rl->starved[is_sync] = 0;

	priv = !test_bit(QUEUE_FLAG_ELVSWITCH, &q->queue_flags);
	if (priv)
		rl->elvpriv++;

	spin_unlock_irq(q->queue_lock);

	rq = blk_alloc_request(q, rw_flags, priv, gfp_mask);
	if (unlikely(!rq)) {
		/*
		 * Allocation failed presumably due to memory. Undo anything
		 * we might have messed up.
		 *
		 * Allocating task should really be put onto the front of the
		 * wait queue, but this is pretty rare.
		 */
		spin_lock_irq(q->queue_lock);
		freed_request(q, is_sync, priv);

		/*
		 * in the very unlikely event that allocation failed and no
		 * requests for this direction was pending, mark us starved
		 * so that freeing of a request in the other direction will
		 * notice us. another possible fix would be to split the
		 * rq mempool into READ and WRITE
		 */
rq_starved:
		if (unlikely(rl->count[is_sync] == 0))
			rl->starved[is_sync] = 1;

		goto out;
	}

	/*
	 * ioc may be NULL here, and ioc_batching will be false. That's
	 * OK, if the queue is under the request limit then requests need
	 * not count toward the nr_batch_requests limit. There will always
	 * be some limit enforced by BLK_BATCH_TIME.
	 */
	if (ioc_batching(q, ioc))
		ioc->nr_batch_requests--;

	trace_block_getrq(q, bio, rw_flags & 1);
out:
	return rq;
}

/*
 * No available requests for this queue, unplug the device and wait for some
 * requests to become available.
 *
 * Called with q->queue_lock held, and returns with it unlocked.
 */
static struct request *get_request_wait(struct request_queue *q, int rw_flags,
					struct bio *bio)
{
	const bool is_sync = rw_is_sync(rw_flags) != 0;
	struct request *rq;

	rq = get_request(q, rw_flags, bio, GFP_NOIO);
	while (!rq) {
		DEFINE_WAIT(wait);
		struct io_context *ioc;
		struct request_list *rl = &q->rq;

		prepare_to_wait_exclusive(&rl->wait[is_sync], &wait,
				TASK_UNINTERRUPTIBLE);

		trace_block_sleeprq(q, bio, rw_flags & 1);

		__generic_unplug_device(q);
		spin_unlock_irq(q->queue_lock);
		io_schedule();

		/*
		 * After sleeping, we become a "batching" process and
		 * will be able to allocate at least one request, and
		 * up to a big batch of them for a small period time.
		 * See ioc_batching, ioc_set_batching
		 */
		ioc = current_io_context(GFP_NOIO, q->node);
		ioc_set_batching(q, ioc);

		spin_lock_irq(q->queue_lock);
		finish_wait(&rl->wait[is_sync], &wait);

		rq = get_request(q, rw_flags, bio, GFP_NOIO);
	};

	return rq;
}

struct request *blk_get_request(struct request_queue *q, int rw, gfp_t gfp_mask)
{
	struct request *rq;

	BUG_ON(rw != READ && rw != WRITE);

	spin_lock_irq(q->queue_lock);
	if (gfp_mask & __GFP_WAIT) {
		rq = get_request_wait(q, rw, NULL);
	} else {
		rq = get_request(q, rw, NULL, gfp_mask);
		if (!rq)
			spin_unlock_irq(q->queue_lock);
	}
	/* q->queue_lock is unlocked at this point */

	return rq;
}
EXPORT_SYMBOL(blk_get_request);

/**
 * blk_start_queueing - initiate dispatch of requests to device
 * @q:		request queue to kick into gear
 *
 * This is basically a helper to remove the need to know whether a queue
 * is plugged or not if someone just wants to initiate dispatch of requests
 * for this queue. Should be used to start queueing on a device outside
 * of ->request_fn() context. Also see @blk_run_queue.
 *
 * The queue lock must be held with interrupts disabled.
 */
void blk_start_queueing(struct request_queue *q)
{
	if (!blk_queue_plugged(q)) {
		if (unlikely(blk_queue_stopped(q)))
			return;
		q->request_fn(q);
	} else
		__generic_unplug_device(q);
}
EXPORT_SYMBOL(blk_start_queueing);

/**
 * blk_requeue_request - put a request back on queue
 * @q:		request queue where request should be inserted
 * @rq:		request to be inserted
 *
 * Description:
 *    Drivers often keep queueing requests until the hardware cannot accept
 *    more, when that condition happens we need to put the request back
 *    on the queue. Must be called with queue lock held.
 */
void blk_requeue_request(struct request_queue *q, struct request *rq)
{
	blk_delete_timer(rq);
	blk_clear_rq_complete(rq);
	trace_block_rq_requeue(q, rq);

	if (blk_rq_tagged(rq))
		blk_queue_end_tag(q, rq);

	elv_requeue_request(q, rq);
}
EXPORT_SYMBOL(blk_requeue_request);

/**
 * blk_insert_request - insert a special request into a request queue
 * @q:		request queue where request should be inserted
 * @rq:		request to be inserted
 * @at_head:	insert request at head or tail of queue
 * @data:	private data
 *
 * Description:
 *    Many block devices need to execute commands asynchronously, so they don't
 *    block the whole kernel from preemption during request execution.  This is
 *    accomplished normally by inserting aritficial requests tagged as
 *    REQ_TYPE_SPECIAL in to the corresponding request queue, and letting them
 *    be scheduled for actual execution by the request queue.
 *
 *    We have the option of inserting the head or the tail of the queue.
 *    Typically we use the tail for new ioctls and so forth.  We use the head
 *    of the queue for things like a QUEUE_FULL message from a device, or a
 *    host that is unable to accept a particular command.
 */
void blk_insert_request(struct request_queue *q, struct request *rq,
			int at_head, void *data)
{
	int where = at_head ? ELEVATOR_INSERT_FRONT : ELEVATOR_INSERT_BACK;
	unsigned long flags;

	/*
	 * tell I/O scheduler that this isn't a regular read/write (ie it
	 * must not attempt merges on this) and that it acts as a soft
	 * barrier
	 */
	rq->cmd_type = REQ_TYPE_SPECIAL;
	rq->cmd_flags |= REQ_SOFTBARRIER;

	rq->special = data;

	spin_lock_irqsave(q->queue_lock, flags);

	/*
	 * If command is tagged, release the tag
	 */
	if (blk_rq_tagged(rq))
		blk_queue_end_tag(q, rq);

	drive_stat_acct(rq, 1);
	__elv_add_request(q, rq, where, 0);
	blk_start_queueing(q);
	spin_unlock_irqrestore(q->queue_lock, flags);
}
EXPORT_SYMBOL(blk_insert_request);

/*
 * add-request adds a request to the linked list.
 * queue lock is held and interrupts disabled, as we muck with the
 * request queue list.
 */
static inline void add_request(struct request_queue *q, struct request *req)
{
	drive_stat_acct(req, 1);

	/*
	 * elevator indicated where it wants this request to be
	 * inserted at elevator_merge time
	 */
	__elv_add_request(q, req, ELEVATOR_INSERT_SORT, 0);
}

static void part_round_stats_single(int cpu, struct hd_struct *part,
				    unsigned long now)
{
	if (now == part->stamp)
		return;

	if (part->in_flight) {
		__part_stat_add(cpu, part, time_in_queue,
				part->in_flight * (now - part->stamp));
		__part_stat_add(cpu, part, io_ticks, (now - part->stamp));
	}
	part->stamp = now;
}

/**
 * part_round_stats() - Round off the performance stats on a struct disk_stats.
 * @cpu: cpu number for stats access
 * @part: target partition
 *
 * The average IO queue length and utilisation statistics are maintained
 * by observing the current state of the queue length and the amount of
 * time it has been in this state for.
 *
 * Normally, that accounting is done on IO completion, but that can result
 * in more than a second's worth of IO being accounted for within any one
 * second, leading to >100% utilisation.  To deal with that, we call this
 * function to do a round-off before returning the results when reading
 * /proc/diskstats.  This accounts immediately for all queue usage up to
 * the current jiffies and restarts the counters again.
 */
void part_round_stats(int cpu, struct hd_struct *part)
{
	unsigned long now = jiffies;

	if (part->partno)
		part_round_stats_single(cpu, &part_to_disk(part)->part0, now);
	part_round_stats_single(cpu, part, now);
}
EXPORT_SYMBOL_GPL(part_round_stats);

/*
 * queue lock must be held
 */
void __blk_put_request(struct request_queue *q, struct request *req)
{
	if (unlikely(!q))
		return;
	if (unlikely(--req->ref_count))
		return;

	elv_completed_request(q, req);

	/* this is a bio leak */
	WARN_ON(req->bio != NULL);

	/*
	 * Request may not have originated from ll_rw_blk. if not,
	 * it didn't come out of our reserved rq pools
	 */
	if (req->cmd_flags & REQ_ALLOCED) {
		int is_sync = rq_is_sync(req) != 0;
		int priv = req->cmd_flags & REQ_ELVPRIV;

		BUG_ON(!list_empty(&req->queuelist));
		BUG_ON(!hlist_unhashed(&req->hash));

		blk_free_request(q, req);
		freed_request(q, is_sync, priv);
	}
}
EXPORT_SYMBOL_GPL(__blk_put_request);

void blk_put_request(struct request *req)
{
	unsigned long flags;
	struct request_queue *q = req->q;

	spin_lock_irqsave(q->queue_lock, flags);
	__blk_put_request(q, req);
	spin_unlock_irqrestore(q->queue_lock, flags);
}
EXPORT_SYMBOL(blk_put_request);

void init_request_from_bio(struct request *req, struct bio *bio)
{
	req->cpu = bio->bi_comp_cpu;
	req->cmd_type = REQ_TYPE_FS;

	/*
	 * inherit FAILFAST from bio (for read-ahead, and explicit FAILFAST)
	 */
	if (bio_rw_ahead(bio))
		req->cmd_flags |= (REQ_FAILFAST_DEV | REQ_FAILFAST_TRANSPORT |
				   REQ_FAILFAST_DRIVER);
	if (bio_failfast_dev(bio))
		req->cmd_flags |= REQ_FAILFAST_DEV;
	if (bio_failfast_transport(bio))
		req->cmd_flags |= REQ_FAILFAST_TRANSPORT;
	if (bio_failfast_driver(bio))
		req->cmd_flags |= REQ_FAILFAST_DRIVER;

	/*
	 * REQ_BARRIER implies no merging, but lets make it explicit
	 */
	if (unlikely(bio_discard(bio))) {
		req->cmd_flags |= REQ_DISCARD;
		if (bio_barrier(bio))
			req->cmd_flags |= REQ_SOFTBARRIER;
		req->q->prepare_discard_fn(req->q, req);
	} else if (unlikely(bio_barrier(bio)))
		req->cmd_flags |= (REQ_HARDBARRIER | REQ_NOMERGE);

	if (bio_sync(bio))
		req->cmd_flags |= REQ_RW_SYNC;
	if (bio_unplug(bio))
		req->cmd_flags |= REQ_UNPLUG;
	if (bio_rw_meta(bio))
		req->cmd_flags |= REQ_RW_META;
	if (bio_noidle(bio))
		req->cmd_flags |= REQ_NOIDLE;

	req->errors = 0;
	req->hard_sector = req->sector = bio->bi_sector;
	req->ioprio = bio_prio(bio);
	req->start_time = jiffies;
	blk_rq_bio_prep(req->q, req, bio);
}

/*
 * Only disabling plugging for non-rotational devices if it does tagging
 * as well, otherwise we do need the proper merging
 */
static inline bool queue_should_plug(struct request_queue *q)
{
	return !(blk_queue_nonrot(q) && blk_queue_tagged(q));
}

static int __make_request(struct request_queue *q, struct bio *bio)
{
	struct request *req;
	int el_ret, nr_sectors;
	const unsigned short prio = bio_prio(bio);
	const int sync = bio_sync(bio);
	const int unplug = bio_unplug(bio);
	int rw_flags;

	nr_sectors = bio_sectors(bio);

	/*
	 * low level driver can indicate that it wants pages above a
	 * certain limit bounced to low memory (ie for highmem, or even
	 * ISA dma in theory)
	 */
	blk_queue_bounce(q, &bio);

	spin_lock_irq(q->queue_lock);

	if (unlikely(bio_barrier(bio)) || elv_queue_empty(q))
		goto get_rq;

	el_ret = elv_merge(q, &req, bio);
	switch (el_ret) {
	case ELEVATOR_BACK_MERGE:
		BUG_ON(!rq_mergeable(req));

		if (!ll_back_merge_fn(q, req, bio))
			break;

		trace_block_bio_backmerge(q, bio);

		req->biotail->bi_next = bio;
		req->biotail = bio;
		req->nr_sectors = req->hard_nr_sectors += nr_sectors;
		req->ioprio = ioprio_best(req->ioprio, prio);
		if (!blk_rq_cpu_valid(req))
			req->cpu = bio->bi_comp_cpu;
		drive_stat_acct(req, 0);
		if (!attempt_back_merge(q, req))
			elv_merged_request(q, req, el_ret);
		goto out;

	case ELEVATOR_FRONT_MERGE:
		BUG_ON(!rq_mergeable(req));

		if (!ll_front_merge_fn(q, req, bio))
			break;

		trace_block_bio_frontmerge(q, bio);

		bio->bi_next = req->bio;
		req->bio = bio;

		/*
		 * may not be valid. if the low level driver said
		 * it didn't need a bounce buffer then it better
		 * not touch req->buffer either...
		 */
		req->buffer = bio_data(bio);
		req->current_nr_sectors = bio_cur_sectors(bio);
		req->hard_cur_sectors = req->current_nr_sectors;
		req->sector = req->hard_sector = bio->bi_sector;
		req->nr_sectors = req->hard_nr_sectors += nr_sectors;
		req->ioprio = ioprio_best(req->ioprio, prio);
		if (!blk_rq_cpu_valid(req))
			req->cpu = bio->bi_comp_cpu;
		drive_stat_acct(req, 0);
		if (!attempt_front_merge(q, req))
			elv_merged_request(q, req, el_ret);
		goto out;

	/* ELV_NO_MERGE: elevator says don't/can't merge. */
	default:
		;
	}

get_rq:
	/*
	 * This sync check and mask will be re-done in init_request_from_bio(),
	 * but we need to set it earlier to expose the sync flag to the
	 * rq allocator and io schedulers.
	 */
	rw_flags = bio_data_dir(bio);
	if (sync)
		rw_flags |= REQ_RW_SYNC;

	/*
	 * Grab a free request. This is might sleep but can not fail.
	 * Returns with the queue unlocked.
	 */
	req = get_request_wait(q, rw_flags, bio);

	/*
	 * After dropping the lock and possibly sleeping here, our request
	 * may now be mergeable after it had proven unmergeable (above).
	 * We don't worry about that case for efficiency. It won't happen
	 * often, and the elevators are able to handle it.
	 */
	init_request_from_bio(req, bio);

	spin_lock_irq(q->queue_lock);
	if (test_bit(QUEUE_FLAG_SAME_COMP, &q->queue_flags) ||
	    bio_flagged(bio, BIO_CPU_AFFINE))
		req->cpu = blk_cpu_to_group(smp_processor_id());
	if (queue_should_plug(q) && elv_queue_empty(q))
		blk_plug_device(q);
	add_request(q, req);
out:
	if (unplug || !queue_should_plug(q))
		__generic_unplug_device(q);
	spin_unlock_irq(q->queue_lock);
	return 0;
}

/*
 * If bio->bi_dev is a partition, remap the location
 */
static inline void blk_partition_remap(struct bio *bio)
{
	struct block_device *bdev = bio->bi_bdev;

	if (bio_sectors(bio) && bdev != bdev->bd_contains) {
		struct hd_struct *p = bdev->bd_part;

		bio->bi_sector += p->start_sect;
		bio->bi_bdev = bdev->bd_contains;

		trace_block_remap(bdev_get_queue(bio->bi_bdev), bio,
				    bdev->bd_dev, bio->bi_sector,
				    bio->bi_sector - p->start_sect);
	}
}

static void handle_bad_sector(struct bio *bio)
{
	char b[BDEVNAME_SIZE];

	printk(KERN_INFO "attempt to access beyond end of device\n");
	printk(KERN_INFO "%s: rw=%ld, want=%Lu, limit=%Lu\n",
			bdevname(bio->bi_bdev, b),
			bio->bi_rw,
			(unsigned long long)bio->bi_sector + bio_sectors(bio),
			(long long)(bio->bi_bdev->bd_inode->i_size >> 9));

	set_bit(BIO_EOF, &bio->bi_flags);
}

#ifdef CONFIG_FAIL_MAKE_REQUEST

static DECLARE_FAULT_ATTR(fail_make_request);

static int __init setup_fail_make_request(char *str)
{
	return setup_fault_attr(&fail_make_request, str);
}
__setup("fail_make_request=", setup_fail_make_request);

static int should_fail_request(struct bio *bio)
{
	struct hd_struct *part = bio->bi_bdev->bd_part;

	if (part_to_disk(part)->part0.make_it_fail || part->make_it_fail)
		return should_fail(&fail_make_request, bio->bi_size);

	return 0;
}

static int __init fail_make_request_debugfs(void)
{
	return init_fault_attr_dentries(&fail_make_request,
					"fail_make_request");
}

late_initcall(fail_make_request_debugfs);

#else /* CONFIG_FAIL_MAKE_REQUEST */

static inline int should_fail_request(struct bio *bio)
{
	return 0;
}

#endif /* CONFIG_FAIL_MAKE_REQUEST */

/*
 * Check whether this bio extends beyond the end of the device.
 */
static inline int bio_check_eod(struct bio *bio, unsigned int nr_sectors)
{
	sector_t maxsector;

	if (!nr_sectors)
		return 0;

	/* Test device or partition size, when known. */
	maxsector = bio->bi_bdev->bd_inode->i_size >> 9;
	if (maxsector) {
		sector_t sector = bio->bi_sector;

		if (maxsector < nr_sectors || maxsector - nr_sectors < sector) {
			/*
			 * This may well happen - the kernel calls bread()
			 * without checking the size of the device, e.g., when
			 * mounting a device.
			 */
			handle_bad_sector(bio);
			return 1;
		}
	}

	return 0;
}

/**
 * generic_make_request - hand a buffer to its device driver for I/O
 * @bio:  The bio describing the location in memory and on the device.
 *
 * generic_make_request() is used to make I/O requests of block
 * devices. It is passed a &struct bio, which describes the I/O that needs
 * to be done.
 *
 * generic_make_request() does not return any status.  The
 * success/failure status of the request, along with notification of
 * completion, is delivered asynchronously through the bio->bi_end_io
 * function described (one day) else where.
 *
 * The caller of generic_make_request must make sure that bi_io_vec
 * are set to describe the memory buffer, and that bi_dev and bi_sector are
 * set to describe the device address, and the
 * bi_end_io and optionally bi_private are set to describe how
 * completion notification should be signaled.
 *
 * generic_make_request and the drivers it calls may use bi_next if this
 * bio happens to be merged with someone else, and may change bi_dev and
 * bi_sector for remaps as it sees fit.  So the values of these fields
 * should NOT be depended on after the call to generic_make_request.
 */
static inline void __generic_make_request(struct bio *bio)
{
	struct request_queue *q;
	sector_t old_sector;
	int ret, nr_sectors = bio_sectors(bio);
	dev_t old_dev;
	int err = -EIO;

	might_sleep();

	if (bio_check_eod(bio, nr_sectors))
		goto end_io;

	/*
	 * Resolve the mapping until finished. (drivers are
	 * still free to implement/resolve their own stacking
	 * by explicitly returning 0)
	 *
	 * NOTE: we don't repeat the blk_size check for each new device.
	 * Stacking drivers are expected to know what they are doing.
	 */
	old_sector = -1;
	old_dev = 0;
	do {
		char b[BDEVNAME_SIZE];

		q = bdev_get_queue(bio->bi_bdev);
		if (unlikely(!q)) {
			printk(KERN_ERR
			       "generic_make_request: Trying to access "
				"nonexistent block-device %s (%Lu)\n",
				bdevname(bio->bi_bdev, b),
				(long long) bio->bi_sector);
			goto end_io;
		}

		if (unlikely(nr_sectors > q->max_hw_sectors)) {
			printk(KERN_ERR "bio too big device %s (%u > %u)\n",
				bdevname(bio->bi_bdev, b),
				bio_sectors(bio),
				q->max_hw_sectors);
			goto end_io;
		}

		if (unlikely(test_bit(QUEUE_FLAG_DEAD, &q->queue_flags)))
			goto end_io;

		if (should_fail_request(bio))
			goto end_io;

		/*
		 * If this device has partitions, remap block n
		 * of partition p to block n+start(p) of the disk.
		 */
		blk_partition_remap(bio);

		if (bio_integrity_enabled(bio) && bio_integrity_prep(bio))
			goto end_io;

		if (old_sector != -1)
			trace_block_remap(q, bio, old_dev, bio->bi_sector,
					    old_sector);

		trace_block_bio_queue(q, bio);

		old_sector = bio->bi_sector;
		old_dev = bio->bi_bdev->bd_dev;

		if (bio_check_eod(bio, nr_sectors))
			goto end_io;

		if (bio_discard(bio) && !q->prepare_discard_fn) {
			err = -EOPNOTSUPP;
			goto end_io;
		}
		if (bio_barrier(bio) && bio_has_data(bio) &&
		    (q->next_ordered == QUEUE_ORDERED_NONE)) {
			err = -EOPNOTSUPP;
			goto end_io;
		}

		ret = q->make_request_fn(q, bio);
	} while (ret);

	return;

end_io:
	bio_endio(bio, err);
}

/*
 * We only want one ->make_request_fn to be active at a time,
 * else stack usage with stacked devices could be a problem.
 * So use current->bio_{list,tail} to keep a list of requests
 * submited by a make_request_fn function.
 * current->bio_tail is also used as a flag to say if
 * generic_make_request is currently active in this task or not.
 * If it is NULL, then no make_request is active.  If it is non-NULL,
 * then a make_request is active, and new requests should be added
 * at the tail
 */
void generic_make_request(struct bio *bio)
{
	if (current->bio_tail) {
		/* make_request is active */
		*(current->bio_tail) = bio;
		bio->bi_next = NULL;
		current->bio_tail = &bio->bi_next;
		return;
	}
	/* following loop may be a bit non-obvious, and so deserves some
	 * explanation.
	 * Before entering the loop, bio->bi_next is NULL (as all callers
	 * ensure that) so we have a list with a single bio.
	 * We pretend that we have just taken it off a longer list, so
	 * we assign bio_list to the next (which is NULL) and bio_tail
	 * to &bio_list, thus initialising the bio_list of new bios to be
	 * added.  __generic_make_request may indeed add some more bios
	 * through a recursive call to generic_make_request.  If it
	 * did, we find a non-NULL value in bio_list and re-enter the loop
	 * from the top.  In this case we really did just take the bio
	 * of the top of the list (no pretending) and so fixup bio_list and
	 * bio_tail or bi_next, and call into __generic_make_request again.
	 *
	 * The loop was structured like this to make only one call to
	 * __generic_make_request (which is important as it is large and
	 * inlined) and to keep the structure simple.
	 */
	BUG_ON(bio->bi_next);
	do {
		current->bio_list = bio->bi_next;
		if (bio->bi_next == NULL)
			current->bio_tail = &current->bio_list;
		else
			bio->bi_next = NULL;
		__generic_make_request(bio);
		bio = current->bio_list;
	} while (bio);
	current->bio_tail = NULL; /* deactivate */
}
EXPORT_SYMBOL(generic_make_request);

/**
 * submit_bio - submit a bio to the block device layer for I/O
 * @rw: whether to %READ or %WRITE, or maybe to %READA (read ahead)
 * @bio: The &struct bio which describes the I/O
 *
 * submit_bio() is very similar in purpose to generic_make_request(), and
 * uses that function to do most of the work. Both are fairly rough
 * interfaces; @bio must be presetup and ready for I/O.
 *
 */
void submit_bio(int rw, struct bio *bio)
{
	int count = bio_sectors(bio);

	bio->bi_rw |= rw;

	/*
	 * If it's a regular read/write or a barrier with data attached,
	 * go through the normal accounting stuff before submission.
	 */
	if (bio_has_data(bio)) {
		if (rw & WRITE) {
			count_vm_events(PGPGOUT, count);
		} else {
			task_io_account_read(bio->bi_size);
			count_vm_events(PGPGIN, count);
		}

		if (unlikely(block_dump)) {
			char b[BDEVNAME_SIZE];
			printk(KERN_DEBUG "%s(%d): %s block %Lu on %s\n",
			current->comm, task_pid_nr(current),
				(rw & WRITE) ? "WRITE" : "READ",
				(unsigned long long)bio->bi_sector,
				bdevname(bio->bi_bdev, b));
		}
	}

	generic_make_request(bio);
}
EXPORT_SYMBOL(submit_bio);

/**
 * blk_rq_check_limits - Helper function to check a request for the queue limit
 * @q:  the queue
 * @rq: the request being checked
 *
 * Description:
 *    @rq may have been made based on weaker limitations of upper-level queues
 *    in request stacking drivers, and it may violate the limitation of @q.
 *    Since the block layer and the underlying device driver trust @rq
 *    after it is inserted to @q, it should be checked against @q before
 *    the insertion using this generic function.
 *
 *    This function should also be useful for request stacking drivers
 *    in some cases below, so export this fuction.
 *    Request stacking drivers like request-based dm may change the queue
 *    limits while requests are in the queue (e.g. dm's table swapping).
 *    Such request stacking drivers should check those requests agaist
 *    the new queue limits again when they dispatch those requests,
 *    although such checkings are also done against the old queue limits
 *    when submitting requests.
 */
int blk_rq_check_limits(struct request_queue *q, struct request *rq)
{
	if (rq->nr_sectors > q->max_sectors ||
	    rq->data_len > q->max_hw_sectors << 9) {
		printk(KERN_ERR "%s: over max size limit.\n", __func__);
		return -EIO;
	}

	/*
	 * queue's settings related to segment counting like q->bounce_pfn
	 * may differ from that of other stacking queues.
	 * Recalculate it to check the request correctly on this queue's
	 * limitation.
	 */
	blk_recalc_rq_segments(rq);
	if (rq->nr_phys_segments > q->max_phys_segments ||
	    rq->nr_phys_segments > q->max_hw_segments) {
		printk(KERN_ERR "%s: over max segments limit.\n", __func__);
		return -EIO;
	}

	return 0;
}
EXPORT_SYMBOL_GPL(blk_rq_check_limits);

/**
 * blk_insert_cloned_request - Helper for stacking drivers to submit a request
 * @q:  the queue to submit the request
 * @rq: the request being queued
 */
int blk_insert_cloned_request(struct request_queue *q, struct request *rq)
{
	unsigned long flags;

	if (blk_rq_check_limits(q, rq))
		return -EIO;

#ifdef CONFIG_FAIL_MAKE_REQUEST
	if (rq->rq_disk && rq->rq_disk->part0.make_it_fail &&
	    should_fail(&fail_make_request, blk_rq_bytes(rq)))
		return -EIO;
#endif

	spin_lock_irqsave(q->queue_lock, flags);

	/*
	 * Submitting request must be dequeued before calling this function
	 * because it will be linked to another request_queue
	 */
	BUG_ON(blk_queued_rq(rq));

	drive_stat_acct(rq, 1);
	__elv_add_request(q, rq, ELEVATOR_INSERT_BACK, 0);

	spin_unlock_irqrestore(q->queue_lock, flags);

	return 0;
}
EXPORT_SYMBOL_GPL(blk_insert_cloned_request);

/**
 * blkdev_dequeue_request - dequeue request and start timeout timer
 * @req: request to dequeue
 *
 * Dequeue @req and start timeout timer on it.  This hands off the
 * request to the driver.
 *
 * Block internal functions which don't want to start timer should
 * call elv_dequeue_request().
 */
void blkdev_dequeue_request(struct request *req)
{
	elv_dequeue_request(req->q, req);

	/*
	 * We are now handing the request to the hardware, add the
	 * timeout handler.
	 */
	blk_add_timer(req);
}
EXPORT_SYMBOL(blkdev_dequeue_request);

static void blk_account_io_completion(struct request *req, unsigned int bytes)
{
	struct gendisk *disk = req->rq_disk;

	if (!disk || !blk_do_io_stat(disk->queue))
		return;

	if (blk_fs_request(req)) {
		const int rw = rq_data_dir(req);
		struct hd_struct *part;
		int cpu;

		cpu = part_stat_lock();
		part = disk_map_sector_rcu(req->rq_disk, req->sector);
		part_stat_add(cpu, part, sectors[rw], bytes >> 9);
		part_stat_unlock();
	}
}

static void blk_account_io_done(struct request *req)
{
	struct gendisk *disk = req->rq_disk;

	if (!disk || !blk_do_io_stat(disk->queue))
		return;

	/*
	 * Account IO completion.  bar_rq isn't accounted as a normal
	 * IO on queueing nor completion.  Accounting the containing
	 * request is enough.
	 */
	if (blk_fs_request(req) && req != &req->q->bar_rq) {
		unsigned long duration = jiffies - req->start_time;
		const int rw = rq_data_dir(req);
		struct hd_struct *part;
		int cpu;

		cpu = part_stat_lock();
		part = disk_map_sector_rcu(disk, req->sector);

		part_stat_inc(cpu, part, ios[rw]);
		part_stat_add(cpu, part, ticks[rw], duration);
		part_round_stats(cpu, part);
		part_dec_in_flight(part);

		part_stat_unlock();
	}
}

/**
 * __end_that_request_first - end I/O on a request
 * @req:      the request being processed
 * @error:    %0 for success, < %0 for error
 * @nr_bytes: number of bytes to complete
 *
 * Description:
 *     Ends I/O on a number of bytes attached to @req, and sets it up
 *     for the next range of segments (if any) in the cluster.
 *
 * Return:
 *     %0 - we are done with this request, call end_that_request_last()
 *     %1 - still buffers pending for this request
 **/
static int __end_that_request_first(struct request *req, int error,
				    int nr_bytes)
{
	int total_bytes, bio_nbytes, next_idx = 0;
	struct bio *bio;

	trace_block_rq_complete(req->q, req);

	/*
	 * for a REQ_TYPE_BLOCK_PC request, we want to carry any eventual
	 * sense key with us all the way through
	 */
	if (!blk_pc_request(req))
		req->errors = 0;

	if (error && (blk_fs_request(req) && !(req->cmd_flags & REQ_QUIET))) {
		printk(KERN_ERR "end_request: I/O error, dev %s, sector %llu\n",
				req->rq_disk ? req->rq_disk->disk_name : "?",
				(unsigned long long)req->sector);
	}

	blk_account_io_completion(req, nr_bytes);

	total_bytes = bio_nbytes = 0;
	while ((bio = req->bio) != NULL) {
		int nbytes;

		if (nr_bytes >= bio->bi_size) {
			req->bio = bio->bi_next;
			nbytes = bio->bi_size;
			req_bio_endio(req, bio, nbytes, error);
			next_idx = 0;
			bio_nbytes = 0;
		} else {
			int idx = bio->bi_idx + next_idx;

			if (unlikely(bio->bi_idx >= bio->bi_vcnt)) {
				blk_dump_rq_flags(req, "__end_that");
				printk(KERN_ERR "%s: bio idx %d >= vcnt %d\n",
				       __func__, bio->bi_idx, bio->bi_vcnt);
				break;
			}

			nbytes = bio_iovec_idx(bio, idx)->bv_len;
			BIO_BUG_ON(nbytes > bio->bi_size);

			/*
			 * not a complete bvec done
			 */
			if (unlikely(nbytes > nr_bytes)) {
				bio_nbytes += nr_bytes;
				total_bytes += nr_bytes;
				break;
			}

			/*
			 * advance to the next vector
			 */
			next_idx++;
			bio_nbytes += nbytes;
		}

		total_bytes += nbytes;
		nr_bytes -= nbytes;

		bio = req->bio;
		if (bio) {
			/*
			 * end more in this run, or just return 'not-done'
			 */
			if (unlikely(nr_bytes <= 0))
				break;
		}
	}

	/*
	 * completely done
	 */
	if (!req->bio)
		return 0;

	/*
	 * if the request wasn't completed, update state
	 */
	if (bio_nbytes) {
		req_bio_endio(req, bio, bio_nbytes, error);
		bio->bi_idx += next_idx;
		bio_iovec(bio)->bv_offset += nr_bytes;
		bio_iovec(bio)->bv_len -= nr_bytes;
	}

	blk_recalc_rq_sectors(req, total_bytes >> 9);
	blk_recalc_rq_segments(req);
	return 1;
}

/*
 * queue lock must be held
 */
static void end_that_request_last(struct request *req, int error)
{
	if (blk_rq_tagged(req))
		blk_queue_end_tag(req->q, req);

	if (blk_queued_rq(req))
		elv_dequeue_request(req->q, req);

	if (unlikely(laptop_mode) && blk_fs_request(req))
		laptop_io_completion();

	blk_delete_timer(req);

	blk_account_io_done(req);

	if (req->end_io)
		req->end_io(req, error);
	else {
		if (blk_bidi_rq(req))
			__blk_put_request(req->next_rq->q, req->next_rq);

		__blk_put_request(req->q, req);
	}
}

/**
 * blk_rq_bytes - Returns bytes left to complete in the entire request
 * @rq: the request being processed
 **/
unsigned int blk_rq_bytes(struct request *rq)
{
	if (blk_fs_request(rq))
		return rq->hard_nr_sectors << 9;

	return rq->data_len;
}
EXPORT_SYMBOL_GPL(blk_rq_bytes);

/**
 * blk_rq_cur_bytes - Returns bytes left to complete in the current segment
 * @rq: the request being processed
 **/
unsigned int blk_rq_cur_bytes(struct request *rq)
{
	if (blk_fs_request(rq))
		return rq->current_nr_sectors << 9;

	if (rq->bio)
		return rq->bio->bi_size;

	return rq->data_len;
}
EXPORT_SYMBOL_GPL(blk_rq_cur_bytes);

/**
 * end_request - end I/O on the current segment of the request
 * @req:	the request being processed
 * @uptodate:	error value or %0/%1 uptodate flag
 *
 * Description:
 *     Ends I/O on the current segment of a request. If that is the only
 *     remaining segment, the request is also completed and freed.
 *
 *     This is a remnant of how older block drivers handled I/O completions.
 *     Modern drivers typically end I/O on the full request in one go, unless
 *     they have a residual value to account for. For that case this function
 *     isn't really useful, unless the residual just happens to be the
 *     full current segment. In other words, don't use this function in new
 *     code. Use blk_end_request() or __blk_end_request() to end a request.
 **/
void end_request(struct request *req, int uptodate)
{
	int error = 0;

	if (uptodate <= 0)
		error = uptodate ? uptodate : -EIO;

	__blk_end_request(req, error, req->hard_cur_sectors << 9);
}
EXPORT_SYMBOL(end_request);

static int end_that_request_data(struct request *rq, int error,
				 unsigned int nr_bytes, unsigned int bidi_bytes)
{
	if (rq->bio) {
		if (__end_that_request_first(rq, error, nr_bytes))
			return 1;

		/* Bidi request must be completed as a whole */
		if (blk_bidi_rq(rq) &&
		    __end_that_request_first(rq->next_rq, error, bidi_bytes))
			return 1;
	}

	return 0;
}

/**
 * blk_end_io - Generic end_io function to complete a request.
 * @rq:           the request being processed
 * @error:        %0 for success, < %0 for error
 * @nr_bytes:     number of bytes to complete @rq
 * @bidi_bytes:   number of bytes to complete @rq->next_rq
 * @drv_callback: function called between completion of bios in the request
 *                and completion of the request.
 *                If the callback returns non %0, this helper returns without
 *                completion of the request.
 *
 * Description:
 *     Ends I/O on a number of bytes attached to @rq and @rq->next_rq.
 *     If @rq has leftover, sets it up for the next range of segments.
 *
 * Return:
 *     %0 - we are done with this request
 *     %1 - this request is not freed yet, it still has pending buffers.
 **/
static int blk_end_io(struct request *rq, int error, unsigned int nr_bytes,
		      unsigned int bidi_bytes,
		      int (drv_callback)(struct request *))
{
	struct request_queue *q = rq->q;
	unsigned long flags = 0UL;

	if (end_that_request_data(rq, error, nr_bytes, bidi_bytes))
		return 1;

	/* Special feature for tricky drivers */
	if (drv_callback && drv_callback(rq))
		return 1;

	add_disk_randomness(rq->rq_disk);

	spin_lock_irqsave(q->queue_lock, flags);
	end_that_request_last(rq, error);
	spin_unlock_irqrestore(q->queue_lock, flags);

	return 0;
}

/**
 * blk_end_request - Helper function for drivers to complete the request.
 * @rq:       the request being processed
 * @error:    %0 for success, < %0 for error
 * @nr_bytes: number of bytes to complete
 *
 * Description:
 *     Ends I/O on a number of bytes attached to @rq.
 *     If @rq has leftover, sets it up for the next range of segments.
 *
 * Return:
 *     %0 - we are done with this request
 *     %1 - still buffers pending for this request
 **/
int blk_end_request(struct request *rq, int error, unsigned int nr_bytes)
{
	return blk_end_io(rq, error, nr_bytes, 0, NULL);
}
EXPORT_SYMBOL_GPL(blk_end_request);

/**
 * __blk_end_request - Helper function for drivers to complete the request.
 * @rq:       the request being processed
 * @error:    %0 for success, < %0 for error
 * @nr_bytes: number of bytes to complete
 *
 * Description:
 *     Must be called with queue lock held unlike blk_end_request().
 *
 * Return:
 *     %0 - we are done with this request
 *     %1 - still buffers pending for this request
 **/
int __blk_end_request(struct request *rq, int error, unsigned int nr_bytes)
{
	if (rq->bio && __end_that_request_first(rq, error, nr_bytes))
		return 1;

	add_disk_randomness(rq->rq_disk);

	end_that_request_last(rq, error);

	return 0;
}
EXPORT_SYMBOL_GPL(__blk_end_request);

/**
 * blk_end_bidi_request - Helper function for drivers to complete bidi request.
 * @rq:         the bidi request being processed
 * @error:      %0 for success, < %0 for error
 * @nr_bytes:   number of bytes to complete @rq
 * @bidi_bytes: number of bytes to complete @rq->next_rq
 *
 * Description:
 *     Ends I/O on a number of bytes attached to @rq and @rq->next_rq.
 *
 * Return:
 *     %0 - we are done with this request
 *     %1 - still buffers pending for this request
 **/
int blk_end_bidi_request(struct request *rq, int error, unsigned int nr_bytes,
			 unsigned int bidi_bytes)
{
	return blk_end_io(rq, error, nr_bytes, bidi_bytes, NULL);
}
EXPORT_SYMBOL_GPL(blk_end_bidi_request);

/**
 * blk_update_request - Special helper function for request stacking drivers
 * @rq:           the request being processed
 * @error:        %0 for success, < %0 for error
 * @nr_bytes:     number of bytes to complete @rq
 *
 * Description:
 *     Ends I/O on a number of bytes attached to @rq, but doesn't complete
 *     the request structure even if @rq doesn't have leftover.
 *     If @rq has leftover, sets it up for the next range of segments.
 *
 *     This special helper function is only for request stacking drivers
 *     (e.g. request-based dm) so that they can handle partial completion.
 *     Actual device drivers should use blk_end_request instead.
 */
void blk_update_request(struct request *rq, int error, unsigned int nr_bytes)
{
	if (!end_that_request_data(rq, error, nr_bytes, 0)) {
		/*
		 * These members are not updated in end_that_request_data()
		 * when all bios are completed.
		 * Update them so that the request stacking driver can find
		 * how many bytes remain in the request later.
		 */
		rq->nr_sectors = rq->hard_nr_sectors = 0;
		rq->current_nr_sectors = rq->hard_cur_sectors = 0;
	}
}
EXPORT_SYMBOL_GPL(blk_update_request);

/**
 * blk_end_request_callback - Special helper function for tricky drivers
 * @rq:           the request being processed
 * @error:        %0 for success, < %0 for error
 * @nr_bytes:     number of bytes to complete
 * @drv_callback: function called between completion of bios in the request
 *                and completion of the request.
 *                If the callback returns non %0, this helper returns without
 *                completion of the request.
 *
 * Description:
 *     Ends I/O on a number of bytes attached to @rq.
 *     If @rq has leftover, sets it up for the next range of segments.
 *
 *     This special helper function is used only for existing tricky drivers.
 *     (e.g. cdrom_newpc_intr() of ide-cd)
 *     This interface will be removed when such drivers are rewritten.
 *     Don't use this interface in other places anymore.
 *
 * Return:
 *     %0 - we are done with this request
 *     %1 - this request is not freed yet.
 *          this request still has pending buffers or
 *          the driver doesn't want to finish this request yet.
 **/
int blk_end_request_callback(struct request *rq, int error,
			     unsigned int nr_bytes,
			     int (drv_callback)(struct request *))
{
	return blk_end_io(rq, error, nr_bytes, 0, drv_callback);
}
EXPORT_SYMBOL_GPL(blk_end_request_callback);

void blk_rq_bio_prep(struct request_queue *q, struct request *rq,
		     struct bio *bio)
{
	/* Bit 0 (R/W) is identical in rq->cmd_flags and bio->bi_rw, and
	   we want BIO_RW_AHEAD (bit 1) to imply REQ_FAILFAST (bit 1). */
	rq->cmd_flags |= (bio->bi_rw & 3);

	if (bio_has_data(bio)) {
		rq->nr_phys_segments = bio_phys_segments(q, bio);
		rq->buffer = bio_data(bio);
	}
	rq->current_nr_sectors = bio_cur_sectors(bio);
	rq->hard_cur_sectors = rq->current_nr_sectors;
	rq->hard_nr_sectors = rq->nr_sectors = bio_sectors(bio);
	rq->data_len = bio->bi_size;

	rq->bio = rq->biotail = bio;

	if (bio->bi_bdev)
		rq->rq_disk = bio->bi_bdev->bd_disk;
}

/**
 * blk_lld_busy - Check if underlying low-level drivers of a device are busy
 * @q : the queue of the device being checked
 *
 * Description:
 *    Check if underlying low-level drivers of a device are busy.
 *    If the drivers want to export their busy state, they must set own
 *    exporting function using blk_queue_lld_busy() first.
 *
 *    Basically, this function is used only by request stacking drivers
 *    to stop dispatching requests to underlying devices when underlying
 *    devices are busy.  This behavior helps more I/O merging on the queue
 *    of the request stacking driver and prevents I/O throughput regression
 *    on burst I/O load.
 *
 * Return:
 *    0 - Not busy (The request stacking driver should dispatch request)
 *    1 - Busy (The request stacking driver should stop dispatching request)
 */
int blk_lld_busy(struct request_queue *q)
{
	if (q->lld_busy_fn)
		return q->lld_busy_fn(q);

	return 0;
}
EXPORT_SYMBOL_GPL(blk_lld_busy);

int kblockd_schedule_work(struct request_queue *q, struct work_struct *work)
{
	return queue_work(kblockd_workqueue, work);
}
EXPORT_SYMBOL(kblockd_schedule_work);

int __init blk_dev_init(void)
{
	kblockd_workqueue = create_workqueue("kblockd");
	if (!kblockd_workqueue)
		panic("Failed to create kblockd\n");

	request_cachep = kmem_cache_create("blkdev_requests",
			sizeof(struct request), 0, SLAB_PANIC, NULL);

	blk_requestq_cachep = kmem_cache_create("blkdev_queue",
			sizeof(struct request_queue), 0, SLAB_PANIC, NULL);

	return 0;
}