/*

   * 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 hd_struct *part;

  	int rw = rq_data_dir(rq);

  	int cpu;

  	if (!blk_fs_request(rq) || !blk_do_io_stat(rq))

  		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
  ",

  			       __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
  ", msg,

  		rq->rq_disk ? rq->rq_disk->disk_name : "?", rq->cmd_type,
  		rq->cmd_flags);

  	printk(KERN_INFO "  sector %llu, nr/cnr %lu/%u
  ",
  						(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
  ",
  						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("
  ");
  	}
  }

  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);

  /**
   * 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_run_queue(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);

  	if (unlikely(blk_queue_stopped(q)))
  		return;
  
  	if (elv_queue_empty(q))
  		return;

  	/*
  	 * Only recurse once to avoid overrunning the stack, let the unplug
  	 * handling reinvoke the handler shortly if we already got there.
  	 */

  	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);
  	}

  }
  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.

   */
  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 flags, 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 = flags | 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++;

  	if (blk_queue_io_stat(q))
  		rw_flags |= REQ_IO_STAT;

  	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_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->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_run_queue(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;

  	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;

  	if (bio_sync(bio))

  		req->cmd_flags |= REQ_RW_SYNC;

  	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
  ");
  	printk(KERN_INFO "%s: rw=%ld, want=%Lu, limit=%Lu
  ",
  			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)
  ",
  				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)
  ",

  				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
  ",

  			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.
  ", __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.
  ", __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)
  {

  	if (!blk_do_io_stat(req))

  		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)
  {

  	if (!blk_do_io_stat(req))

  		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(req->rq_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();
  	}
  }

  /**

   * 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);

  struct request *elv_next_request(struct request_queue *q)
  {
  	struct request *rq;
  	int ret;
  
  	while ((rq = __elv_next_request(q)) != NULL) {
  		if (!(rq->cmd_flags & REQ_STARTED)) {
  			/*
  			 * This is the first time the device driver
  			 * sees this request (possibly after
  			 * requeueing).  Notify IO scheduler.
  			 */
  			if (blk_sorted_rq(rq))
  				elv_activate_rq(q, rq);
  
  			/*
  			 * just mark as started even if we don't start
  			 * it, a request that has been delayed should
  			 * not be passed by new incoming requests
  			 */
  			rq->cmd_flags |= REQ_STARTED;
  			trace_block_rq_issue(q, rq);
  		}
  
  		if (!q->boundary_rq || q->boundary_rq == rq) {
  			q->end_sector = rq_end_sector(rq);
  			q->boundary_rq = NULL;
  		}
  
  		if (rq->cmd_flags & REQ_DONTPREP)
  			break;
  
  		if (q->dma_drain_size && rq->data_len) {
  			/*
  			 * make sure space for the drain appears we
  			 * know we can do this because max_hw_segments
  			 * has been adjusted to be one fewer than the
  			 * device can handle
  			 */
  			rq->nr_phys_segments++;
  		}
  
  		if (!q->prep_rq_fn)
  			break;
  
  		ret = q->prep_rq_fn(q, rq);
  		if (ret == BLKPREP_OK) {
  			break;
  		} else if (ret == BLKPREP_DEFER) {
  			/*
  			 * the request may have been (partially) prepped.
  			 * we need to keep this request in the front to
  			 * avoid resource deadlock.  REQ_STARTED will
  			 * prevent other fs requests from passing this one.
  			 */
  			if (q->dma_drain_size && rq->data_len &&
  			    !(rq->cmd_flags & REQ_DONTPREP)) {
  				/*
  				 * remove the space for the drain we added
  				 * so that we don't add it again
  				 */
  				--rq->nr_phys_segments;
  			}
  
  			rq = NULL;
  			break;
  		} else if (ret == BLKPREP_KILL) {
  			rq->cmd_flags |= REQ_QUIET;
  			__blk_end_request(rq, -EIO, blk_rq_bytes(rq));
  		} else {
  			printk(KERN_ERR "%s: bad return=%d
  ", __func__, ret);
  			break;
  		}
  	}
  
  	return rq;
  }
  EXPORT_SYMBOL(elv_next_request);
  
  void elv_dequeue_request(struct request_queue *q, struct request *rq)
  {
  	BUG_ON(list_empty(&rq->queuelist));
  	BUG_ON(ELV_ON_HASH(rq));
  
  	list_del_init(&rq->queuelist);
  
  	/*
  	 * the time frame between a request being removed from the lists
  	 * and to it is freed is accounted as io that is in progress at
  	 * the driver side.
  	 */
  	if (blk_account_rq(rq))
  		q->in_flight++;
  }

  /**

   * __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 fs requests, rq is just carrier of independent bio's
  	 * and each partial completion should be handled separately.
  	 * Reset per-request error on each partial completion.
  	 *
  	 * TODO: tj: This is too subtle.  It would be better to let
  	 * low level drivers do what they see fit.

  	 */

  	if (blk_fs_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
  ",

  				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
  ",

  				       __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;
  }

  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;
  }

  /*
   * 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);