block: fix inconsistency in I/O stat accounting code

[deliverable/linux.git] / block / blk-core.c

/*
 * 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->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)
{
        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();
        }
}

/**
 * __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;
}