/*
   *  linux/fs/block_dev.c
   *
   *  Copyright (C) 1991, 1992  Linus Torvalds
   *  Copyright (C) 2001  Andrea Arcangeli <andrea@suse.de> SuSE
   */

  #include <linux/init.h>
  #include <linux/mm.h>
  #include <linux/fcntl.h>
  #include <linux/slab.h>
  #include <linux/kmod.h>
  #include <linux/major.h>

  #include <linux/device_cgroup.h>

  #include <linux/highmem.h>
  #include <linux/blkdev.h>
  #include <linux/module.h>
  #include <linux/blkpg.h>

  #include <linux/magic.h>

  #include <linux/buffer_head.h>

  #include <linux/swap.h>

  #include <linux/pagevec.h>

  #include <linux/writeback.h>

  #include <linux/mpage.h>
  #include <linux/mount.h>
  #include <linux/uio.h>
  #include <linux/namei.h>

  #include <linux/log2.h>

  #include <linux/cleancache.h>

  #include <linux/aio.h>

  #include <asm/uaccess.h>

  #include "internal.h"

  
  struct bdev_inode {
  	struct block_device bdev;
  	struct inode vfs_inode;
  };

  static const struct address_space_operations def_blk_aops;

  static inline struct bdev_inode *BDEV_I(struct inode *inode)
  {
  	return container_of(inode, struct bdev_inode, vfs_inode);
  }
  
  inline struct block_device *I_BDEV(struct inode *inode)
  {
  	return &BDEV_I(inode)->bdev;
  }

  EXPORT_SYMBOL(I_BDEV);

  /*

   * Move the inode from its current bdi to a new bdi. If the inode is dirty we
   * need to move it onto the dirty list of @dst so that the inode is always on
   * the right list.

   */
  static void bdev_inode_switch_bdi(struct inode *inode,
  			struct backing_dev_info *dst)
  {

  	struct backing_dev_info *old = inode->i_data.backing_dev_info;

  	bool wakeup_bdi = false;

  
  	if (unlikely(dst == old))		/* deadlock avoidance */
  		return;
  	bdi_lock_two(&old->wb, &dst->wb);

  	spin_lock(&inode->i_lock);

  	inode->i_data.backing_dev_info = dst;

  	if (inode->i_state & I_DIRTY) {
  		if (bdi_cap_writeback_dirty(dst) && !wb_has_dirty_io(&dst->wb))
  			wakeup_bdi = true;

  		list_move(&inode->i_wb_list, &dst->wb.b_dirty);

  	}

  	spin_unlock(&inode->i_lock);

  	spin_unlock(&old->wb.list_lock);
  	spin_unlock(&dst->wb.list_lock);

  
  	if (wakeup_bdi)
  		bdi_wakeup_thread_delayed(dst);

  }

  /* Kill _all_ buffers and pagecache , dirty or not.. */

  void kill_bdev(struct block_device *bdev)

  {

  	struct address_space *mapping = bdev->bd_inode->i_mapping;

  	if (mapping->nrpages == 0 && mapping->nrshadows == 0)

  		return;

  	invalidate_bh_lrus();

  	truncate_inode_pages(mapping, 0);

  }	

  EXPORT_SYMBOL(kill_bdev);
  
  /* Invalidate clean unused buffers and pagecache. */
  void invalidate_bdev(struct block_device *bdev)
  {
  	struct address_space *mapping = bdev->bd_inode->i_mapping;
  
  	if (mapping->nrpages == 0)
  		return;
  
  	invalidate_bh_lrus();
  	lru_add_drain_all();	/* make sure all lru add caches are flushed */
  	invalidate_mapping_pages(mapping, 0, -1);
  	/* 99% of the time, we don't need to flush the cleancache on the bdev.
  	 * But, for the strange corners, lets be cautious
  	 */

  	cleancache_invalidate_inode(mapping);

  }
  EXPORT_SYMBOL(invalidate_bdev);

  
  int set_blocksize(struct block_device *bdev, int size)
  {
  	/* Size must be a power of two, and between 512 and PAGE_SIZE */

  	if (size > PAGE_SIZE || size < 512 || !is_power_of_2(size))

  		return -EINVAL;
  
  	/* Size cannot be smaller than the size supported by the device */

  	if (size < bdev_logical_block_size(bdev))

  		return -EINVAL;
  
  	/* Don't change the size if it is same as current */
  	if (bdev->bd_block_size != size) {
  		sync_blockdev(bdev);
  		bdev->bd_block_size = size;
  		bdev->bd_inode->i_blkbits = blksize_bits(size);
  		kill_bdev(bdev);
  	}
  	return 0;
  }
  
  EXPORT_SYMBOL(set_blocksize);
  
  int sb_set_blocksize(struct super_block *sb, int size)
  {

  	if (set_blocksize(sb->s_bdev, size))
  		return 0;
  	/* If we get here, we know size is power of two
  	 * and it's value is between 512 and PAGE_SIZE */
  	sb->s_blocksize = size;

  	sb->s_blocksize_bits = blksize_bits(size);

  	return sb->s_blocksize;
  }
  
  EXPORT_SYMBOL(sb_set_blocksize);
  
  int sb_min_blocksize(struct super_block *sb, int size)
  {

  	int minsize = bdev_logical_block_size(sb->s_bdev);

  	if (size < minsize)
  		size = minsize;
  	return sb_set_blocksize(sb, size);
  }
  
  EXPORT_SYMBOL(sb_min_blocksize);
  
  static int
  blkdev_get_block(struct inode *inode, sector_t iblock,
  		struct buffer_head *bh, int create)
  {

  	bh->b_bdev = I_BDEV(inode);
  	bh->b_blocknr = iblock;
  	set_buffer_mapped(bh);
  	return 0;
  }

  static ssize_t
  blkdev_direct_IO(int rw, struct kiocb *iocb, const struct iovec *iov,
  			loff_t offset, unsigned long nr_segs)
  {
  	struct file *file = iocb->ki_filp;
  	struct inode *inode = file->f_mapping->host;

  	return __blockdev_direct_IO(rw, iocb, inode, I_BDEV(inode), iov, offset,

  				    nr_segs, blkdev_get_block, NULL, NULL, 0);

  }

  int __sync_blockdev(struct block_device *bdev, int wait)
  {
  	if (!bdev)
  		return 0;
  	if (!wait)
  		return filemap_flush(bdev->bd_inode->i_mapping);
  	return filemap_write_and_wait(bdev->bd_inode->i_mapping);
  }

  /*
   * Write out and wait upon all the dirty data associated with a block
   * device via its mapping.  Does not take the superblock lock.
   */
  int sync_blockdev(struct block_device *bdev)
  {

  	return __sync_blockdev(bdev, 1);

  }
  EXPORT_SYMBOL(sync_blockdev);
  
  /*
   * Write out and wait upon all dirty data associated with this
   * device.   Filesystem data as well as the underlying block
   * device.  Takes the superblock lock.
   */
  int fsync_bdev(struct block_device *bdev)
  {
  	struct super_block *sb = get_super(bdev);
  	if (sb) {

  		int res = sync_filesystem(sb);

  		drop_super(sb);
  		return res;
  	}
  	return sync_blockdev(bdev);
  }

  EXPORT_SYMBOL(fsync_bdev);

  
  /**
   * freeze_bdev  --  lock a filesystem and force it into a consistent state
   * @bdev:	blockdevice to lock
   *

   * If a superblock is found on this device, we take the s_umount semaphore
   * on it to make sure nobody unmounts until the snapshot creation is done.
   * The reference counter (bd_fsfreeze_count) guarantees that only the last
   * unfreeze process can unfreeze the frozen filesystem actually when multiple
   * freeze requests arrive simultaneously. It counts up in freeze_bdev() and
   * count down in thaw_bdev(). When it becomes 0, thaw_bdev() will unfreeze
   * actually.
   */
  struct super_block *freeze_bdev(struct block_device *bdev)
  {
  	struct super_block *sb;
  	int error = 0;
  
  	mutex_lock(&bdev->bd_fsfreeze_mutex);

  	if (++bdev->bd_fsfreeze_count > 1) {
  		/*
  		 * We don't even need to grab a reference - the first call
  		 * to freeze_bdev grab an active reference and only the last
  		 * thaw_bdev drops it.
  		 */

  		sb = get_super(bdev);

  		drop_super(sb);
  		mutex_unlock(&bdev->bd_fsfreeze_mutex);
  		return sb;
  	}
  
  	sb = get_active_super(bdev);
  	if (!sb)
  		goto out;

  	error = freeze_super(sb);
  	if (error) {
  		deactivate_super(sb);
  		bdev->bd_fsfreeze_count--;

  		mutex_unlock(&bdev->bd_fsfreeze_mutex);

  		return ERR_PTR(error);

  	}

  	deactivate_super(sb);

   out:

  	sync_blockdev(bdev);
  	mutex_unlock(&bdev->bd_fsfreeze_mutex);

  	return sb;	/* thaw_bdev releases s->s_umount */

  }
  EXPORT_SYMBOL(freeze_bdev);
  
  /**
   * thaw_bdev  -- unlock filesystem
   * @bdev:	blockdevice to unlock
   * @sb:		associated superblock
   *
   * Unlocks the filesystem and marks it writeable again after freeze_bdev().
   */
  int thaw_bdev(struct block_device *bdev, struct super_block *sb)
  {

  	int error = -EINVAL;

  
  	mutex_lock(&bdev->bd_fsfreeze_mutex);

  	if (!bdev->bd_fsfreeze_count)

  		goto out;

  
  	error = 0;
  	if (--bdev->bd_fsfreeze_count > 0)

  		goto out;

  
  	if (!sb)

  		goto out;

  	error = thaw_super(sb);
  	if (error) {
  		bdev->bd_fsfreeze_count++;
  		mutex_unlock(&bdev->bd_fsfreeze_mutex);
  		return error;
  	}
  out:

  	mutex_unlock(&bdev->bd_fsfreeze_mutex);
  	return 0;
  }
  EXPORT_SYMBOL(thaw_bdev);

  static int blkdev_writepage(struct page *page, struct writeback_control *wbc)
  {
  	return block_write_full_page(page, blkdev_get_block, wbc);
  }
  
  static int blkdev_readpage(struct file * file, struct page * page)
  {
  	return block_read_full_page(page, blkdev_get_block);
  }

  static int blkdev_write_begin(struct file *file, struct address_space *mapping,
  			loff_t pos, unsigned len, unsigned flags,
  			struct page **pagep, void **fsdata)

  {

  	return block_write_begin(mapping, pos, len, flags, pagep,
  				 blkdev_get_block);

  }

  static int blkdev_write_end(struct file *file, struct address_space *mapping,
  			loff_t pos, unsigned len, unsigned copied,
  			struct page *page, void *fsdata)

  {

  	int ret;
  	ret = block_write_end(file, mapping, pos, len, copied, page, fsdata);
  
  	unlock_page(page);
  	page_cache_release(page);
  
  	return ret;

  }
  
  /*
   * private llseek:

   * for a block special file file_inode(file)->i_size is zero

   * so we compute the size by hand (just as in block_read/write above)
   */

  static loff_t block_llseek(struct file *file, loff_t offset, int whence)

  {
  	struct inode *bd_inode = file->f_mapping->host;

  	loff_t retval;

  	mutex_lock(&bd_inode->i_mutex);

  	retval = fixed_size_llseek(file, offset, whence, i_size_read(bd_inode));

  	mutex_unlock(&bd_inode->i_mutex);

  	return retval;
  }
  	

  int blkdev_fsync(struct file *filp, loff_t start, loff_t end, int datasync)

  {

  	struct inode *bd_inode = filp->f_mapping->host;
  	struct block_device *bdev = I_BDEV(bd_inode);

  	int error;

  	
  	error = filemap_write_and_wait_range(filp->f_mapping, start, end);
  	if (error)
  		return error;

  	/*
  	 * There is no need to serialise calls to blkdev_issue_flush with
  	 * i_mutex and doing so causes performance issues with concurrent
  	 * O_SYNC writers to a block device.
  	 */

  	error = blkdev_issue_flush(bdev, GFP_KERNEL, NULL);

  	if (error == -EOPNOTSUPP)
  		error = 0;

  	return error;

  }

  EXPORT_SYMBOL(blkdev_fsync);

  
  /*
   * pseudo-fs
   */
  
  static  __cacheline_aligned_in_smp DEFINE_SPINLOCK(bdev_lock);

  static struct kmem_cache * bdev_cachep __read_mostly;

  
  static struct inode *bdev_alloc_inode(struct super_block *sb)
  {

  	struct bdev_inode *ei = kmem_cache_alloc(bdev_cachep, GFP_KERNEL);

  	if (!ei)
  		return NULL;
  	return &ei->vfs_inode;
  }

  static void bdev_i_callback(struct rcu_head *head)

  {

  	struct inode *inode = container_of(head, struct inode, i_rcu);

  	struct bdev_inode *bdi = BDEV_I(inode);

  	kmem_cache_free(bdev_cachep, bdi);
  }

  static void bdev_destroy_inode(struct inode *inode)
  {
  	call_rcu(&inode->i_rcu, bdev_i_callback);
  }

  static void init_once(void *foo)

  {
  	struct bdev_inode *ei = (struct bdev_inode *) foo;
  	struct block_device *bdev = &ei->bdev;

  	memset(bdev, 0, sizeof(*bdev));
  	mutex_init(&bdev->bd_mutex);

  	INIT_LIST_HEAD(&bdev->bd_inodes);
  	INIT_LIST_HEAD(&bdev->bd_list);

  #ifdef CONFIG_SYSFS
  	INIT_LIST_HEAD(&bdev->bd_holder_disks);
  #endif

  	inode_init_once(&ei->vfs_inode);

  	/* Initialize mutex for freeze. */
  	mutex_init(&bdev->bd_fsfreeze_mutex);

  }
  
  static inline void __bd_forget(struct inode *inode)
  {
  	list_del_init(&inode->i_devices);
  	inode->i_bdev = NULL;
  	inode->i_mapping = &inode->i_data;
  }

  static void bdev_evict_inode(struct inode *inode)

  {
  	struct block_device *bdev = &BDEV_I(inode)->bdev;
  	struct list_head *p;

  	truncate_inode_pages_final(&inode->i_data);

  	invalidate_inode_buffers(inode); /* is it needed here? */

  	clear_inode(inode);

  	spin_lock(&bdev_lock);
  	while ( (p = bdev->bd_inodes.next) != &bdev->bd_inodes ) {
  		__bd_forget(list_entry(p, struct inode, i_devices));
  	}
  	list_del_init(&bdev->bd_list);
  	spin_unlock(&bdev_lock);
  }

  static const struct super_operations bdev_sops = {

  	.statfs = simple_statfs,
  	.alloc_inode = bdev_alloc_inode,
  	.destroy_inode = bdev_destroy_inode,
  	.drop_inode = generic_delete_inode,

  	.evict_inode = bdev_evict_inode,

  };

  static struct dentry *bd_mount(struct file_system_type *fs_type,
  	int flags, const char *dev_name, void *data)

  {

  	return mount_pseudo(fs_type, "bdev:", &bdev_sops, NULL, BDEVFS_MAGIC);

  }
  
  static struct file_system_type bd_type = {
  	.name		= "bdev",

  	.mount		= bd_mount,

  	.kill_sb	= kill_anon_super,
  };

  static struct super_block *blockdev_superblock __read_mostly;

  
  void __init bdev_cache_init(void)
  {
  	int err;

  	static struct vfsmount *bd_mnt;

  	bdev_cachep = kmem_cache_create("bdev_cache", sizeof(struct bdev_inode),

  			0, (SLAB_HWCACHE_ALIGN|SLAB_RECLAIM_ACCOUNT|
  				SLAB_MEM_SPREAD|SLAB_PANIC),

  			init_once);

  	err = register_filesystem(&bd_type);
  	if (err)
  		panic("Cannot register bdev pseudo-fs");
  	bd_mnt = kern_mount(&bd_type);

  	if (IS_ERR(bd_mnt))
  		panic("Cannot create bdev pseudo-fs");

  	blockdev_superblock = bd_mnt->mnt_sb;   /* For writeback */

  }
  
  /*
   * Most likely _very_ bad one - but then it's hardly critical for small
   * /dev and can be fixed when somebody will need really large one.
   * Keep in mind that it will be fed through icache hash function too.
   */
  static inline unsigned long hash(dev_t dev)
  {
  	return MAJOR(dev)+MINOR(dev);
  }
  
  static int bdev_test(struct inode *inode, void *data)
  {
  	return BDEV_I(inode)->bdev.bd_dev == *(dev_t *)data;
  }
  
  static int bdev_set(struct inode *inode, void *data)
  {
  	BDEV_I(inode)->bdev.bd_dev = *(dev_t *)data;
  	return 0;
  }
  
  static LIST_HEAD(all_bdevs);
  
  struct block_device *bdget(dev_t dev)
  {
  	struct block_device *bdev;
  	struct inode *inode;

  	inode = iget5_locked(blockdev_superblock, hash(dev),

  			bdev_test, bdev_set, &dev);
  
  	if (!inode)
  		return NULL;
  
  	bdev = &BDEV_I(inode)->bdev;
  
  	if (inode->i_state & I_NEW) {
  		bdev->bd_contains = NULL;

  		bdev->bd_super = NULL;

  		bdev->bd_inode = inode;
  		bdev->bd_block_size = (1 << inode->i_blkbits);
  		bdev->bd_part_count = 0;
  		bdev->bd_invalidated = 0;
  		inode->i_mode = S_IFBLK;
  		inode->i_rdev = dev;
  		inode->i_bdev = bdev;
  		inode->i_data.a_ops = &def_blk_aops;
  		mapping_set_gfp_mask(&inode->i_data, GFP_USER);
  		inode->i_data.backing_dev_info = &default_backing_dev_info;
  		spin_lock(&bdev_lock);
  		list_add(&bdev->bd_list, &all_bdevs);
  		spin_unlock(&bdev_lock);
  		unlock_new_inode(inode);
  	}
  	return bdev;
  }
  
  EXPORT_SYMBOL(bdget);

  /**
   * bdgrab -- Grab a reference to an already referenced block device
   * @bdev:	Block device to grab a reference to.
   */
  struct block_device *bdgrab(struct block_device *bdev)
  {

  	ihold(bdev->bd_inode);

  	return bdev;
  }

  EXPORT_SYMBOL(bdgrab);

  long nr_blockdev_pages(void)
  {

  	struct block_device *bdev;

  	long ret = 0;
  	spin_lock(&bdev_lock);

  	list_for_each_entry(bdev, &all_bdevs, bd_list) {

  		ret += bdev->bd_inode->i_mapping->nrpages;
  	}
  	spin_unlock(&bdev_lock);
  	return ret;
  }
  
  void bdput(struct block_device *bdev)
  {
  	iput(bdev->bd_inode);
  }
  
  EXPORT_SYMBOL(bdput);
   
  static struct block_device *bd_acquire(struct inode *inode)
  {
  	struct block_device *bdev;

  	spin_lock(&bdev_lock);
  	bdev = inode->i_bdev;

  	if (bdev) {

  		ihold(bdev->bd_inode);

  		spin_unlock(&bdev_lock);
  		return bdev;
  	}
  	spin_unlock(&bdev_lock);

  	bdev = bdget(inode->i_rdev);
  	if (bdev) {
  		spin_lock(&bdev_lock);

  		if (!inode->i_bdev) {
  			/*

  			 * We take an additional reference to bd_inode,

  			 * and it's released in clear_inode() of inode.
  			 * So, we can access it via ->i_mapping always
  			 * without igrab().
  			 */

  			ihold(bdev->bd_inode);

  			inode->i_bdev = bdev;
  			inode->i_mapping = bdev->bd_inode->i_mapping;
  			list_add(&inode->i_devices, &bdev->bd_inodes);
  		}

  		spin_unlock(&bdev_lock);
  	}
  	return bdev;
  }

  int sb_is_blkdev_sb(struct super_block *sb)

  {
  	return sb == blockdev_superblock;
  }

  /* Call when you free inode */
  
  void bd_forget(struct inode *inode)
  {

  	struct block_device *bdev = NULL;

  	spin_lock(&bdev_lock);

  	if (!sb_is_blkdev_sb(inode->i_sb))
  		bdev = inode->i_bdev;
  	__bd_forget(inode);

  	spin_unlock(&bdev_lock);

  
  	if (bdev)
  		iput(bdev->bd_inode);

  }

  /**
   * bd_may_claim - test whether a block device can be claimed
   * @bdev: block device of interest
   * @whole: whole block device containing @bdev, may equal @bdev
   * @holder: holder trying to claim @bdev
   *

   * Test whether @bdev can be claimed by @holder.

   *
   * CONTEXT:
   * spin_lock(&bdev_lock).
   *
   * RETURNS:
   * %true if @bdev can be claimed, %false otherwise.
   */
  static bool bd_may_claim(struct block_device *bdev, struct block_device *whole,
  			 void *holder)

  {

  	if (bdev->bd_holder == holder)

  		return true;	 /* already a holder */

  	else if (bdev->bd_holder != NULL)

  		return false; 	 /* held by someone else */

  	else if (bdev->bd_contains == bdev)

  		return true;  	 /* is a whole device which isn't held */

  	else if (whole->bd_holder == bd_may_claim)

  		return true; 	 /* is a partition of a device that is being partitioned */
  	else if (whole->bd_holder != NULL)
  		return false;	 /* is a partition of a held device */

  	else

  		return true;	 /* is a partition of an un-held device */
  }
  
  /**

   * bd_prepare_to_claim - prepare to claim a block device
   * @bdev: block device of interest
   * @whole: the whole device containing @bdev, may equal @bdev
   * @holder: holder trying to claim @bdev
   *
   * Prepare to claim @bdev.  This function fails if @bdev is already
   * claimed by another holder and waits if another claiming is in
   * progress.  This function doesn't actually claim.  On successful
   * return, the caller has ownership of bd_claiming and bd_holder[s].
   *
   * CONTEXT:
   * spin_lock(&bdev_lock).  Might release bdev_lock, sleep and regrab
   * it multiple times.
   *
   * RETURNS:
   * 0 if @bdev can be claimed, -EBUSY otherwise.
   */
  static int bd_prepare_to_claim(struct block_device *bdev,
  			       struct block_device *whole, void *holder)
  {
  retry:
  	/* if someone else claimed, fail */
  	if (!bd_may_claim(bdev, whole, holder))
  		return -EBUSY;

  	/* if claiming is already in progress, wait for it to finish */
  	if (whole->bd_claiming) {

  		wait_queue_head_t *wq = bit_waitqueue(&whole->bd_claiming, 0);
  		DEFINE_WAIT(wait);
  
  		prepare_to_wait(wq, &wait, TASK_UNINTERRUPTIBLE);
  		spin_unlock(&bdev_lock);
  		schedule();
  		finish_wait(wq, &wait);
  		spin_lock(&bdev_lock);
  		goto retry;
  	}
  
  	/* yay, all mine */
  	return 0;
  }
  
  /**
   * bd_start_claiming - start claiming a block device
   * @bdev: block device of interest
   * @holder: holder trying to claim @bdev
   *
   * @bdev is about to be opened exclusively.  Check @bdev can be opened
   * exclusively and mark that an exclusive open is in progress.  Each
   * successful call to this function must be matched with a call to

   * either bd_finish_claiming() or bd_abort_claiming() (which do not
   * fail).
   *
   * This function is used to gain exclusive access to the block device
   * without actually causing other exclusive open attempts to fail. It
   * should be used when the open sequence itself requires exclusive
   * access but may subsequently fail.

   *
   * CONTEXT:
   * Might sleep.
   *
   * RETURNS:
   * Pointer to the block device containing @bdev on success, ERR_PTR()
   * value on failure.
   */
  static struct block_device *bd_start_claiming(struct block_device *bdev,
  					      void *holder)
  {
  	struct gendisk *disk;
  	struct block_device *whole;
  	int partno, err;
  
  	might_sleep();
  
  	/*
  	 * @bdev might not have been initialized properly yet, look up
  	 * and grab the outer block device the hard way.
  	 */
  	disk = get_gendisk(bdev->bd_dev, &partno);
  	if (!disk)
  		return ERR_PTR(-ENXIO);

  	/*
  	 * Normally, @bdev should equal what's returned from bdget_disk()
  	 * if partno is 0; however, some drivers (floppy) use multiple
  	 * bdev's for the same physical device and @bdev may be one of the
  	 * aliases.  Keep @bdev if partno is 0.  This means claimer
  	 * tracking is broken for those devices but it has always been that
  	 * way.
  	 */
  	if (partno)
  		whole = bdget_disk(disk, 0);
  	else
  		whole = bdgrab(bdev);

  	module_put(disk->fops->owner);

  	put_disk(disk);
  	if (!whole)
  		return ERR_PTR(-ENOMEM);
  
  	/* prepare to claim, if successful, mark claiming in progress */
  	spin_lock(&bdev_lock);
  
  	err = bd_prepare_to_claim(bdev, whole, holder);
  	if (err == 0) {
  		whole->bd_claiming = holder;
  		spin_unlock(&bdev_lock);
  		return whole;
  	} else {
  		spin_unlock(&bdev_lock);
  		bdput(whole);
  		return ERR_PTR(err);
  	}
  }

  #ifdef CONFIG_SYSFS

  struct bd_holder_disk {
  	struct list_head	list;
  	struct gendisk		*disk;
  	int			refcnt;
  };
  
  static struct bd_holder_disk *bd_find_holder_disk(struct block_device *bdev,
  						  struct gendisk *disk)
  {
  	struct bd_holder_disk *holder;
  
  	list_for_each_entry(holder, &bdev->bd_holder_disks, list)
  		if (holder->disk == disk)
  			return holder;
  	return NULL;
  }

  static int add_symlink(struct kobject *from, struct kobject *to)

  {

  	return sysfs_create_link(from, to, kobject_name(to));

  }
  
  static void del_symlink(struct kobject *from, struct kobject *to)
  {

  	sysfs_remove_link(from, kobject_name(to));
  }

  /**

   * bd_link_disk_holder - create symlinks between holding disk and slave bdev
   * @bdev: the claimed slave bdev
   * @disk: the holding disk

   *

   * DON'T USE THIS UNLESS YOU'RE ALREADY USING IT.
   *

   * This functions creates the following sysfs symlinks.

   *

   * - from "slaves" directory of the holder @disk to the claimed @bdev
   * - from "holders" directory of the @bdev to the holder @disk

   *

   * For example, if /dev/dm-0 maps to /dev/sda and disk for dm-0 is
   * passed to bd_link_disk_holder(), then:

   *

   *   /sys/block/dm-0/slaves/sda --> /sys/block/sda
   *   /sys/block/sda/holders/dm-0 --> /sys/block/dm-0

   *

   * The caller must have claimed @bdev before calling this function and
   * ensure that both @bdev and @disk are valid during the creation and
   * lifetime of these symlinks.

   *

   * CONTEXT:
   * Might sleep.

   *

   * RETURNS:
   * 0 on success, -errno on failure.

   */

  int bd_link_disk_holder(struct block_device *bdev, struct gendisk *disk)

  {

  	struct bd_holder_disk *holder;

  	int ret = 0;

  	mutex_lock(&bdev->bd_mutex);

  	WARN_ON_ONCE(!bdev->bd_holder);

  	/* FIXME: remove the following once add_disk() handles errors */
  	if (WARN_ON(!disk->slave_dir || !bdev->bd_part->holder_dir))
  		goto out_unlock;

  	holder = bd_find_holder_disk(bdev, disk);
  	if (holder) {
  		holder->refcnt++;

  		goto out_unlock;

  	}

  	holder = kzalloc(sizeof(*holder), GFP_KERNEL);
  	if (!holder) {
  		ret = -ENOMEM;

  		goto out_unlock;
  	}

  	INIT_LIST_HEAD(&holder->list);
  	holder->disk = disk;
  	holder->refcnt = 1;
  
  	ret = add_symlink(disk->slave_dir, &part_to_dev(bdev->bd_part)->kobj);
  	if (ret)
  		goto out_free;
  
  	ret = add_symlink(bdev->bd_part->holder_dir, &disk_to_dev(disk)->kobj);
  	if (ret)
  		goto out_del;

  	/*
  	 * bdev could be deleted beneath us which would implicitly destroy
  	 * the holder directory.  Hold on to it.
  	 */
  	kobject_get(bdev->bd_part->holder_dir);

  
  	list_add(&holder->list, &bdev->bd_holder_disks);
  	goto out_unlock;
  
  out_del:
  	del_symlink(disk->slave_dir, &part_to_dev(bdev->bd_part)->kobj);
  out_free:
  	kfree(holder);

  out_unlock:

  	mutex_unlock(&bdev->bd_mutex);

  	return ret;

  }

  EXPORT_SYMBOL_GPL(bd_link_disk_holder);

  /**
   * bd_unlink_disk_holder - destroy symlinks created by bd_link_disk_holder()
   * @bdev: the calimed slave bdev
   * @disk: the holding disk
   *
   * DON'T USE THIS UNLESS YOU'RE ALREADY USING IT.
   *
   * CONTEXT:
   * Might sleep.
   */
  void bd_unlink_disk_holder(struct block_device *bdev, struct gendisk *disk)

  {

  	struct bd_holder_disk *holder;

  	mutex_lock(&bdev->bd_mutex);

  	holder = bd_find_holder_disk(bdev, disk);
  
  	if (!WARN_ON_ONCE(holder == NULL) && !--holder->refcnt) {
  		del_symlink(disk->slave_dir, &part_to_dev(bdev->bd_part)->kobj);
  		del_symlink(bdev->bd_part->holder_dir,
  			    &disk_to_dev(disk)->kobj);

  		kobject_put(bdev->bd_part->holder_dir);

  		list_del_init(&holder->list);
  		kfree(holder);
  	}
  
  	mutex_unlock(&bdev->bd_mutex);

  }

  EXPORT_SYMBOL_GPL(bd_unlink_disk_holder);

  #endif

  /**

   * flush_disk - invalidates all buffer-cache entries on a disk
   *
   * @bdev:      struct block device to be flushed

   * @kill_dirty: flag to guide handling of dirty inodes

   *
   * Invalidates all buffer-cache entries on a disk. It should be called
   * when a disk has been changed -- either by a media change or online
   * resize.
   */

  static void flush_disk(struct block_device *bdev, bool kill_dirty)

  {

  	if (__invalidate_device(bdev, kill_dirty)) {

  		char name[BDEVNAME_SIZE] = "";
  
  		if (bdev->bd_disk)
  			disk_name(bdev->bd_disk, 0, name);
  		printk(KERN_WARNING "VFS: busy inodes on changed media or "
  		       "resized disk %s
  ", name);
  	}
  
  	if (!bdev->bd_disk)
  		return;

  	if (disk_part_scan_enabled(bdev->bd_disk))

  		bdev->bd_invalidated = 1;
  }
  
  /**

   * check_disk_size_change - checks for disk size change and adjusts bdev size.

   * @disk: struct gendisk to check
   * @bdev: struct bdev to adjust.
   *
   * This routine checks to see if the bdev size does not match the disk size
   * and adjusts it if it differs.
   */
  void check_disk_size_change(struct gendisk *disk, struct block_device *bdev)
  {
  	loff_t disk_size, bdev_size;
  
  	disk_size = (loff_t)get_capacity(disk) << 9;
  	bdev_size = i_size_read(bdev->bd_inode);
  	if (disk_size != bdev_size) {
  		char name[BDEVNAME_SIZE];
  
  		disk_name(disk, 0, name);
  		printk(KERN_INFO
  		       "%s: detected capacity change from %lld to %lld
  ",
  		       name, bdev_size, disk_size);
  		i_size_write(bdev->bd_inode, disk_size);

  		flush_disk(bdev, false);

  	}
  }
  EXPORT_SYMBOL(check_disk_size_change);
  
  /**

   * revalidate_disk - wrapper for lower-level driver's revalidate_disk call-back

   * @disk: struct gendisk to be revalidated
   *
   * This routine is a wrapper for lower-level driver's revalidate_disk
   * call-backs.  It is used to do common pre and post operations needed
   * for all revalidate_disk operations.
   */
  int revalidate_disk(struct gendisk *disk)
  {

  	struct block_device *bdev;

  	int ret = 0;
  
  	if (disk->fops->revalidate_disk)
  		ret = disk->fops->revalidate_disk(disk);

  	bdev = bdget_disk(disk, 0);
  	if (!bdev)
  		return ret;
  
  	mutex_lock(&bdev->bd_mutex);
  	check_disk_size_change(disk, bdev);

  	bdev->bd_invalidated = 0;

  	mutex_unlock(&bdev->bd_mutex);
  	bdput(bdev);

  	return ret;
  }
  EXPORT_SYMBOL(revalidate_disk);

  /*
   * This routine checks whether a removable media has been changed,
   * and invalidates all buffer-cache-entries in that case. This
   * is a relatively slow routine, so we have to try to minimize using
   * it. Thus it is called only upon a 'mount' or 'open'. This
   * is the best way of combining speed and utility, I think.
   * People changing diskettes in the middle of an operation deserve
   * to lose :-)
   */
  int check_disk_change(struct block_device *bdev)
  {
  	struct gendisk *disk = bdev->bd_disk;

  	const struct block_device_operations *bdops = disk->fops;

  	unsigned int events;

  	events = disk_clear_events(disk, DISK_EVENT_MEDIA_CHANGE |
  				   DISK_EVENT_EJECT_REQUEST);
  	if (!(events & DISK_EVENT_MEDIA_CHANGE))

  		return 0;

  	flush_disk(bdev, true);

  	if (bdops->revalidate_disk)
  		bdops->revalidate_disk(bdev->bd_disk);

  	return 1;
  }
  
  EXPORT_SYMBOL(check_disk_change);
  
  void bd_set_size(struct block_device *bdev, loff_t size)
  {

  	unsigned bsize = bdev_logical_block_size(bdev);

  	mutex_lock(&bdev->bd_inode->i_mutex);
  	i_size_write(bdev->bd_inode, size);
  	mutex_unlock(&bdev->bd_inode->i_mutex);

  	while (bsize < PAGE_CACHE_SIZE) {
  		if (size & bsize)
  			break;
  		bsize <<= 1;
  	}
  	bdev->bd_block_size = bsize;
  	bdev->bd_inode->i_blkbits = blksize_bits(bsize);
  }
  EXPORT_SYMBOL(bd_set_size);

  static void __blkdev_put(struct block_device *bdev, fmode_t mode, int for_part);

  /*
   * bd_mutex locking:
   *
   *  mutex_lock(part->bd_mutex)
   *    mutex_lock_nested(whole->bd_mutex, 1)
   */

  static int __blkdev_get(struct block_device *bdev, fmode_t mode, int for_part)

  {

  	struct gendisk *disk;

  	struct module *owner;

  	int ret;

  	int partno;

  	int perm = 0;

  	if (mode & FMODE_READ)

  		perm |= MAY_READ;

  	if (mode & FMODE_WRITE)

  		perm |= MAY_WRITE;
  	/*
  	 * hooks: /n/, see "layering violations".
  	 */

  	if (!for_part) {
  		ret = devcgroup_inode_permission(bdev->bd_inode, perm);
  		if (ret != 0) {
  			bdput(bdev);
  			return ret;
  		}

  	}

   restart:

  	ret = -ENXIO;

  	disk = get_gendisk(bdev->bd_dev, &partno);

  	if (!disk)

  		goto out;

  	owner = disk->fops->owner;

  	disk_block_events(disk);

  	mutex_lock_nested(&bdev->bd_mutex, for_part);

  	if (!bdev->bd_openers) {
  		bdev->bd_disk = disk;

  		bdev->bd_queue = disk->queue;

  		bdev->bd_contains = bdev;

  		if (!partno) {

  			struct backing_dev_info *bdi;

  
  			ret = -ENXIO;
  			bdev->bd_part = disk_get_part(disk, partno);
  			if (!bdev->bd_part)
  				goto out_clear;

  			ret = 0;

  			if (disk->fops->open) {

  				ret = disk->fops->open(bdev, mode);

  				if (ret == -ERESTARTSYS) {
  					/* Lost a race with 'disk' being
  					 * deleted, try again.
  					 * See md.c
  					 */
  					disk_put_part(bdev->bd_part);
  					bdev->bd_part = NULL;

  					bdev->bd_disk = NULL;

  					bdev->bd_queue = NULL;

  					mutex_unlock(&bdev->bd_mutex);

  					disk_unblock_events(disk);

  					put_disk(disk);

  					module_put(owner);

  					goto restart;
  				}

  			}

  			if (!ret) {

  				bd_set_size(bdev,(loff_t)get_capacity(disk)<<9);
  				bdi = blk_get_backing_dev_info(bdev);
  				if (bdi == NULL)
  					bdi = &default_backing_dev_info;
  				bdev_inode_switch_bdi(bdev->bd_inode, bdi);
  			}

  			/*
  			 * If the device is invalidated, rescan partition
  			 * if open succeeded or failed with -ENOMEDIUM.
  			 * The latter is necessary to prevent ghost
  			 * partitions on a removed medium.
  			 */

  			if (bdev->bd_invalidated) {
  				if (!ret)
  					rescan_partitions(disk, bdev);
  				else if (ret == -ENOMEDIUM)
  					invalidate_partitions(disk, bdev);
  			}

  			if (ret)
  				goto out_clear;

  		} else {

  			struct block_device *whole;
  			whole = bdget_disk(disk, 0);
  			ret = -ENOMEM;
  			if (!whole)

  				goto out_clear;

  			BUG_ON(for_part);

  			ret = __blkdev_get(whole, mode, 1);

  			if (ret)

  				goto out_clear;

  			bdev->bd_contains = whole;

  			bdev_inode_switch_bdi(bdev->bd_inode,
  				whole->bd_inode->i_data.backing_dev_info);

  			bdev->bd_part = disk_get_part(disk, partno);

  			if (!(disk->flags & GENHD_FL_UP) ||

  			    !bdev->bd_part || !bdev->bd_part->nr_sects) {

  				ret = -ENXIO;

  				goto out_clear;

  			}

  			bd_set_size(bdev, (loff_t)bdev->bd_part->nr_sects << 9);

  		}
  	} else {

  		if (bdev->bd_contains == bdev) {

  			ret = 0;
  			if (bdev->bd_disk->fops->open)

  				ret = bdev->bd_disk->fops->open(bdev, mode);

  			/* the same as first opener case, read comment there */

  			if (bdev->bd_invalidated) {
  				if (!ret)
  					rescan_partitions(bdev->bd_disk, bdev);
  				else if (ret == -ENOMEDIUM)
  					invalidate_partitions(bdev->bd_disk, bdev);
  			}

  			if (ret)
  				goto out_unlock_bdev;

  		}

  		/* only one opener holds refs to the module and disk */

  		put_disk(disk);

  		module_put(owner);

  	}
  	bdev->bd_openers++;

  	if (for_part)
  		bdev->bd_part_count++;

  	mutex_unlock(&bdev->bd_mutex);

  	disk_unblock_events(disk);

  	return 0;

   out_clear:

  	disk_put_part(bdev->bd_part);

  	bdev->bd_disk = NULL;

  	bdev->bd_part = NULL;

  	bdev->bd_queue = NULL;

  	bdev_inode_switch_bdi(bdev->bd_inode, &default_backing_dev_info);

  	if (bdev != bdev->bd_contains)

  		__blkdev_put(bdev->bd_contains, mode, 1);

  	bdev->bd_contains = NULL;

   out_unlock_bdev:

  	mutex_unlock(&bdev->bd_mutex);

  	disk_unblock_events(disk);

  	put_disk(disk);

  	module_put(owner);

   out:

  	bdput(bdev);

  	return ret;
  }

  /**
   * blkdev_get - open a block device
   * @bdev: block_device to open
   * @mode: FMODE_* mask
   * @holder: exclusive holder identifier
   *
   * Open @bdev with @mode.  If @mode includes %FMODE_EXCL, @bdev is
   * open with exclusive access.  Specifying %FMODE_EXCL with %NULL
   * @holder is invalid.  Exclusive opens may nest for the same @holder.
   *
   * On success, the reference count of @bdev is unchanged.  On failure,
   * @bdev is put.
   *
   * CONTEXT:
   * Might sleep.
   *
   * RETURNS:
   * 0 on success, -errno on failure.
   */

  int blkdev_get(struct block_device *bdev, fmode_t mode, void *holder)

  {

  	struct block_device *whole = NULL;
  	int res;
  
  	WARN_ON_ONCE((mode & FMODE_EXCL) && !holder);
  
  	if ((mode & FMODE_EXCL) && holder) {
  		whole = bd_start_claiming(bdev, holder);
  		if (IS_ERR(whole)) {
  			bdput(bdev);
  			return PTR_ERR(whole);
  		}
  	}
  
  	res = __blkdev_get(bdev, mode, 0);
  
  	if (whole) {

  		struct gendisk *disk = whole->bd_disk;

  		/* finish claiming */

  		mutex_lock(&bdev->bd_mutex);

  		spin_lock(&bdev_lock);

  		if (!res) {

  			BUG_ON(!bd_may_claim(bdev, whole, holder));
  			/*
  			 * Note that for a whole device bd_holders
  			 * will be incremented twice, and bd_holder
  			 * will be set to bd_may_claim before being
  			 * set to holder
  			 */
  			whole->bd_holders++;
  			whole->bd_holder = bd_may_claim;
  			bdev->bd_holders++;
  			bdev->bd_holder = holder;
  		}
  
  		/* tell others that we're done */
  		BUG_ON(whole->bd_claiming != holder);
  		whole->bd_claiming = NULL;
  		wake_up_bit(&whole->bd_claiming, 0);
  
  		spin_unlock(&bdev_lock);

  
  		/*

  		 * Block event polling for write claims if requested.  Any
  		 * write holder makes the write_holder state stick until
  		 * all are released.  This is good enough and tracking
  		 * individual writeable reference is too fragile given the
  		 * way @mode is used in blkdev_get/put().

  		 */

  		if (!res && (mode & FMODE_WRITE) && !bdev->bd_write_holder &&
  		    (disk->flags & GENHD_FL_BLOCK_EVENTS_ON_EXCL_WRITE)) {

  			bdev->bd_write_holder = true;

  			disk_block_events(disk);

  		}
  
  		mutex_unlock(&bdev->bd_mutex);

  		bdput(whole);

  	}
  
  	return res;

  }

  EXPORT_SYMBOL(blkdev_get);

  /**
   * blkdev_get_by_path - open a block device by name
   * @path: path to the block device to open
   * @mode: FMODE_* mask
   * @holder: exclusive holder identifier
   *
   * Open the blockdevice described by the device file at @path.  @mode
   * and @holder are identical to blkdev_get().
   *
   * On success, the returned block_device has reference count of one.
   *
   * CONTEXT:
   * Might sleep.
   *
   * RETURNS:
   * Pointer to block_device on success, ERR_PTR(-errno) on failure.
   */
  struct block_device *blkdev_get_by_path(const char *path, fmode_t mode,
  					void *holder)
  {
  	struct block_device *bdev;
  	int err;
  
  	bdev = lookup_bdev(path);
  	if (IS_ERR(bdev))
  		return bdev;
  
  	err = blkdev_get(bdev, mode, holder);
  	if (err)
  		return ERR_PTR(err);

  	if ((mode & FMODE_WRITE) && bdev_read_only(bdev)) {
  		blkdev_put(bdev, mode);
  		return ERR_PTR(-EACCES);
  	}

  	return bdev;
  }
  EXPORT_SYMBOL(blkdev_get_by_path);
  
  /**
   * blkdev_get_by_dev - open a block device by device number
   * @dev: device number of block device to open
   * @mode: FMODE_* mask
   * @holder: exclusive holder identifier
   *
   * Open the blockdevice described by device number @dev.  @mode and
   * @holder are identical to blkdev_get().
   *
   * Use it ONLY if you really do not have anything better - i.e. when
   * you are behind a truly sucky interface and all you are given is a
   * device number.  _Never_ to be used for internal purposes.  If you
   * ever need it - reconsider your API.
   *
   * On success, the returned block_device has reference count of one.
   *
   * CONTEXT:
   * Might sleep.
   *
   * RETURNS:
   * Pointer to block_device on success, ERR_PTR(-errno) on failure.
   */
  struct block_device *blkdev_get_by_dev(dev_t dev, fmode_t mode, void *holder)
  {
  	struct block_device *bdev;
  	int err;
  
  	bdev = bdget(dev);
  	if (!bdev)
  		return ERR_PTR(-ENOMEM);
  
  	err = blkdev_get(bdev, mode, holder);
  	if (err)
  		return ERR_PTR(err);
  
  	return bdev;
  }
  EXPORT_SYMBOL(blkdev_get_by_dev);

  static int blkdev_open(struct inode * inode, struct file * filp)
  {
  	struct block_device *bdev;

  
  	/*
  	 * Preserve backwards compatibility and allow large file access
  	 * even if userspace doesn't ask for it explicitly. Some mkfs
  	 * binary needs it. We might want to drop this workaround
  	 * during an unstable branch.
  	 */
  	filp->f_flags |= O_LARGEFILE;

  	if (filp->f_flags & O_NDELAY)
  		filp->f_mode |= FMODE_NDELAY;
  	if (filp->f_flags & O_EXCL)
  		filp->f_mode |= FMODE_EXCL;
  	if ((filp->f_flags & O_ACCMODE) == 3)
  		filp->f_mode |= FMODE_WRITE_IOCTL;

  	bdev = bd_acquire(inode);

  	if (bdev == NULL)
  		return -ENOMEM;

  	filp->f_mapping = bdev->bd_inode->i_mapping;

  	return blkdev_get(bdev, filp->f_mode, filp);

  }

  static void __blkdev_put(struct block_device *bdev, fmode_t mode, int for_part)

  {

  	struct gendisk *disk = bdev->bd_disk;

  	struct block_device *victim = NULL;

  	mutex_lock_nested(&bdev->bd_mutex, for_part);

  	if (for_part)
  		bdev->bd_part_count--;

  	if (!--bdev->bd_openers) {

  		WARN_ON_ONCE(bdev->bd_holders);

  		sync_blockdev(bdev);
  		kill_bdev(bdev);

  		/* ->release can cause the old bdi to disappear,
  		 * so must switch it out first
  		 */
  		bdev_inode_switch_bdi(bdev->bd_inode,
  					&default_backing_dev_info);

  	}
  	if (bdev->bd_contains == bdev) {
  		if (disk->fops->release)

  			disk->fops->release(disk, mode);

  	}
  	if (!bdev->bd_openers) {
  		struct module *owner = disk->fops->owner;

  		disk_put_part(bdev->bd_part);
  		bdev->bd_part = NULL;

  		bdev->bd_disk = NULL;

  		if (bdev != bdev->bd_contains)
  			victim = bdev->bd_contains;

  		bdev->bd_contains = NULL;

  
  		put_disk(disk);
  		module_put(owner);

  	}

  	mutex_unlock(&bdev->bd_mutex);
  	bdput(bdev);

  	if (victim)

  		__blkdev_put(victim, mode, 1);

  }

  void blkdev_put(struct block_device *bdev, fmode_t mode)

  {

  	mutex_lock(&bdev->bd_mutex);

  	if (mode & FMODE_EXCL) {

  		bool bdev_free;
  
  		/*
  		 * Release a claim on the device.  The holder fields
  		 * are protected with bdev_lock.  bd_mutex is to
  		 * synchronize disk_holder unlinking.
  		 */

  		spin_lock(&bdev_lock);
  
  		WARN_ON_ONCE(--bdev->bd_holders < 0);
  		WARN_ON_ONCE(--bdev->bd_contains->bd_holders < 0);
  
  		/* bd_contains might point to self, check in a separate step */
  		if ((bdev_free = !bdev->bd_holders))
  			bdev->bd_holder = NULL;
  		if (!bdev->bd_contains->bd_holders)
  			bdev->bd_contains->bd_holder = NULL;
  
  		spin_unlock(&bdev_lock);

  		/*
  		 * If this was the last claim, remove holder link and
  		 * unblock evpoll if it was a write holder.
  		 */

  		if (bdev_free && bdev->bd_write_holder) {
  			disk_unblock_events(bdev->bd_disk);
  			bdev->bd_write_holder = false;

  		}

  	}

  	/*
  	 * Trigger event checking and tell drivers to flush MEDIA_CHANGE
  	 * event.  This is to ensure detection of media removal commanded
  	 * from userland - e.g. eject(1).
  	 */
  	disk_flush_events(bdev->bd_disk, DISK_EVENT_MEDIA_CHANGE);
  
  	mutex_unlock(&bdev->bd_mutex);

  	__blkdev_put(bdev, mode, 0);

  }

  EXPORT_SYMBOL(blkdev_put);

  static int blkdev_close(struct inode * inode, struct file * filp)
  {
  	struct block_device *bdev = I_BDEV(filp->f_mapping->host);

  	blkdev_put(bdev, filp->f_mode);
  	return 0;

  }

  static long block_ioctl(struct file *file, unsigned cmd, unsigned long arg)

  {

  	struct block_device *bdev = I_BDEV(file->f_mapping->host);
  	fmode_t mode = file->f_mode;

  
  	/*
  	 * O_NDELAY can be altered using fcntl(.., F_SETFL, ..), so we have
  	 * to updated it before every ioctl.
  	 */

  	if (file->f_flags & O_NDELAY)

  		mode |= FMODE_NDELAY;
  	else
  		mode &= ~FMODE_NDELAY;

  	return blkdev_ioctl(bdev, mode, cmd, arg);

  }

  /*

   * Write data to the block device.  Only intended for the block device itself
   * and the raw driver which basically is a fake block device.
   *
   * Does not take i_mutex for the write and thus is not for general purpose
   * use.
   */
  ssize_t blkdev_aio_write(struct kiocb *iocb, const struct iovec *iov,
  			 unsigned long nr_segs, loff_t pos)
  {
  	struct file *file = iocb->ki_filp;

  	struct blk_plug plug;

  	ssize_t ret;
  
  	BUG_ON(iocb->ki_pos != pos);

  	blk_start_plug(&plug);

  	ret = __generic_file_aio_write(iocb, iov, nr_segs);

  	if (ret > 0) {

  		ssize_t err;
  
  		err = generic_write_sync(file, pos, ret);

  		if (err < 0)

  			ret = err;
  	}

  	blk_finish_plug(&plug);

  	return ret;
  }
  EXPORT_SYMBOL_GPL(blkdev_aio_write);

  static ssize_t blkdev_aio_read(struct kiocb *iocb, const struct iovec *iov,
  			 unsigned long nr_segs, loff_t pos)
  {
  	struct file *file = iocb->ki_filp;
  	struct inode *bd_inode = file->f_mapping->host;
  	loff_t size = i_size_read(bd_inode);
  
  	if (pos >= size)
  		return 0;
  
  	size -= pos;

  	if (size < iocb->ki_nbytes)

  		nr_segs = iov_shorten((struct iovec *)iov, nr_segs, size);
  	return generic_file_aio_read(iocb, iov, nr_segs, pos);
  }

  /*

   * Try to release a page associated with block device when the system
   * is under memory pressure.
   */
  static int blkdev_releasepage(struct page *page, gfp_t wait)
  {
  	struct super_block *super = BDEV_I(page->mapping->host)->bdev.bd_super;
  
  	if (super && super->s_op->bdev_try_to_free_page)
  		return super->s_op->bdev_try_to_free_page(super, page, wait);
  
  	return try_to_free_buffers(page);
  }

  static const struct address_space_operations def_blk_aops = {

  	.readpage	= blkdev_readpage,
  	.writepage	= blkdev_writepage,

  	.write_begin	= blkdev_write_begin,
  	.write_end	= blkdev_write_end,

  	.writepages	= generic_writepages,

  	.releasepage	= blkdev_releasepage,

  	.direct_IO	= blkdev_direct_IO,

  	.is_dirty_writeback = buffer_check_dirty_writeback,

  };

  const struct file_operations def_blk_fops = {

  	.open		= blkdev_open,
  	.release	= blkdev_close,
  	.llseek		= block_llseek,

  	.read		= do_sync_read,
  	.write		= do_sync_write,

  	.aio_read	= blkdev_aio_read,

  	.aio_write	= blkdev_aio_write,

  	.mmap		= generic_file_mmap,

  	.fsync		= blkdev_fsync,

  	.unlocked_ioctl	= block_ioctl,

  #ifdef CONFIG_COMPAT
  	.compat_ioctl	= compat_blkdev_ioctl,
  #endif

  	.splice_read	= generic_file_splice_read,
  	.splice_write	= generic_file_splice_write,

  };
  
  int ioctl_by_bdev(struct block_device *bdev, unsigned cmd, unsigned long arg)
  {
  	int res;
  	mm_segment_t old_fs = get_fs();
  	set_fs(KERNEL_DS);

  	res = blkdev_ioctl(bdev, 0, cmd, arg);

  	set_fs(old_fs);
  	return res;
  }
  
  EXPORT_SYMBOL(ioctl_by_bdev);
  
  /**
   * lookup_bdev  - lookup a struct block_device by name

   * @pathname:	special file representing the block device

   *

   * Get a reference to the blockdevice at @pathname in the current

   * namespace if possible and return it.  Return ERR_PTR(error)
   * otherwise.
   */

  struct block_device *lookup_bdev(const char *pathname)

  {
  	struct block_device *bdev;
  	struct inode *inode;

  	struct path path;

  	int error;

  	if (!pathname || !*pathname)

  		return ERR_PTR(-EINVAL);

  	error = kern_path(pathname, LOOKUP_FOLLOW, &path);

  	if (error)
  		return ERR_PTR(error);

  	inode = path.dentry->d_inode;

  	error = -ENOTBLK;
  	if (!S_ISBLK(inode->i_mode))
  		goto fail;
  	error = -EACCES;

  	if (path.mnt->mnt_flags & MNT_NODEV)

  		goto fail;
  	error = -ENOMEM;
  	bdev = bd_acquire(inode);
  	if (!bdev)
  		goto fail;
  out:

  	path_put(&path);

  	return bdev;
  fail:
  	bdev = ERR_PTR(error);
  	goto out;
  }

  EXPORT_SYMBOL(lookup_bdev);

  int __invalidate_device(struct block_device *bdev, bool kill_dirty)

  {
  	struct super_block *sb = get_super(bdev);
  	int res = 0;
  
  	if (sb) {
  		/*
  		 * no need to lock the super, get_super holds the
  		 * read mutex so the filesystem cannot go away
  		 * under us (->put_super runs with the write lock
  		 * hold).
  		 */
  		shrink_dcache_sb(sb);

  		res = invalidate_inodes(sb, kill_dirty);

  		drop_super(sb);
  	}

  	invalidate_bdev(bdev);

  	return res;
  }
  EXPORT_SYMBOL(__invalidate_device);

  
  void iterate_bdevs(void (*func)(struct block_device *, void *), void *arg)
  {
  	struct inode *inode, *old_inode = NULL;
  
  	spin_lock(&inode_sb_list_lock);
  	list_for_each_entry(inode, &blockdev_superblock->s_inodes, i_sb_list) {
  		struct address_space *mapping = inode->i_mapping;
  
  		spin_lock(&inode->i_lock);
  		if (inode->i_state & (I_FREEING|I_WILL_FREE|I_NEW) ||
  		    mapping->nrpages == 0) {
  			spin_unlock(&inode->i_lock);
  			continue;
  		}
  		__iget(inode);
  		spin_unlock(&inode->i_lock);
  		spin_unlock(&inode_sb_list_lock);
  		/*
  		 * We hold a reference to 'inode' so it couldn't have been
  		 * removed from s_inodes list while we dropped the
  		 * inode_sb_list_lock.  We cannot iput the inode now as we can
  		 * be holding the last reference and we cannot iput it under
  		 * inode_sb_list_lock. So we keep the reference and iput it
  		 * later.
  		 */
  		iput(old_inode);
  		old_inode = inode;
  
  		func(I_BDEV(inode), arg);
  
  		spin_lock(&inode_sb_list_lock);
  	}
  	spin_unlock(&inode_sb_list_lock);
  	iput(old_inode);
  }