root/fs/block_dev.c

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

#include <linux/config.h>
#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/devfs_fs_kernel.h>
#include <linux/smp_lock.h>
#include <linux/highmem.h>
#include <linux/blkdev.h>
#include <linux/module.h>
#include <linux/blkpg.h>
#include <linux/buffer_head.h>
#include <linux/mpage.h>
#include <linux/mount.h>
#include <linux/uio.h>
#include <linux/namei.h>
#include <asm/uaccess.h>

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

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);

static sector_t max_block(struct block_device *bdev)
{
        sector_t retval = ~((sector_t)0);
        loff_t sz = i_size_read(bdev->bd_inode);

        if (sz) {
                unsigned int size = block_size(bdev);
                unsigned int sizebits = blksize_bits(size);
                retval = (sz >> sizebits);
        }
        return retval;
}

/* Kill _all_ buffers, dirty or not.. */
static void kill_bdev(struct block_device *bdev)
{
        invalidate_bdev(bdev, 1);
        truncate_inode_pages(bdev->bd_inode->i_mapping, 0);
}       

int set_blocksize(struct block_device *bdev, int size)
{
        int oldsize;

        /* Size must be a power of two, and between 512 and PAGE_SIZE */
        if (size > PAGE_SIZE || size < 512 || (size & (size-1)))
                return -EINVAL;

        /* Size cannot be smaller than the size supported by the device */
        if (size < bdev_hardsect_size(bdev))
                return -EINVAL;

        oldsize = bdev->bd_block_size;
        if (oldsize == size)
                return 0;

        /* Ok, we're actually changing the blocksize.. */
        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)
{
        int bits;
        if (set_blocksize(sb->s_bdev, size) < 0)
                return 0;
        sb->s_blocksize = size;
        for (bits = 9, size >>= 9; size >>= 1; bits++)
                ;
        sb->s_blocksize_bits = bits;
        return sb->s_blocksize;
}

EXPORT_SYMBOL(sb_set_blocksize);

int sb_min_blocksize(struct super_block *sb, int size)
{
        int minsize = bdev_hardsect_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)
{
        if (iblock >= max_block(I_BDEV(inode))) {
                if (create)
                        return -EIO;

                /*
                 * for reads, we're just trying to fill a partial page.
                 * return a hole, they will have to call get_block again
                 * before they can fill it, and they will get -EIO at that
                 * time
                 */
                return 0;
        }
        bh->b_bdev = I_BDEV(inode);
        bh->b_blocknr = iblock;
        set_buffer_mapped(bh);
        return 0;
}

static int
blkdev_get_blocks(struct inode *inode, sector_t iblock,
                unsigned long max_blocks, struct buffer_head *bh, int create)
{
        if ((iblock + max_blocks) > max_block(I_BDEV(inode)))
                return -EIO;

        bh->b_bdev = I_BDEV(inode);
        bh->b_blocknr = iblock;
        bh->b_size = max_blocks << inode->i_blkbits;
        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_no_locking(rw, iocb, inode, I_BDEV(inode),
                                iov, offset, nr_segs, blkdev_get_blocks, NULL);
}

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_prepare_write(struct file *file, struct page *page, unsigned from, unsigned to)
{
        return block_prepare_write(page, from, to, blkdev_get_block);
}

static int blkdev_commit_write(struct file *file, struct page *page, unsigned from, unsigned to)
{
        return block_commit_write(page, from, to);
}

/*
 * private llseek:
 * for a block special file file->f_dentry->d_inode->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 origin)
{
        struct inode *bd_inode = file->f_mapping->host;
        loff_t size;
        loff_t retval;

        down(&bd_inode->i_sem);
        size = i_size_read(bd_inode);

        switch (origin) {
                case 2:
                        offset += size;
                        break;
                case 1:
                        offset += file->f_pos;
        }
        retval = -EINVAL;
        if (offset >= 0 && offset <= size) {
                if (offset != file->f_pos) {
                        file->f_pos = offset;
                }
                retval = offset;
        }
        up(&bd_inode->i_sem);
        return retval;
}
        
/*
 *      Filp is never NULL; the only case when ->fsync() is called with
 *      NULL first argument is nfsd_sync_dir() and that's not a directory.
 */
 
static int block_fsync(struct file *filp, struct dentry *dentry, int datasync)
{
        return sync_blockdev(I_BDEV(filp->f_mapping->host));
}

/*
 * pseudo-fs
 */

static spinlock_t bdev_lock __cacheline_aligned_in_smp = SPIN_LOCK_UNLOCKED;
static kmem_cache_t * bdev_cachep;

static struct inode *bdev_alloc_inode(struct super_block *sb)
{
        struct bdev_inode *ei = kmem_cache_alloc(bdev_cachep, SLAB_KERNEL);
        if (!ei)
                return NULL;
        return &ei->vfs_inode;
}

static void bdev_destroy_inode(struct inode *inode)
{
        struct bdev_inode *bdi = BDEV_I(inode);

        bdi->bdev.bd_inode_backing_dev_info = NULL;
        kmem_cache_free(bdev_cachep, bdi);
}

static void init_once(void * foo, kmem_cache_t * cachep, unsigned long flags)
{
        struct bdev_inode *ei = (struct bdev_inode *) foo;
        struct block_device *bdev = &ei->bdev;

        if ((flags & (SLAB_CTOR_VERIFY|SLAB_CTOR_CONSTRUCTOR)) ==
            SLAB_CTOR_CONSTRUCTOR)
        {
                memset(bdev, 0, sizeof(*bdev));
                sema_init(&bdev->bd_sem, 1);
                sema_init(&bdev->bd_mount_sem, 1);
                INIT_LIST_HEAD(&bdev->bd_inodes);
                INIT_LIST_HEAD(&bdev->bd_list);
                inode_init_once(&ei->vfs_inode);
        }
}

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_clear_inode(struct inode *inode)
{
        struct block_device *bdev = &BDEV_I(inode)->bdev;
        struct list_head *p;
        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 struct super_operations bdev_sops = {
        .statfs = simple_statfs,
        .alloc_inode = bdev_alloc_inode,
        .destroy_inode = bdev_destroy_inode,
        .drop_inode = generic_delete_inode,
        .clear_inode = bdev_clear_inode,
};

static struct super_block *bd_get_sb(struct file_system_type *fs_type,
        int flags, const char *dev_name, void *data)
{
        return get_sb_pseudo(fs_type, "bdev:", &bdev_sops, 0x62646576);
}

static struct file_system_type bd_type = {
        .name           = "bdev",
        .get_sb         = bd_get_sb,
        .kill_sb        = kill_anon_super,
};

static struct vfsmount *bd_mnt;
struct super_block *blockdev_superblock;

void __init bdev_cache_init(void)
{
        int err;
        bdev_cachep = kmem_cache_create("bdev_cache", sizeof(struct bdev_inode),
                        0, SLAB_HWCACHE_ALIGN|SLAB_RECLAIM_ACCOUNT|SLAB_PANIC,
                        init_once, NULL);
        err = register_filesystem(&bd_type);
        if (err)
                panic("Cannot register bdev pseudo-fs");
        bd_mnt = kern_mount(&bd_type);
        err = PTR_ERR(bd_mnt);
        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(bd_mnt->mnt_sb, 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_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);

long nr_blockdev_pages(void)
{
        struct list_head *p;
        long ret = 0;
        spin_lock(&bdev_lock);
        list_for_each(p, &all_bdevs) {
                struct block_device *bdev;
                bdev = list_entry(p, struct block_device, 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 && igrab(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)
                        __bd_forget(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;
}

/* Call when you free inode */

void bd_forget(struct inode *inode)
{
        spin_lock(&bdev_lock);
        if (inode->i_bdev)
                __bd_forget(inode);
        spin_unlock(&bdev_lock);
}

int bd_claim(struct block_device *bdev, void *holder)
{
        int res;
        spin_lock(&bdev_lock);

        /* first decide result */
        if (bdev->bd_holder == holder)
                res = 0;         /* already a holder */
        else if (bdev->bd_holder != NULL)
                res = -EBUSY;    /* held by someone else */
        else if (bdev->bd_contains == bdev)
                res = 0;         /* is a whole device which isn't held */

        else if (bdev->bd_contains->bd_holder == bd_claim)
                res = 0;         /* is a partition of a device that is being partitioned */
        else if (bdev->bd_contains->bd_holder != NULL)
                res = -EBUSY;    /* is a partition of a held device */
        else
                res = 0;         /* is a partition of an un-held device */

        /* now impose change */
        if (res==0) {
                /* note that for a whole device bd_holders
                 * will be incremented twice, and bd_holder will
                 * be set to bd_claim before being set to holder
                 */
                bdev->bd_contains->bd_holders ++;
                bdev->bd_contains->bd_holder = bd_claim;
                bdev->bd_holders++;
                bdev->bd_holder = holder;
        }
        spin_unlock(&bdev_lock);
        return res;
}

EXPORT_SYMBOL(bd_claim);

void bd_release(struct block_device *bdev)
{
        spin_lock(&bdev_lock);
        if (!--bdev->bd_contains->bd_holders)
                bdev->bd_contains->bd_holder = NULL;
        if (!--bdev->bd_holders)
                bdev->bd_holder = NULL;
        spin_unlock(&bdev_lock);
}

EXPORT_SYMBOL(bd_release);

/*
 * Tries to open block device by device number.  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.
 */
struct block_device *open_by_devnum(dev_t dev, unsigned mode)
{
        struct block_device *bdev = bdget(dev);
        int err = -ENOMEM;
        int flags = mode & FMODE_WRITE ? O_RDWR : O_RDONLY;
        if (bdev)
                err = blkdev_get(bdev, mode, flags);
        return err ? ERR_PTR(err) : bdev;
}

EXPORT_SYMBOL(open_by_devnum);

/*
 * 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;
        struct block_device_operations * bdops = disk->fops;

        if (!bdops->media_changed)
                return 0;
        if (!bdops->media_changed(bdev->bd_disk))
                return 0;

        if (__invalidate_device(bdev, 0))
                printk("VFS: busy inodes on changed media.\n");

        if (bdops->revalidate_disk)
                bdops->revalidate_disk(bdev->bd_disk);
        if (bdev->bd_disk->minors > 1)
                bdev->bd_invalidated = 1;
        return 1;
}

EXPORT_SYMBOL(check_disk_change);

void bd_set_size(struct block_device *bdev, loff_t size)
{
        unsigned bsize = bdev_hardsect_size(bdev);

        bdev->bd_inode->i_size = size;
        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 int do_open(struct block_device *bdev, struct file *file)
{
        struct module *owner = NULL;
        struct gendisk *disk;
        int ret = -ENXIO;
        int part;

        file->f_mapping = bdev->bd_inode->i_mapping;
        lock_kernel();
        disk = get_gendisk(bdev->bd_dev, &part);
        if (!disk) {
                unlock_kernel();
                bdput(bdev);
                return ret;
        }
        owner = disk->fops->owner;

        down(&bdev->bd_sem);
        if (!bdev->bd_openers) {
                bdev->bd_disk = disk;
                bdev->bd_contains = bdev;
                if (!part) {
                        struct backing_dev_info *bdi;
                        if (disk->fops->open) {
                                ret = disk->fops->open(bdev->bd_inode, file);
                                if (ret)
                                        goto out_first;
                        }
                        if (!bdev->bd_openers) {
                                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->bd_inode->i_data.backing_dev_info = bdi;
                        }
                        if (bdev->bd_invalidated)
                                rescan_partitions(disk, bdev);
                } else {
                        struct hd_struct *p;
                        struct block_device *whole;
                        whole = bdget_disk(disk, 0);
                        ret = -ENOMEM;
                        if (!whole)
                                goto out_first;
                        ret = blkdev_get(whole, file->f_mode, file->f_flags);
                        if (ret)
                                goto out_first;
                        bdev->bd_contains = whole;
                        down(&whole->bd_sem);
                        whole->bd_part_count++;
                        p = disk->part[part - 1];
                        bdev->bd_inode->i_data.backing_dev_info =
                           whole->bd_inode->i_data.backing_dev_info;
                        if (!(disk->flags & GENHD_FL_UP) || !p || !p->nr_sects) {
                                whole->bd_part_count--;
                                up(&whole->bd_sem);
                                ret = -ENXIO;
                                goto out_first;
                        }
                        kobject_get(&p->kobj);
                        bdev->bd_part = p;
                        bd_set_size(bdev, (loff_t) p->nr_sects << 9);
                        up(&whole->bd_sem);
                }
        } else {
                put_disk(disk);
                module_put(owner);
                if (bdev->bd_contains == bdev) {
                        if (bdev->bd_disk->fops->open) {
                                ret = bdev->bd_disk->fops->open(bdev->bd_inode, file);
                                if (ret)
                                        goto out;
                        }
                        if (bdev->bd_invalidated)
                                rescan_partitions(bdev->bd_disk, bdev);
                } else {
                        down(&bdev->bd_contains->bd_sem);
                        bdev->bd_contains->bd_part_count++;
                        up(&bdev->bd_contains->bd_sem);
                }
        }
        bdev->bd_openers++;
        up(&bdev->bd_sem);
        unlock_kernel();
        return 0;

out_first:
        bdev->bd_disk = NULL;
        bdev->bd_inode->i_data.backing_dev_info = &default_backing_dev_info;
        if (bdev != bdev->bd_contains)
                blkdev_put(bdev->bd_contains);
        bdev->bd_contains = NULL;
        put_disk(disk);
        module_put(owner);
out:
        up(&bdev->bd_sem);
        unlock_kernel();
        if (ret)
                bdput(bdev);
        return ret;
}

int blkdev_get(struct block_device *bdev, mode_t mode, unsigned flags)
{
        /*
         * This crockload is due to bad choice of ->open() type.
         * It will go away.
         * For now, block device ->open() routine must _not_
         * examine anything in 'inode' argument except ->i_rdev.
         */
        struct file fake_file = {};
        struct dentry fake_dentry = {};
        fake_file.f_mode = mode;
        fake_file.f_flags = flags;
        fake_file.f_dentry = &fake_dentry;
        fake_dentry.d_inode = bdev->bd_inode;

        return do_open(bdev, &fake_file);
}

EXPORT_SYMBOL(blkdev_get);

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

        /*
         * 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;

        bdev = bd_acquire(inode);

        res = do_open(bdev, filp);
        if (res)
                return res;

        if (!(filp->f_flags & O_EXCL) )
                return 0;

        if (!(res = bd_claim(bdev, filp)))
                return 0;

        blkdev_put(bdev);
        return res;
}

EXPORT_SYMBOL(blkdev_open);

int blkdev_put(struct block_device *bdev)
{
        int ret = 0;
        struct inode *bd_inode = bdev->bd_inode;
        struct gendisk *disk = bdev->bd_disk;

        down(&bdev->bd_sem);
        lock_kernel();
        if (!--bdev->bd_openers) {
                sync_blockdev(bdev);
                kill_bdev(bdev);
        }
        if (bdev->bd_contains == bdev) {
                if (disk->fops->release)
                        ret = disk->fops->release(bd_inode, NULL);
        } else {
                down(&bdev->bd_contains->bd_sem);
                bdev->bd_contains->bd_part_count--;
                up(&bdev->bd_contains->bd_sem);
        }
        if (!bdev->bd_openers) {
                struct module *owner = disk->fops->owner;

                put_disk(disk);
                module_put(owner);

                if (bdev->bd_contains != bdev) {
                        kobject_put(&bdev->bd_part->kobj);
                        bdev->bd_part = NULL;
                }
                bdev->bd_disk = NULL;
                bdev->bd_inode->i_data.backing_dev_info = &default_backing_dev_info;
                if (bdev != bdev->bd_contains) {
                        blkdev_put(bdev->bd_contains);
                }
                bdev->bd_contains = NULL;
        }
        unlock_kernel();
        up(&bdev->bd_sem);
        bdput(bdev);
        return ret;
}

EXPORT_SYMBOL(blkdev_put);

static int blkdev_close(struct inode * inode, struct file * filp)
{
        struct block_device *bdev = I_BDEV(filp->f_mapping->host);
        if (bdev->bd_holder == filp)
                bd_release(bdev);
        return blkdev_put(bdev);
}

static ssize_t blkdev_file_write(struct file *file, const char __user *buf,
                                   size_t count, loff_t *ppos)
{
        struct iovec local_iov = { .iov_base = (void __user *)buf, .iov_len = count };

        return generic_file_write_nolock(file, &local_iov, 1, ppos);
}

static ssize_t blkdev_file_aio_write(struct kiocb *iocb, const char __user *buf,
                                   size_t count, loff_t pos)
{
        struct iovec local_iov = { .iov_base = (void __user *)buf, .iov_len = count };

        return generic_file_aio_write_nolock(iocb, &local_iov, 1, &iocb->ki_pos);
}

static int block_ioctl(struct inode *inode, struct file *file, unsigned cmd,
                        unsigned long arg)
{
        return blkdev_ioctl(file->f_mapping->host, file, cmd, arg);
}

struct address_space_operations def_blk_aops = {
        .readpage       = blkdev_readpage,
        .writepage      = blkdev_writepage,
        .sync_page      = block_sync_page,
        .prepare_write  = blkdev_prepare_write,
        .commit_write   = blkdev_commit_write,
        .writepages     = generic_writepages,
        .direct_IO      = blkdev_direct_IO,
};

struct file_operations def_blk_fops = {
        .open           = blkdev_open,
        .release        = blkdev_close,
        .llseek         = block_llseek,
        .read           = generic_file_read,
        .write          = blkdev_file_write,
        .aio_read       = generic_file_aio_read,
        .aio_write      = blkdev_file_aio_write, 
        .mmap           = generic_file_mmap,
        .fsync          = block_fsync,
        .ioctl          = block_ioctl,
        .readv          = generic_file_readv,
        .writev         = generic_file_write_nolock,
        .sendfile       = generic_file_sendfile,
};

EXPORT_SYMBOL(def_blk_fops);

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->bd_inode, NULL, cmd, arg);
        set_fs(old_fs);
        return res;
}

EXPORT_SYMBOL(ioctl_by_bdev);

/**
 * lookup_bdev  - lookup a struct block_device by name
 *
 * @path:       special file representing the block device
 *
 * Get a reference to the blockdevice at @path in the current
 * namespace if possible and return it.  Return ERR_PTR(error)
 * otherwise.
 */
struct block_device *lookup_bdev(const char *path)
{
        struct block_device *bdev;
        struct inode *inode;
        struct nameidata nd;
        int error;

        if (!path || !*path)
                return ERR_PTR(-EINVAL);

        error = path_lookup(path, LOOKUP_FOLLOW, &nd);
        if (error)
                return ERR_PTR(error);

        inode = nd.dentry->d_inode;
        error = -ENOTBLK;
        if (!S_ISBLK(inode->i_mode))
                goto fail;
        error = -EACCES;
        if (nd.mnt->mnt_flags & MNT_NODEV)
                goto fail;
        error = -ENOMEM;
        bdev = bd_acquire(inode);
        if (!bdev)
                goto fail;
out:
        path_release(&nd);
        return bdev;
fail:
        bdev = ERR_PTR(error);
        goto out;
}

/**
 * open_bdev_excl  -  open a block device by name and set it up for use
 *
 * @path:       special file representing the block device
 * @flags:      %MS_RDONLY for opening read-only
 * @holder:     owner for exclusion
 *
 * Open the blockdevice described by the special file at @path, claim it
 * for the @holder.
 */
struct block_device *open_bdev_excl(const char *path, int flags, void *holder)
{
        struct block_device *bdev;
        mode_t mode = FMODE_READ;
        int error = 0;

        bdev = lookup_bdev(path);
        if (IS_ERR(bdev))
                return bdev;

        if (!(flags & MS_RDONLY))
                mode |= FMODE_WRITE;
        error = blkdev_get(bdev, mode, 0);
        if (error)
                return ERR_PTR(error);
        error = -EACCES;
        if (!(flags & MS_RDONLY) && bdev_read_only(bdev))
                goto blkdev_put;
        error = bd_claim(bdev, holder);
        if (error)
                goto blkdev_put;

        return bdev;
        
blkdev_put:
        blkdev_put(bdev);
        return ERR_PTR(error);
}

EXPORT_SYMBOL(open_bdev_excl);

/**
 * close_bdev_excl  -  release a blockdevice openen by open_bdev_excl()
 *
 * @bdev:       blockdevice to close
 *
 * This is the counterpart to open_bdev_excl().
 */
void close_bdev_excl(struct block_device *bdev)
{
        bd_release(bdev);
        blkdev_put(bdev);
}

EXPORT_SYMBOL(close_bdev_excl);