Linux 2.2.8

[davej-history.git] / fs / buffer.c

/*
 *  linux/fs/buffer.c
 *
 *  Copyright (C) 1991, 1992  Linus Torvalds
 */

/*
 *  'buffer.c' implements the buffer-cache functions. Race-conditions have
 * been avoided by NEVER letting an interrupt change a buffer (except for the
 * data, of course), but instead letting the caller do it.
 */

/* Start bdflush() with kernel_thread not syscall - Paul Gortmaker, 12/95 */

/* Removed a lot of unnecessary code and simplified things now that
 * the buffer cache isn't our primary cache - Andrew Tridgell 12/96
 */

/* Speed up hash, lru, and free list operations.  Use gfp() for allocating
 * hash table, use SLAB cache for buffer heads. -DaveM
 */

/* Added 32k buffer block sizes - these are required older ARM systems.
 * - RMK
 */

#include <linux/malloc.h>
#include <linux/locks.h>
#include <linux/errno.h>
#include <linux/swap.h>
#include <linux/swapctl.h>
#include <linux/smp_lock.h>
#include <linux/vmalloc.h>
#include <linux/blkdev.h>
#include <linux/sysrq.h>
#include <linux/file.h>
#include <linux/init.h>
#include <linux/quotaops.h>

#include <asm/uaccess.h>
#include <asm/io.h>
#include <asm/bitops.h>

#define NR_SIZES 7
static char buffersize_index[65] =
{-1,  0,  1, -1,  2, -1, -1, -1, 3, -1, -1, -1, -1, -1, -1, -1,
  4, -1, -1, -1, -1, -1, -1, -1, -1,-1, -1, -1, -1, -1, -1, -1,
  5, -1, -1, -1, -1, -1, -1, -1, -1,-1, -1, -1, -1, -1, -1, -1,
 -1, -1, -1, -1, -1, -1, -1, -1, -1,-1, -1, -1, -1, -1, -1, -1,
  6};

#define BUFSIZE_INDEX(X) ((int) buffersize_index[(X)>>9])
#define MAX_BUF_PER_PAGE (PAGE_SIZE / 512)
#define NR_RESERVED (2*MAX_BUF_PER_PAGE)
#define MAX_UNUSED_BUFFERS NR_RESERVED+20 /* don't ever have more than this 
                                             number of unused buffer heads */

/*
 * Hash table mask..
 */
static unsigned long bh_hash_mask = 0;

static int grow_buffers(int size);

static struct buffer_head ** hash_table;
static struct buffer_head * lru_list[NR_LIST] = {NULL, };
static struct buffer_head * free_list[NR_SIZES] = {NULL, };

static kmem_cache_t *bh_cachep;

static struct buffer_head * unused_list = NULL;
static struct buffer_head * reuse_list = NULL;
static struct wait_queue * buffer_wait = NULL;

static int nr_buffers = 0;
static int nr_buffers_type[NR_LIST] = {0,};
static int nr_buffer_heads = 0;
static int nr_unused_buffer_heads = 0;
static int nr_hashed_buffers = 0;

/* This is used by some architectures to estimate available memory. */
int buffermem = 0;

/* Here is the parameter block for the bdflush process. If you add or
 * remove any of the parameters, make sure to update kernel/sysctl.c.
 */

#define N_PARAM 9

/* The dummy values in this structure are left in there for compatibility
 * with old programs that play with the /proc entries.
 */
union bdflush_param{
        struct {
                int nfract;  /* Percentage of buffer cache dirty to 
                                activate bdflush */
                int ndirty;  /* Maximum number of dirty blocks to write out per
                                wake-cycle */
                int nrefill; /* Number of clean buffers to try to obtain
                                each time we call refill */
                int nref_dirt; /* Dirty buffer threshold for activating bdflush
                                  when trying to refill buffers. */
                int interval;  /* Interval (seconds) between spontaneous
                                  bdflush runs */
                int age_buffer;  /* Time for normal buffer to age before 
                                    we flush it */
                int age_super;  /* Time for superblock to age before we 
                                   flush it */
                int dummy2;    /* unused */
                int dummy3;    /* unused */
        } b_un;
        unsigned int data[N_PARAM];
} bdf_prm = {{40, 500, 64, 256, 5, 30*HZ, 5*HZ, 1884, 2}};

/* These are the min and max parameter values that we will allow to be assigned */
int bdflush_min[N_PARAM] = {  0,  10,    5,   25,  1,   1*HZ,   1*HZ, 1, 1};
int bdflush_max[N_PARAM] = {100,5000, 2000, 2000,100, 600*HZ, 600*HZ, 2047, 5};

void wakeup_bdflush(int);

/*
 * Rewrote the wait-routines to use the "new" wait-queue functionality,
 * and getting rid of the cli-sti pairs. The wait-queue routines still
 * need cli-sti, but now it's just a couple of 386 instructions or so.
 *
 * Note that the real wait_on_buffer() is an inline function that checks
 * if 'b_wait' is set before calling this, so that the queues aren't set
 * up unnecessarily.
 */
void __wait_on_buffer(struct buffer_head * bh)
{
        struct task_struct *tsk = current;
        struct wait_queue wait;

        bh->b_count++;
        wait.task = tsk;
        add_wait_queue(&bh->b_wait, &wait);
repeat:
        tsk->state = TASK_UNINTERRUPTIBLE;
        run_task_queue(&tq_disk);
        if (buffer_locked(bh)) {
                schedule();
                goto repeat;
        }
        tsk->state = TASK_RUNNING;
        remove_wait_queue(&bh->b_wait, &wait);
        bh->b_count--;
}

/* Call sync_buffers with wait!=0 to ensure that the call does not
 * return until all buffer writes have completed.  Sync() may return
 * before the writes have finished; fsync() may not.
 */

/* Godamity-damn.  Some buffers (bitmaps for filesystems)
 * spontaneously dirty themselves without ever brelse being called.
 * We will ultimately want to put these in a separate list, but for
 * now we search all of the lists for dirty buffers.
 */
static int sync_buffers(kdev_t dev, int wait)
{
        int i, retry, pass = 0, err = 0;
        struct buffer_head * bh, *next;

        /* One pass for no-wait, three for wait:
         * 0) write out all dirty, unlocked buffers;
         * 1) write out all dirty buffers, waiting if locked;
         * 2) wait for completion by waiting for all buffers to unlock.
         */
        do {
                retry = 0;
repeat:
                /* We search all lists as a failsafe mechanism, not because we expect
                 * there to be dirty buffers on any of the other lists.
                 */
                bh = lru_list[BUF_DIRTY];
                if (!bh)
                        goto repeat2;
                for (i = nr_buffers_type[BUF_DIRTY]*2 ; i-- > 0 ; bh = next) {
                        if (bh->b_list != BUF_DIRTY)
                                goto repeat;
                        next = bh->b_next_free;
                        if (!lru_list[BUF_DIRTY])
                                break;
                        if (dev && bh->b_dev != dev)
                                continue;
                        if (buffer_locked(bh)) {
                                /* Buffer is locked; skip it unless wait is
                                 * requested AND pass > 0.
                                 */
                                if (!wait || !pass) {
                                        retry = 1;
                                        continue;
                                }
                                wait_on_buffer (bh);
                                goto repeat;
                        }

                        /* If an unlocked buffer is not uptodate, there has
                         * been an IO error. Skip it.
                         */
                        if (wait && buffer_req(bh) && !buffer_locked(bh) &&
                            !buffer_dirty(bh) && !buffer_uptodate(bh)) {
                                err = -EIO;
                                continue;
                        }

                        /* Don't write clean buffers.  Don't write ANY buffers
                         * on the third pass.
                         */
                        if (!buffer_dirty(bh) || pass >= 2)
                                continue;

                        /* Don't bother about locked buffers.
                         *
                         * XXX We checked if it was locked above and there is no
                         * XXX way we could have slept in between. -DaveM
                         */
                        if (buffer_locked(bh))
                                continue;
                        bh->b_count++;
                        next->b_count++;
                        bh->b_flushtime = 0;
                        ll_rw_block(WRITE, 1, &bh);
                        bh->b_count--;
                        next->b_count--;
                        retry = 1;
                }

    repeat2:
                bh = lru_list[BUF_LOCKED];
                if (!bh)
                        break;
                for (i = nr_buffers_type[BUF_LOCKED]*2 ; i-- > 0 ; bh = next) {
                        if (bh->b_list != BUF_LOCKED)
                                goto repeat2;
                        next = bh->b_next_free;
                        if (!lru_list[BUF_LOCKED])
                                break;
                        if (dev && bh->b_dev != dev)
                                continue;
                        if (buffer_locked(bh)) {
                                /* Buffer is locked; skip it unless wait is
                                 * requested AND pass > 0.
                                 */
                                if (!wait || !pass) {
                                        retry = 1;
                                        continue;
                                }
                                wait_on_buffer (bh);
                                goto repeat2;
                        }
                }

                /* If we are waiting for the sync to succeed, and if any dirty
                 * blocks were written, then repeat; on the second pass, only
                 * wait for buffers being written (do not pass to write any
                 * more buffers on the second pass).
                 */
        } while (wait && retry && ++pass<=2);
        return err;
}

void sync_dev(kdev_t dev)
{
        sync_buffers(dev, 0);
        sync_supers(dev);
        sync_inodes(dev);
        sync_buffers(dev, 0);
        DQUOT_SYNC(dev);
        /*
         * FIXME(eric) we need to sync the physical devices here.
         * This is because some (scsi) controllers have huge amounts of
         * cache onboard (hundreds of Mb), and we need to instruct
         * them to commit all of the dirty memory to disk, and we should
         * not return until this has happened.
         *
         * This would need to get implemented by going through the assorted
         * layers so that each block major number can be synced, and this
         * would call down into the upper and mid-layer scsi.
         */
}

int fsync_dev(kdev_t dev)
{
        sync_buffers(dev, 0);
        sync_supers(dev);
        sync_inodes(dev);
        DQUOT_SYNC(dev);
        return sync_buffers(dev, 1);
}

asmlinkage int sys_sync(void)
{
        lock_kernel();
        fsync_dev(0);
        unlock_kernel();
        return 0;
}

/*
 *      filp may be NULL if called via the msync of a vma.
 */
 
int file_fsync(struct file *filp, struct dentry *dentry)
{
        struct inode * inode = dentry->d_inode;
        struct super_block * sb;
        kdev_t dev;

        /* sync the inode to buffers */
        write_inode_now(inode);

        /* sync the superblock to buffers */
        sb = inode->i_sb;
        wait_on_super(sb);
        if (sb->s_op && sb->s_op->write_super)
                sb->s_op->write_super(sb);

        /* .. finally sync the buffers to disk */
        dev = inode->i_dev;
        return sync_buffers(dev, 1);
}

asmlinkage int sys_fsync(unsigned int fd)
{
        struct file * file;
        struct dentry * dentry;
        struct inode * inode;
        int err;

        lock_kernel();
        err = -EBADF;
        file = fget(fd);
        if (!file)
                goto out;

        dentry = file->f_dentry;
        if (!dentry)
                goto out_putf;

        inode = dentry->d_inode;
        if (!inode)
                goto out_putf;

        err = -EINVAL;
        if (!file->f_op || !file->f_op->fsync)
                goto out_putf;

        /* We need to protect against concurrent writers.. */
        down(&inode->i_sem);
        err = file->f_op->fsync(file, dentry);
        up(&inode->i_sem);

out_putf:
        fput(file);
out:
        unlock_kernel();
        return err;
}

asmlinkage int sys_fdatasync(unsigned int fd)
{
        struct file * file;
        struct dentry * dentry;
        struct inode * inode;
        int err;

        lock_kernel();
        err = -EBADF;
        file = fget(fd);
        if (!file)
                goto out;

        dentry = file->f_dentry;
        if (!dentry)
                goto out_putf;

        inode = dentry->d_inode;
        if (!inode)
                goto out_putf;

        err = -EINVAL;
        if (!file->f_op || !file->f_op->fsync)
                goto out_putf;

        /* this needs further work, at the moment it is identical to fsync() */
        down(&inode->i_sem);
        err = file->f_op->fsync(file, dentry);
        up(&inode->i_sem);

out_putf:
        fput(file);
out:
        unlock_kernel();
        return err;
}

void invalidate_buffers(kdev_t dev)
{
        int i;
        int nlist;
        struct buffer_head * bh;

        for(nlist = 0; nlist < NR_LIST; nlist++) {
                bh = lru_list[nlist];
                for (i = nr_buffers_type[nlist]*2 ; --i > 0 ; bh = bh->b_next_free) {
                        if (bh->b_dev != dev)
                                continue;
                        wait_on_buffer(bh);
                        if (bh->b_dev != dev)
                                continue;
                        if (bh->b_count)
                                continue;
                        bh->b_flushtime = 0;
                        clear_bit(BH_Protected, &bh->b_state);
                        clear_bit(BH_Uptodate, &bh->b_state);
                        clear_bit(BH_Dirty, &bh->b_state);
                        clear_bit(BH_Req, &bh->b_state);
                }
        }
}

#define _hashfn(dev,block) (((unsigned)(HASHDEV(dev)^block)) & bh_hash_mask)
#define hash(dev,block) hash_table[_hashfn(dev,block)]

static inline void remove_from_hash_queue(struct buffer_head * bh)
{
        struct buffer_head **pprev = bh->b_pprev;
        if (pprev) {
                struct buffer_head * next = bh->b_next;
                if (next) {
                        next->b_pprev = pprev;
                        bh->b_next = NULL;
                }
                *pprev = next;
                bh->b_pprev = NULL;
        }
        nr_hashed_buffers--;
}

static inline void remove_from_lru_list(struct buffer_head * bh)
{
        if (!(bh->b_prev_free) || !(bh->b_next_free))
                panic("VFS: LRU block list corrupted");
        if (bh->b_dev == B_FREE)
                panic("LRU list corrupted");
        bh->b_prev_free->b_next_free = bh->b_next_free;
        bh->b_next_free->b_prev_free = bh->b_prev_free;

        if (lru_list[bh->b_list] == bh)
                 lru_list[bh->b_list] = bh->b_next_free;
        if (lru_list[bh->b_list] == bh)
                 lru_list[bh->b_list] = NULL;
        bh->b_next_free = bh->b_prev_free = NULL;
}

static inline void remove_from_free_list(struct buffer_head * bh)
{
        int isize = BUFSIZE_INDEX(bh->b_size);
        if (!(bh->b_prev_free) || !(bh->b_next_free))
                panic("VFS: Free block list corrupted");
        if(bh->b_dev != B_FREE)
                panic("Free list corrupted");
        if(!free_list[isize])
                panic("Free list empty");
        if(bh->b_next_free == bh)
                 free_list[isize] = NULL;
        else {
                bh->b_prev_free->b_next_free = bh->b_next_free;
                bh->b_next_free->b_prev_free = bh->b_prev_free;
                if (free_list[isize] == bh)
                         free_list[isize] = bh->b_next_free;
        }
        bh->b_next_free = bh->b_prev_free = NULL;
}

static void remove_from_queues(struct buffer_head * bh)
{
        if(bh->b_dev == B_FREE) {
                remove_from_free_list(bh); /* Free list entries should not be
                                              in the hash queue */
                return;
        }
        nr_buffers_type[bh->b_list]--;
        remove_from_hash_queue(bh);
        remove_from_lru_list(bh);
}

static inline void put_last_free(struct buffer_head * bh)
{
        if (bh) {
                struct buffer_head **bhp = &free_list[BUFSIZE_INDEX(bh->b_size)];

                bh->b_dev = B_FREE;  /* So it is obvious we are on the free list. */

                /* Add to back of free list. */
                if(!*bhp) {
                        *bhp = bh;
                        bh->b_prev_free = bh;
                }

                bh->b_next_free = *bhp;
                bh->b_prev_free = (*bhp)->b_prev_free;
                (*bhp)->b_prev_free->b_next_free = bh;
                (*bhp)->b_prev_free = bh;
        }
}

static void insert_into_queues(struct buffer_head * bh)
{
        /* put at end of free list */
        if(bh->b_dev == B_FREE) {
                put_last_free(bh);
        } else {
                struct buffer_head **bhp = &lru_list[bh->b_list];

                if(!*bhp) {
                        *bhp = bh;
                        bh->b_prev_free = bh;
                }

                if (bh->b_next_free)
                        panic("VFS: buffer LRU pointers corrupted");

                bh->b_next_free = *bhp;
                bh->b_prev_free = (*bhp)->b_prev_free;
                (*bhp)->b_prev_free->b_next_free = bh;
                (*bhp)->b_prev_free = bh;

                nr_buffers_type[bh->b_list]++;

                /* Put the buffer in new hash-queue if it has a device. */
                bh->b_next = NULL;
                bh->b_pprev = NULL;
                if (bh->b_dev) {
                        struct buffer_head **bhp = &hash(bh->b_dev, bh->b_blocknr);
                        struct buffer_head *next = *bhp;

                        if (next) {
                                bh->b_next = next;
                                next->b_pprev = &bh->b_next;
                        }
                        *bhp = bh;
                        bh->b_pprev = bhp;
                }
                nr_hashed_buffers++;
        }
}

struct buffer_head * find_buffer(kdev_t dev, int block, int size)
{               
        struct buffer_head * next;

        next = hash(dev,block);
        for (;;) {
                struct buffer_head *tmp = next;
                if (!next)
                        break;
                next = tmp->b_next;
                if (tmp->b_blocknr != block || tmp->b_size != size || tmp->b_dev != dev)
                        continue;
                next = tmp;
                break;
        }
        return next;
}

/*
 * Why like this, I hear you say... The reason is race-conditions.
 * As we don't lock buffers (unless we are reading them, that is),
 * something might happen to it while we sleep (ie a read-error
 * will force it bad). This shouldn't really happen currently, but
 * the code is ready.
 */
struct buffer_head * get_hash_table(kdev_t dev, int block, int size)
{
        struct buffer_head * bh;
        bh = find_buffer(dev,block,size);
        if (bh)
                bh->b_count++;
        return bh;
}

unsigned int get_hardblocksize(kdev_t dev)
{
        /*
         * Get the hard sector size for the given device.  If we don't know
         * what it is, return 0.
         */
        if (hardsect_size[MAJOR(dev)] != NULL) {
                int blksize = hardsect_size[MAJOR(dev)][MINOR(dev)];
                if (blksize != 0)
                        return blksize;
        }

        /*
         * We don't know what the hardware sector size for this device is.
         * Return 0 indicating that we don't know.
         */
        return 0;
}

void set_blocksize(kdev_t dev, int size)
{
        extern int *blksize_size[];
        int i, nlist;
        struct buffer_head * bh, *bhnext;

        if (!blksize_size[MAJOR(dev)])
                return;

        /* Size must be a power of two, and between 512 and PAGE_SIZE */
        if (size > PAGE_SIZE || size < 512 || (size & (size-1)))
                panic("Invalid blocksize passed to set_blocksize");

        if (blksize_size[MAJOR(dev)][MINOR(dev)] == 0 && size == BLOCK_SIZE) {
                blksize_size[MAJOR(dev)][MINOR(dev)] = size;
                return;
        }
        if (blksize_size[MAJOR(dev)][MINOR(dev)] == size)
                return;
        sync_buffers(dev, 2);
        blksize_size[MAJOR(dev)][MINOR(dev)] = size;

        /* We need to be quite careful how we do this - we are moving entries
         * around on the free list, and we can get in a loop if we are not careful.
         */
        for(nlist = 0; nlist < NR_LIST; nlist++) {
                bh = lru_list[nlist];
                for (i = nr_buffers_type[nlist]*2 ; --i > 0 ; bh = bhnext) {
                        if(!bh)
                                break;

                        bhnext = bh->b_next_free; 
                        if (bh->b_dev != dev)
                                 continue;
                        if (bh->b_size == size)
                                 continue;
                        bhnext->b_count++;
                        wait_on_buffer(bh);
                        bhnext->b_count--;
                        if (bh->b_dev == dev && bh->b_size != size) {
                                clear_bit(BH_Dirty, &bh->b_state);
                                clear_bit(BH_Uptodate, &bh->b_state);
                                clear_bit(BH_Req, &bh->b_state);
                                bh->b_flushtime = 0;
                        }
                        remove_from_hash_queue(bh);
                }
        }
}

/*
 * We used to try various strange things. Let's not.
 */
static void refill_freelist(int size)
{
        if (!grow_buffers(size)) {
                wakeup_bdflush(1);
                current->policy |= SCHED_YIELD;
                schedule();
        }
}

void init_buffer(struct buffer_head *bh, kdev_t dev, int block,
                 bh_end_io_t *handler, void *dev_id)
{
        bh->b_count = 1;
        bh->b_list = BUF_CLEAN;
        bh->b_flushtime = 0;
        bh->b_dev = dev;
        bh->b_blocknr = block;
        bh->b_end_io = handler;
        bh->b_dev_id = dev_id;
}

static void end_buffer_io_sync(struct buffer_head *bh, int uptodate)
{
        mark_buffer_uptodate(bh, uptodate);
        unlock_buffer(bh);
}

/*
 * Ok, this is getblk, and it isn't very clear, again to hinder
 * race-conditions. Most of the code is seldom used, (ie repeating),
 * so it should be much more efficient than it looks.
 *
 * The algorithm is changed: hopefully better, and an elusive bug removed.
 *
 * 14.02.92: changed it to sync dirty buffers a bit: better performance
 * when the filesystem starts to get full of dirty blocks (I hope).
 */
struct buffer_head * getblk(kdev_t dev, int block, int size)
{
        struct buffer_head * bh;
        int isize;

repeat:
        bh = get_hash_table(dev, block, size);
        if (bh) {
                if (!buffer_dirty(bh)) {
                        bh->b_flushtime = 0;
                }
                return bh;
        }

        isize = BUFSIZE_INDEX(size);
get_free:
        bh = free_list[isize];
        if (!bh)
                goto refill;
        remove_from_free_list(bh);

        /* OK, FINALLY we know that this buffer is the only one of its kind,
         * and that it's unused (b_count=0), unlocked, and clean.
         */
        init_buffer(bh, dev, block, end_buffer_io_sync, NULL);
        bh->b_state=0;
        insert_into_queues(bh);
        return bh;

        /*
         * If we block while refilling the free list, somebody may
         * create the buffer first ... search the hashes again.
         */
refill:
        refill_freelist(size);
        if (!find_buffer(dev,block,size))
                goto get_free;
        goto repeat;
}

void set_writetime(struct buffer_head * buf, int flag)
{
        int newtime;

        if (buffer_dirty(buf)) {
                /* Move buffer to dirty list if jiffies is clear. */
                newtime = jiffies + (flag ? bdf_prm.b_un.age_super : 
                                     bdf_prm.b_un.age_buffer);
                if(!buf->b_flushtime || buf->b_flushtime > newtime)
                         buf->b_flushtime = newtime;
        } else {
                buf->b_flushtime = 0;
        }
}


/*
 * Put a buffer into the appropriate list, without side-effects.
 */
static inline void file_buffer(struct buffer_head *bh, int list)
{
        remove_from_queues(bh);
        bh->b_list = list;
        insert_into_queues(bh);
}

/*
 * A buffer may need to be moved from one buffer list to another
 * (e.g. in case it is not shared any more). Handle this.
 */
void refile_buffer(struct buffer_head * buf)
{
        int dispose;

        if(buf->b_dev == B_FREE) {
                printk("Attempt to refile free buffer\n");
                return;
        }
        if (buffer_dirty(buf))
                dispose = BUF_DIRTY;
        else if (buffer_locked(buf))
                dispose = BUF_LOCKED;
        else
                dispose = BUF_CLEAN;
        if(dispose != buf->b_list) {
                file_buffer(buf, dispose);
                if(dispose == BUF_DIRTY) {
                        int too_many = (nr_buffers * bdf_prm.b_un.nfract/100);

                        /* This buffer is dirty, maybe we need to start flushing.
                         * If too high a percentage of the buffers are dirty...
                         */
                        if (nr_buffers_type[BUF_DIRTY] > too_many)
                                wakeup_bdflush(1);

                        /* If this is a loop device, and
                         * more than half of the buffers are dirty...
                         * (Prevents no-free-buffers deadlock with loop device.)
                         */
                        if (MAJOR(buf->b_dev) == LOOP_MAJOR &&
                            nr_buffers_type[BUF_DIRTY]*2>nr_buffers)
                                wakeup_bdflush(1);
                }
        }
}

/*
 * Release a buffer head
 */
void __brelse(struct buffer_head * buf)
{
        /* If dirty, mark the time this buffer should be written back. */
        set_writetime(buf, 0);
        refile_buffer(buf);
        touch_buffer(buf);

        if (buf->b_count) {
                buf->b_count--;
                return;
        }
        printk("VFS: brelse: Trying to free free buffer\n");
}

/*
 * bforget() is like brelse(), except it puts the buffer on the
 * free list if it can.. We can NOT free the buffer if:
 *  - there are other users of it
 *  - it is locked and thus can have active IO
 */
void __bforget(struct buffer_head * buf)
{
        if (buf->b_count != 1 || buffer_locked(buf)) {
                __brelse(buf);
                return;
        }
        buf->b_count = 0;
        buf->b_state = 0;
        remove_from_queues(buf);
        put_last_free(buf);
}

/*
 * bread() reads a specified block and returns the buffer that contains
 * it. It returns NULL if the block was unreadable.
 */
struct buffer_head * bread(kdev_t dev, int block, int size)
{
        struct buffer_head * bh;

        bh = getblk(dev, block, size);
        if (buffer_uptodate(bh))
                return bh;
        ll_rw_block(READ, 1, &bh);
        wait_on_buffer(bh);
        if (buffer_uptodate(bh))
                return bh;
        brelse(bh);
        return NULL;
}

/*
 * Ok, breada can be used as bread, but additionally to mark other
 * blocks for reading as well. End the argument list with a negative
 * number.
 */

#define NBUF 16

struct buffer_head * breada(kdev_t dev, int block, int bufsize,
        unsigned int pos, unsigned int filesize)
{
        struct buffer_head * bhlist[NBUF];
        unsigned int blocks;
        struct buffer_head * bh;
        int index;
        int i, j;

        if (pos >= filesize)
                return NULL;

        if (block < 0)
                return NULL;

        bh = getblk(dev, block, bufsize);
        index = BUFSIZE_INDEX(bh->b_size);

        if (buffer_uptodate(bh))
                return(bh);   
        else ll_rw_block(READ, 1, &bh);

        blocks = (filesize - pos) >> (9+index);

        if (blocks < (read_ahead[MAJOR(dev)] >> index))
                blocks = read_ahead[MAJOR(dev)] >> index;
        if (blocks > NBUF) 
                blocks = NBUF;

/*      if (blocks) printk("breada (new) %d blocks\n",blocks); */


        bhlist[0] = bh;
        j = 1;
        for(i=1; i<blocks; i++) {
                bh = getblk(dev,block+i,bufsize);
                if (buffer_uptodate(bh)) {
                        brelse(bh);
                        break;
                }
                else bhlist[j++] = bh;
        }

        /* Request the read for these buffers, and then release them. */
        if (j>1)  
                ll_rw_block(READA, (j-1), bhlist+1); 
        for(i=1; i<j; i++)
                brelse(bhlist[i]);

        /* Wait for this buffer, and then continue on. */
        bh = bhlist[0];
        wait_on_buffer(bh);
        if (buffer_uptodate(bh))
                return bh;
        brelse(bh);
        return NULL;
}

/*
 * Note: the caller should wake up the buffer_wait list if needed.
 */
static void put_unused_buffer_head(struct buffer_head * bh)
{
        if (nr_unused_buffer_heads >= MAX_UNUSED_BUFFERS) {
                nr_buffer_heads--;
                kmem_cache_free(bh_cachep, bh);
                return;
        }

        memset(bh,0,sizeof(*bh));
        nr_unused_buffer_heads++;
        bh->b_next_free = unused_list;
        unused_list = bh;
}

/* 
 * We can't put completed temporary IO buffer_heads directly onto the
 * unused_list when they become unlocked, since the device driver
 * end_request routines still expect access to the buffer_head's
 * fields after the final unlock.  So, the device driver puts them on
 * the reuse_list instead once IO completes, and we recover these to
 * the unused_list here.
 *
 * Note that we don't do a wakeup here, but return a flag indicating
 * whether we got any buffer heads. A task ready to sleep can check
 * the returned value, and any tasks already sleeping will have been
 * awakened when the buffer heads were added to the reuse list.
 */
static inline int recover_reusable_buffer_heads(void)
{
        struct buffer_head *head = xchg(&reuse_list, NULL);
        int found = 0;
        
        if (head) {
                do {
                        struct buffer_head *bh = head;
                        head = head->b_next_free;
                        put_unused_buffer_head(bh);
                } while (head);
                found = 1;
        }
        return found;
}

/*
 * Reserve NR_RESERVED buffer heads for async IO requests to avoid
 * no-buffer-head deadlock.  Return NULL on failure; waiting for
 * buffer heads is now handled in create_buffers().
 */ 
static struct buffer_head * get_unused_buffer_head(int async)
{
        struct buffer_head * bh;

        recover_reusable_buffer_heads();
        if (nr_unused_buffer_heads > NR_RESERVED) {
                bh = unused_list;
                unused_list = bh->b_next_free;
                nr_unused_buffer_heads--;
                return bh;
        }

        /* This is critical.  We can't swap out pages to get
         * more buffer heads, because the swap-out may need
         * more buffer-heads itself.  Thus SLAB_BUFFER.
         */
        if((bh = kmem_cache_alloc(bh_cachep, SLAB_BUFFER)) != NULL) {
                memset(bh, 0, sizeof(*bh));
                nr_buffer_heads++;
                return bh;
        }

        /*
         * If we need an async buffer, use the reserved buffer heads.
         */
        if (async && unused_list) {
                bh = unused_list;
                unused_list = bh->b_next_free;
                nr_unused_buffer_heads--;
                return bh;
        }

#if 0
        /*
         * (Pending further analysis ...)
         * Ordinary (non-async) requests can use a different memory priority
         * to free up pages. Any swapping thus generated will use async
         * buffer heads.
         */
        if(!async &&
           (bh = kmem_cache_alloc(bh_cachep, SLAB_KERNEL)) != NULL) {
                memset(bh, 0, sizeof(*bh));
                nr_buffer_heads++;
                return bh;
        }
#endif

        return NULL;
}

/*
 * Create the appropriate buffers when given a page for data area and
 * the size of each buffer.. Use the bh->b_this_page linked list to
 * follow the buffers created.  Return NULL if unable to create more
 * buffers.
 * The async flag is used to differentiate async IO (paging, swapping)
 * from ordinary buffer allocations, and only async requests are allowed
 * to sleep waiting for buffer heads. 
 */
static struct buffer_head * create_buffers(unsigned long page, 
                                                unsigned long size, int async)
{
        struct wait_queue wait = { current, NULL };
        struct buffer_head *bh, *head;
        long offset;

try_again:
        head = NULL;
        offset = PAGE_SIZE;
        while ((offset -= size) >= 0) {
                bh = get_unused_buffer_head(async);
                if (!bh)
                        goto no_grow;

                bh->b_dev = B_FREE;  /* Flag as unused */
                bh->b_this_page = head;
                head = bh;

                bh->b_state = 0;
                bh->b_next_free = NULL;
                bh->b_count = 0;
                bh->b_size = size;

                bh->b_data = (char *) (page+offset);
                bh->b_list = 0;
        }
        return head;
/*
 * In case anything failed, we just free everything we got.
 */
no_grow:
        if (head) {
                do {
                        bh = head;
                        head = head->b_this_page;
                        put_unused_buffer_head(bh);
                } while (head);

                /* Wake up any waiters ... */
                wake_up(&buffer_wait);
        }

        /*
         * Return failure for non-async IO requests.  Async IO requests
         * are not allowed to fail, so we have to wait until buffer heads
         * become available.  But we don't want tasks sleeping with 
         * partially complete buffers, so all were released above.
         */
        if (!async)
                return NULL;

        /* We're _really_ low on memory. Now we just
         * wait for old buffer heads to become free due to
         * finishing IO.  Since this is an async request and
         * the reserve list is empty, we're sure there are 
         * async buffer heads in use.
         */
        run_task_queue(&tq_disk);

        /* 
         * Set our state for sleeping, then check again for buffer heads.
         * This ensures we won't miss a wake_up from an interrupt.
         */
        add_wait_queue(&buffer_wait, &wait);
        current->state = TASK_UNINTERRUPTIBLE;
        if (!recover_reusable_buffer_heads())
                schedule();
        remove_wait_queue(&buffer_wait, &wait);
        current->state = TASK_RUNNING;
        goto try_again;
}

/* Run the hooks that have to be done when a page I/O has completed. */
static inline void after_unlock_page (struct page * page)
{
        if (test_and_clear_bit(PG_decr_after, &page->flags)) {
                atomic_dec(&nr_async_pages);
#ifdef DEBUG_SWAP
                printk ("DebugVM: Finished IO on page %p, nr_async_pages %d\n",
                        (char *) page_address(page), 
                        atomic_read(&nr_async_pages));
#endif
        }
        if (test_and_clear_bit(PG_swap_unlock_after, &page->flags))
                swap_after_unlock_page(page->offset);
        if (test_and_clear_bit(PG_free_after, &page->flags))
                __free_page(page);
}

/*
 * Free all temporary buffers belonging to a page.
 * This needs to be called with interrupts disabled.
 */
static inline void free_async_buffers (struct buffer_head * bh)
{
        struct buffer_head *tmp, *tail;

        /*
         * Link all the buffers into the b_next_free list,
         * so we only have to do one xchg() operation ...
         */
        tail = bh;
        while ((tmp = tail->b_this_page) != bh) {
                tail->b_next_free = tmp;
                tail = tmp;
        };

        /* Update the reuse list */
        tail->b_next_free = xchg(&reuse_list, NULL);
        reuse_list = bh;

        /* Wake up any waiters ... */
        wake_up(&buffer_wait);
}

static void end_buffer_io_async(struct buffer_head * bh, int uptodate)
{
        unsigned long flags;
        struct buffer_head *tmp;
        struct page *page;

        mark_buffer_uptodate(bh, uptodate);
        unlock_buffer(bh);

        /* This is a temporary buffer used for page I/O. */
        page = mem_map + MAP_NR(bh->b_data);
        if (!PageLocked(page))
                goto not_locked;
        if (bh->b_count != 1)
                goto bad_count;

        if (!test_bit(BH_Uptodate, &bh->b_state))
                set_bit(PG_error, &page->flags);

        /*
         * Be _very_ careful from here on. Bad things can happen if
         * two buffer heads end IO at almost the same time and both
         * decide that the page is now completely done.
         *
         * Async buffer_heads are here only as labels for IO, and get
         * thrown away once the IO for this page is complete.  IO is
         * deemed complete once all buffers have been visited
         * (b_count==0) and are now unlocked. We must make sure that
         * only the _last_ buffer that decrements its count is the one
         * that free's the page..
         */
        save_flags(flags);
        cli();
        bh->b_count--;
        tmp = bh;
        do {
                if (tmp->b_count)
                        goto still_busy;
                tmp = tmp->b_this_page;
        } while (tmp != bh);

        /* OK, the async IO on this page is complete. */
        free_async_buffers(bh);
        restore_flags(flags);
        clear_bit(PG_locked, &page->flags);
        wake_up(&page->wait);
        after_unlock_page(page);
        return;

still_busy:
        restore_flags(flags);
        return;

not_locked:
        printk ("Whoops: end_buffer_io_async: async io complete on unlocked page\n");
        return;

bad_count:
        printk ("Whoops: end_buffer_io_async: b_count != 1 on async io.\n");
        return;
}

/*
 * Start I/O on a page.
 * This function expects the page to be locked and may return before I/O is complete.
 * You then have to check page->locked, page->uptodate, and maybe wait on page->wait.
 */
int brw_page(int rw, struct page *page, kdev_t dev, int b[], int size, int bmap)
{
        struct buffer_head *bh, *prev, *next, *arr[MAX_BUF_PER_PAGE];
        int block, nr;

        if (!PageLocked(page))
                panic("brw_page: page not locked for I/O");
        clear_bit(PG_uptodate, &page->flags);
        clear_bit(PG_error, &page->flags);
        /*
         * Allocate async buffer heads pointing to this page, just for I/O.
         * They do _not_ show up in the buffer hash table!
         * They are _not_ registered in page->buffers either!
         */
        bh = create_buffers(page_address(page), size, 1);
        if (!bh) {
                /* WSH: exit here leaves page->count incremented */
                clear_bit(PG_locked, &page->flags);
                wake_up(&page->wait);
                return -ENOMEM;
        }
        nr = 0;
        next = bh;
        do {
                struct buffer_head * tmp;
                block = *(b++);

                init_buffer(next, dev, block, end_buffer_io_async, NULL);
                set_bit(BH_Uptodate, &next->b_state);

                /*
                 * When we use bmap, we define block zero to represent
                 * a hole.  ll_rw_page, however, may legitimately
                 * access block zero, and we need to distinguish the
                 * two cases.
                 */
                if (bmap && !block) {
                        memset(next->b_data, 0, size);
                        next->b_count--;
                        continue;
                }
                tmp = get_hash_table(dev, block, size);
                if (tmp) {
                        if (!buffer_uptodate(tmp)) {
                                if (rw == READ)
                                        ll_rw_block(READ, 1, &tmp);
                                wait_on_buffer(tmp);
                        }
                        if (rw == READ) 
                                memcpy(next->b_data, tmp->b_data, size);
                        else {
                                memcpy(tmp->b_data, next->b_data, size);
                                mark_buffer_dirty(tmp, 0);
                        }
                        brelse(tmp);
                        next->b_count--;
                        continue;
                }
                if (rw == READ)
                        clear_bit(BH_Uptodate, &next->b_state);
                else
                        set_bit(BH_Dirty, &next->b_state);
                arr[nr++] = next;
        } while (prev = next, (next = next->b_this_page) != NULL);
        prev->b_this_page = bh;
        
        if (nr) {
                ll_rw_block(rw, nr, arr);
                /* The rest of the work is done in mark_buffer_uptodate()
                 * and unlock_buffer(). */
        } else {
                unsigned long flags;
                clear_bit(PG_locked, &page->flags);
                set_bit(PG_uptodate, &page->flags);
                wake_up(&page->wait);
                save_flags(flags);
                cli();
                free_async_buffers(bh);
                restore_flags(flags);
                after_unlock_page(page);
        }
        ++current->maj_flt;
        return 0;
}

/*
 * This is called by end_request() when I/O has completed.
 */
void mark_buffer_uptodate(struct buffer_head * bh, int on)
{
        if (on) {
                struct buffer_head *tmp = bh;
                set_bit(BH_Uptodate, &bh->b_state);
                /* If a page has buffers and all these buffers are uptodate,
                 * then the page is uptodate. */
                do {
                        if (!test_bit(BH_Uptodate, &tmp->b_state))
                                return;
                        tmp=tmp->b_this_page;
                } while (tmp && tmp != bh);
                set_bit(PG_uptodate, &mem_map[MAP_NR(bh->b_data)].flags);
                return;
        }
        clear_bit(BH_Uptodate, &bh->b_state);
}

/*
 * Generic "readpage" function for block devices that have the normal
 * bmap functionality. This is most of the block device filesystems.
 * Reads the page asynchronously --- the unlock_buffer() and
 * mark_buffer_uptodate() functions propagate buffer state into the
 * page struct once IO has completed.
 */
int generic_readpage(struct file * file, struct page * page)
{
        struct dentry *dentry = file->f_dentry;
        struct inode *inode = dentry->d_inode;
        unsigned long block;
        int *p, nr[PAGE_SIZE/512];
        int i;

        atomic_inc(&page->count);
        set_bit(PG_locked, &page->flags);
        set_bit(PG_free_after, &page->flags);
        
        i = PAGE_SIZE >> inode->i_sb->s_blocksize_bits;
        block = page->offset >> inode->i_sb->s_blocksize_bits;
        p = nr;
        do {
                *p = inode->i_op->bmap(inode, block);
                i--;
                block++;
                p++;
        } while (i > 0);

        /* IO start */
        brw_page(READ, page, inode->i_dev, nr, inode->i_sb->s_blocksize, 1);
        return 0;
}

/*
 * Try to increase the number of buffers available: the size argument
 * is used to determine what kind of buffers we want.
 */
static int grow_buffers(int size)
{
        unsigned long page;
        struct buffer_head *bh, *tmp;
        struct buffer_head * insert_point;
        int isize;

        if ((size & 511) || (size > PAGE_SIZE)) {
                printk("VFS: grow_buffers: size = %d\n",size);
                return 0;
        }

        if (!(page = __get_free_page(GFP_BUFFER)))
                return 0;
        bh = create_buffers(page, size, 0);
        if (!bh) {
                free_page(page);
                return 0;
        }

        isize = BUFSIZE_INDEX(size);
        insert_point = free_list[isize];

        tmp = bh;
        while (1) {
                if (insert_point) {
                        tmp->b_next_free = insert_point->b_next_free;
                        tmp->b_prev_free = insert_point;
                        insert_point->b_next_free->b_prev_free = tmp;
                        insert_point->b_next_free = tmp;
                } else {
                        tmp->b_prev_free = tmp;
                        tmp->b_next_free = tmp;
                }
                insert_point = tmp;
                ++nr_buffers;
                if (tmp->b_this_page)
                        tmp = tmp->b_this_page;
                else
                        break;
        }
        tmp->b_this_page = bh;
        free_list[isize] = bh;
        mem_map[MAP_NR(page)].buffers = bh;
        buffermem += PAGE_SIZE;
        return 1;
}

/*
 * Can the buffer be thrown out?
 */
#define BUFFER_BUSY_BITS        ((1<<BH_Dirty) | (1<<BH_Lock) | (1<<BH_Protected))
#define buffer_busy(bh)         ((bh)->b_count || ((bh)->b_state & BUFFER_BUSY_BITS))

/*
 * try_to_free_buffers() checks if all the buffers on this particular page
 * are unused, and free's the page if so.
 *
 * Wake up bdflush() if this fails - if we're running low on memory due
 * to dirty buffers, we need to flush them out as quickly as possible.
 */
int try_to_free_buffers(struct page * page_map)
{
        struct buffer_head * tmp, * bh = page_map->buffers;

        tmp = bh;
        do {
                struct buffer_head * p = tmp;

                tmp = tmp->b_this_page;
                if (!buffer_busy(p))
                        continue;

                wakeup_bdflush(0);
                return 0;
        } while (tmp != bh);

        tmp = bh;
        do {
                struct buffer_head * p = tmp;
                tmp = tmp->b_this_page;
                nr_buffers--;
                remove_from_queues(p);
                put_unused_buffer_head(p);
        } while (tmp != bh);

        /* Wake up anyone waiting for buffer heads */
        wake_up(&buffer_wait);

        /* And free the page */
        buffermem -= PAGE_SIZE;
        page_map->buffers = NULL;
        __free_page(page_map);
        return 1;
}

/* ================== Debugging =================== */

void show_buffers(void)
{
        struct buffer_head * bh;
        int found = 0, locked = 0, dirty = 0, used = 0, lastused = 0;
        int protected = 0;
        int nlist;
        static char *buf_types[NR_LIST] = {"CLEAN","LOCKED","DIRTY"};

        printk("Buffer memory:   %6dkB\n",buffermem>>10);
        printk("Buffer heads:    %6d\n",nr_buffer_heads);
        printk("Buffer blocks:   %6d\n",nr_buffers);
        printk("Buffer hashed:   %6d\n",nr_hashed_buffers);

        for(nlist = 0; nlist < NR_LIST; nlist++) {
          found = locked = dirty = used = lastused = protected = 0;
          bh = lru_list[nlist];
          if(!bh) continue;

          do {
                found++;
                if (buffer_locked(bh))
                        locked++;
                if (buffer_protected(bh))
                        protected++;
                if (buffer_dirty(bh))
                        dirty++;
                if (bh->b_count)
                        used++, lastused = found;
                bh = bh->b_next_free;
          } while (bh != lru_list[nlist]);
          printk("%8s: %d buffers, %d used (last=%d), "
                 "%d locked, %d protected, %d dirty\n",
                 buf_types[nlist], found, used, lastused,
                 locked, protected, dirty);
        };
}


/* ===================== Init ======================= */

/*
 * allocate the hash table and init the free list
 * Use gfp() for the hash table to decrease TLB misses, use
 * SLAB cache for buffer heads.
 */
void __init buffer_init(unsigned long memory_size)
{
        int order;
        unsigned int nr_hash;

        /* we need to guess at the right sort of size for a buffer cache.
           the heuristic from working with large databases and getting
           fsync times (ext2) manageable, is the following */

        memory_size >>= 20;
        for (order = 5; (1UL << order) < memory_size; order++);

        /* try to allocate something until we get it or we're asking
           for something that is really too small */

        do {
                nr_hash = (1UL << order) * PAGE_SIZE /
                    sizeof(struct buffer_head *);
                hash_table = (struct buffer_head **)
                    __get_free_pages(GFP_ATOMIC, order);
        } while (hash_table == NULL && --order > 4);
        
        if (!hash_table)
                panic("Failed to allocate buffer hash table\n");
        memset(hash_table, 0, nr_hash * sizeof(struct buffer_head *));
        bh_hash_mask = nr_hash-1;

        bh_cachep = kmem_cache_create("buffer_head",
                                      sizeof(struct buffer_head),
                                      0,
                                      SLAB_HWCACHE_ALIGN, NULL, NULL);
        if(!bh_cachep)
                panic("Cannot create buffer head SLAB cache\n");
        /*
         * Allocate the reserved buffer heads.
         */
        while (nr_buffer_heads < NR_RESERVED) {
                struct buffer_head * bh;

                bh = kmem_cache_alloc(bh_cachep, SLAB_ATOMIC);
                if (!bh)
                        break;
                put_unused_buffer_head(bh);
                nr_buffer_heads++;
        }

        lru_list[BUF_CLEAN] = 0;
        grow_buffers(BLOCK_SIZE);
}


/* ====================== bdflush support =================== */

/* This is a simple kernel daemon, whose job it is to provide a dynamic
 * response to dirty buffers.  Once this process is activated, we write back
 * a limited number of buffers to the disks and then go back to sleep again.
 */
static struct wait_queue * bdflush_done = NULL;
struct task_struct *bdflush_tsk = 0;

void wakeup_bdflush(int wait)
{
        if (current == bdflush_tsk)
                return;
        wake_up_process(bdflush_tsk);
        if (wait) {
                run_task_queue(&tq_disk);
                sleep_on(&bdflush_done);
        }
}


/* 
 * Here we attempt to write back old buffers.
 * To prevent deadlocks for a loop device:
 * 1) Do non-blocking writes to loop (avoids deadlock with running
 *      out of request blocks).
 * 2) But do a blocking write if the only dirty buffers are loop buffers
 *      (otherwise we go into an infinite busy-loop).
 * 3) Quit writing loop blocks if a freelist went low (avoids deadlock
 *      with running out of free buffers for loop's "real" device).
*/

static inline void sync_old_buffers(void)
{
        int i;
        int ndirty = 0;
        int wrta_cmd = WRITEA;
#ifdef DEBUG
        int ncount = 0, nwritten = 0;
#endif
        struct buffer_head * bh, *next;

#ifdef DEBUG
        bh = lru_list[BUF_CLEAN];
        if(bh)
                for(i = nr_buffers_type[BUF_CLEAN]; --i > 0; bh = next) {
                        next = bh->b_next_free;

                        /* Dirty/locked buffer on clean list?  Refile it */
                        if (buffer_locked(bh) || buffer_dirty(bh)) {
                                ncount++;
                                refile_buffer(bh);
                        }
                }
#endif

        bh = lru_list[BUF_LOCKED];
        if(bh)
                for(i = nr_buffers_type[BUF_LOCKED]; --i > 0; bh = next) {
                        next = bh->b_next_free;

                        /* Unlocked buffer on locked list?  Refile it */
                        if (!buffer_locked(bh))
                                refile_buffer(bh);
                }

 restart:
        bh = lru_list[BUF_DIRTY];
        if(bh) 
                for (i = nr_buffers_type[BUF_DIRTY];
                     i-- > 0 && ndirty < bdf_prm.b_un.ndirty; 
                     bh = next) {
                        /* We may have stalled while waiting for
                           I/O to complete. */
                        if(bh->b_list != BUF_DIRTY)
                                goto restart;
                        next = bh->b_next_free;
                        if(!lru_list[BUF_DIRTY]) {
                                printk("Dirty list empty %d\n", i);
                                break;
                        }
                                          
                        /* Clean buffer on dirty list?  Refile it */
                        if (!buffer_dirty(bh)) {
                                refile_buffer(bh);
                                continue;
                        }
                                          
                        if (buffer_locked(bh))
                                continue;
                        /* Should we write back buffers that are
                           shared or not??  Currently dirty buffers
                           are not shared, so it does not matter */
                        next->b_count++;
                        bh->b_count++;
                        ndirty++;
                        bh->b_flushtime = 0;
                        if (MAJOR(bh->b_dev) == LOOP_MAJOR) {
                                ll_rw_block(wrta_cmd,1, &bh);
                                wrta_cmd = WRITEA;
                                if (buffer_dirty(bh))
                                        --ndirty;
                        }
                        else
                                ll_rw_block(WRITE, 1, &bh);
                        bh->b_count--;
                        next->b_count--;
                }
        /* If we didn't write anything, but there are still
         * dirty buffers, then make the next write to a
         * loop device to be a blocking write.
         * This lets us block--which we _must_ do! */
        if (ndirty == 0
            && nr_buffers_type[BUF_DIRTY] > 0 && wrta_cmd != WRITE) {
                wrta_cmd = WRITE;
                goto restart;
        }

#ifdef DEBUG
        if (ncount) printk("sync_old_buffers: %d dirty buffers not on dirty list\n", ncount);
        printk("wrote %d/%d buffers...", nwritten, ndirty);
#endif
        run_task_queue(&tq_disk);
}


/* This is the interface to bdflush.  As we get more sophisticated, we can
 * pass tuning parameters to this "process", to adjust how it behaves. 
 * We would want to verify each parameter, however, to make sure that it 
 * is reasonable. */

asmlinkage int sys_bdflush(int func, long data)
{
        int i, error = -EPERM;

        lock_kernel();
        if (!capable(CAP_SYS_ADMIN))
                goto out;

        if (func == 1)
                /* Func 1 used to call sync_old_buffers; a user space
                   daemon would call it periodically.  This is no
                   longer necessary.  Returning -EPERM here makes the
                   daemon silently exit.  */
                goto out;

        /* Basically func 1 means read param 1, 2 means write param 1, etc */
        if (func >= 2) {
                i = (func-2) >> 1;
                error = -EINVAL;
                if (i < 0 || i >= N_PARAM)
                        goto out;
                if((func & 1) == 0) {
                        error = put_user(bdf_prm.data[i], (int*)data);
                        goto out;
                }
                if (data < bdflush_min[i] || data > bdflush_max[i])
                        goto out;
                bdf_prm.data[i] = data;
                error = 0;
                goto out;
        };

        /* Having func 0 used to launch the actual bdflush and then never
         * return (unless explicitly killed). We return zero here to 
         * remain semi-compatible with present update(8) programs.
         */
        error = 0;
out:
        unlock_kernel();
        return error;
}

/* This is the actual bdflush daemon itself. It used to be started
 * from the syscall above, but now we launch it ourselves internally
 * with kernel_thread(...)  directly after the first thread in
 * init/main.c.  Every so often, or when woken up by another task that
 * needs memory, we call sync_old_buffers to partially clear the dirty list.
 */

int bdflush(void * unused) 
{
        long remaining = HZ * bdf_prm.b_un.interval;
        struct task_struct *tsk = current;

        /*
         *      We have a bare-bones task_struct, and really should fill
         *      in a few more things so "top" and /proc/2/{exe,root,cwd}
         *      display semi-sane things. Not real crucial though...  
         */

        tsk->session = 1;
        tsk->pgrp = 1;
        tsk->dumpable = 0;  /* inhibit ptrace() */
        strcpy(tsk->comm, "kflushd");
        sigfillset(&tsk->blocked);
        bdflush_tsk = tsk;

        /*
         *      As a kernel thread we want to tamper with system buffers
         *      and other internals and thus be subject to the SMP locking
         *      rules. (On a uniprocessor box this does nothing).
         */
        lock_kernel();
                 
        for (;;) {
                tsk->state = TASK_INTERRUPTIBLE;
                remaining = schedule_timeout(remaining);

#ifdef DEBUG
                printk("bdflush() activated...");
#endif
                CHECK_EMERGENCY_SYNC

                if (remaining == 0) {
                        /*
                         * Also try to flush inodes and supers, since
                         * otherwise there would be no way of ensuring
                         * that these quantities ever get written
                         * back.  Ideally, we would have a timestamp
                         * on the inodes and superblocks so that we
                         * could write back only the old ones.
                         */
                        sync_supers(0);
                        sync_inodes(0);
                        remaining = HZ * bdf_prm.b_un.interval;
                }
                        
                /* Keep flushing till there aren't very many dirty buffers */
                do {
                        sync_old_buffers();
                } while(nr_buffers_type[BUF_DIRTY] > nr_buffers * bdf_prm.b_un.nfract/100);

                wake_up(&bdflush_done);
#ifdef DEBUG
                printk("sleeping again.\n");
#endif
        }
}