bitmap.c

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/*
 *  linux/fs/hfs/bitmap.c
 *
 * Copyright (C) 1996-1997  Paul H. Hargrove
 * (C) 2003 Ardis Technologies <[email protected]>
 * This file may be distributed under the terms of the GNU General Public License.
 *
 * Based on GPLed code Copyright (C) 1995  Michael Dreher
 *
 * This file contains the code to modify the volume bitmap:
 * search/set/clear bits.
 */

#include "hfs_fs.h"

/*
 * hfs_find_zero_bit()
 *
 * Description:
 *  Given a block of memory, its length in bits, and a starting bit number,
 *  determine the number of the first zero bits (in left-to-right ordering)
 *  in that range.
 *
 *  Returns >= 'size' if no zero bits are found in the range.
 *
 *  Accesses memory in 32-bit aligned chunks of 32-bits and thus
 *  may read beyond the 'size'th bit.
 */
static u32 hfs_find_set_zero_bits(__be32 *bitmap, u32 size, u32 offset, u32 *max)
{
    __be32 *curr, *end;
    u32 mask, start, len, n;
    __be32 val;
    int i;

    len = *max;
    if (!len)
        return size;

    curr = bitmap + (offset / 32);
    end = bitmap + ((size + 31) / 32);

    /* scan the first partial u32 for zero bits */
    val = *curr;
    if (~val) {
        n = be32_to_cpu(val);
        i = offset % 32;
        mask = (1U << 31) >> i;
        for (; i < 32; mask >>= 1, i++) {
            if (!(n & mask))
                goto found;
        }
    }

    /* scan complete u32s for the first zero bit */
    while (++curr < end) {
        val = *curr;
        if (~val) {
            n = be32_to_cpu(val);
            mask = 1 << 31;
            for (i = 0; i < 32; mask >>= 1, i++) {
                if (!(n & mask))
                    goto found;
            }
        }
    }
    return size;

found:
    start = (curr - bitmap) * 32 + i;
    if (start >= size)
        return start;
    /* do any partial u32 at the start */
    len = min(size - start, len);
    while (1) {
        n |= mask;
        if (++i >= 32)
            break;
        mask >>= 1;
        if (!--len || n & mask)
            goto done;
    }
    if (!--len)
        goto done;
    *curr++ = cpu_to_be32(n);
    /* do full u32s */
    while (1) {
        n = be32_to_cpu(*curr);
        if (len < 32)
            break;
        if (n) {
            len = 32;
            break;
        }
        *curr++ = cpu_to_be32(0xffffffff);
        len -= 32;
    }
    /* do any partial u32 at end */
    mask = 1U << 31;
    for (i = 0; i < len; i++) {
        if (n & mask)
            break;
        n |= mask;
        mask >>= 1;
    }
done:
    *curr = cpu_to_be32(n);
    *max = (curr - bitmap) * 32 + i - start;
    return start;
}

/*
 * hfs_vbm_search_free()
 *
 * Description:
 *   Search for 'num_bits' consecutive cleared bits in the bitmap blocks of
 *   the hfs MDB. 'mdb' had better be locked or the returned range
 *   may be no longer free, when this functions returns!
 *   XXX Currently the search starts from bit 0, but it should start with
 *   the bit number stored in 's_alloc_ptr' of the MDB.
 * Input Variable(s):
 *   struct hfs_mdb *mdb: Pointer to the hfs MDB
 *   u16 *num_bits: Pointer to the number of cleared bits
 *     to search for
 * Output Variable(s):
 *   u16 *num_bits: The number of consecutive clear bits of the
 *     returned range. If the bitmap is fragmented, this will be less than
 *     requested and it will be zero, when the disk is full.
 * Returns:
 *   The number of the first bit of the range of cleared bits which has been
 *   found. When 'num_bits' is zero, this is invalid!
 * Preconditions:
 *   'mdb' points to a "valid" (struct hfs_mdb).
 *   'num_bits' points to a variable of type (u16), which contains
 *  the number of cleared bits to find.
 * Postconditions:
 *   'num_bits' is set to the length of the found sequence.
 */
u32 hfs_vbm_search_free(struct super_block *sb, u32 goal, u32 *num_bits)
{
    void *bitmap;
    u32 pos;

    /* make sure we have actual work to perform */
    if (!*num_bits)
        return 0;

    mutex_lock(&HFS_SB(sb)->bitmap_lock);
    bitmap = HFS_SB(sb)->bitmap;

    pos = hfs_find_set_zero_bits(bitmap, HFS_SB(sb)->fs_ablocks, goal, num_bits);
    if (pos >= HFS_SB(sb)->fs_ablocks) {
        if (goal)
            pos = hfs_find_set_zero_bits(bitmap, goal, 0, num_bits);
        if (pos >= HFS_SB(sb)->fs_ablocks) {
            *num_bits = pos = 0;
            goto out;
        }
    }

    dprint(DBG_BITMAP, "alloc_bits: %u,%u\n", pos, *num_bits);
    HFS_SB(sb)->free_ablocks -= *num_bits;
    hfs_bitmap_dirty(sb);
out:
    mutex_unlock(&HFS_SB(sb)->bitmap_lock);
    return pos;
}


/*
 * hfs_clear_vbm_bits()
 *
 * Description:
 *   Clear the requested bits in the volume bitmap of the hfs filesystem
 * Input Variable(s):
 *   struct hfs_mdb *mdb: Pointer to the hfs MDB
 *   u16 start: The offset of the first bit
 *   u16 count: The number of bits
 * Output Variable(s):
 *   None
 * Returns:
 *    0: no error
 *   -1: One of the bits was already clear.  This is a strange
 *   error and when it happens, the filesystem must be repaired!
 *   -2: One or more of the bits are out of range of the bitmap.
 * Preconditions:
 *   'mdb' points to a "valid" (struct hfs_mdb).
 * Postconditions:
 *   Starting with bit number 'start', 'count' bits in the volume bitmap
 *   are cleared. The affected bitmap blocks are marked "dirty", the free
 *   block count of the MDB is updated and the MDB is marked dirty.
 */
int hfs_clear_vbm_bits(struct super_block *sb, u16 start, u16 count)
{
    __be32 *curr;
    u32 mask;
    int i, len;

    /* is there any actual work to be done? */
    if (!count)
        return 0;

    dprint(DBG_BITMAP, "clear_bits: %u,%u\n", start, count);
    /* are all of the bits in range? */
    if ((start + count) > HFS_SB(sb)->fs_ablocks)
        return -2;

    mutex_lock(&HFS_SB(sb)->bitmap_lock);
    /* bitmap is always on a 32-bit boundary */
    curr = HFS_SB(sb)->bitmap + (start / 32);
    len = count;

    /* do any partial u32 at the start */
    i = start % 32;
    if (i) {
        int j = 32 - i;
        mask = 0xffffffffU << j;
        if (j > count) {
            mask |= 0xffffffffU >> (i + count);
            *curr &= cpu_to_be32(mask);
            goto out;
        }
        *curr++ &= cpu_to_be32(mask);
        count -= j;
    }

    /* do full u32s */
    while (count >= 32) {
        *curr++ = 0;
        count -= 32;
    }
    /* do any partial u32 at end */
    if (count) {
        mask = 0xffffffffU >> count;
        *curr &= cpu_to_be32(mask);
    }
out:
    HFS_SB(sb)->free_ablocks += len;
    mutex_unlock(&HFS_SB(sb)->bitmap_lock);
    hfs_bitmap_dirty(sb);

    return 0;
}