bitmap.c

一份精简的linux内核源代码

/*
 *  linux/fs/bitmap.c
 *
 *  (C) 1991  Linus Torvalds
 */

/* bitmap.c contains the code that handles the inode and block bitmaps */
#include <string.h>

#include <linux/sched.h>
#include <linux/kernel.h>

#define clear_block(addr) \ /* 清除块中内容 */
__asm__("cld\n\t" \
	"rep\n\t" \
	"stosl" \
	::"a" (0),"c" (BLOCK_SIZE/4),"D" ((long) (addr)):"cx","di")

#define set_bit(nr,addr) ({\ /* 把addr+nr处的bit位置位，并返回该值 */
register int res __asm__("ax"); \
__asm__ __volatile__("btsl %2,%3\n\tsetb %%al": \
"=a" (res):"0" (0),"r" (nr),"m" (*(addr))); \
res;})

#define clear_bit(nr,addr) ({\ /* 把addr+nr处的bit位复位，并返回该值的反 */
register int res __asm__("ax"); \
__asm__ __volatile__("btrl %2,%3\n\tsetnb %%al": \
"=a" (res):"0" (0),"r" (nr),"m" (*(addr))); \
res;})

#define find_first_zero(addr) ({ \  /*寻找子addr起的第一个为0的比特位，并返回其偏移值 */
int __res; \
__asm__("cld\n" \
	"1:\tlodsl\n\t" \
	"notl %%eax\n\t" \
	"bsfl %%eax,%%edx\n\t" \
	"je 2f\n\t" \
	"addl %%edx,%%ecx\n\t" \
	"jmp 3f\n" \
	"2:\taddl $32,%%ecx\n\t" \
	"cmpl $8192,%%ecx\n\t" \
	"jl 1b\n" \
	"3:" \
	:"=c" (__res):"c" (0),"S" (addr):"ax","dx","si"); \
__res;})

void free_block(int dev, int block) /* 释放块 */
{
	struct super_block * sb;
	struct buffer_head * bh;

	if (!(sb = get_super(dev))) /* 找到设备的超级块 */
		panic("trying to free block on nonexistent device");
	if (block < sb->s_firstdatazone || block >= sb->s_nzones)/* 判断数据块是否越界*/
		panic("trying to free block not in datazone");
	bh = get_hash_table(dev,block);
	if (bh) { /* 释放缓冲区中对应的缓冲块 */
		if (bh->b_count != 1) {
			printk("trying to free block (%04x:%d), count=%d\n",
				dev,block,bh->b_count);
			return;
		}
		bh->b_dirt=0;
		bh->b_uptodate=0;
		brelse(bh);
	}
	block -= sb->s_firstdatazone - 1 ;/* 复位块位图中的相应位 */
	if (clear_bit(block&8191,sb->s_zmap[block/8192]->b_data)) {
		printk("block (%04x:%d) ",dev,block+sb->s_firstdatazone-1);
		panic("free_block: bit already cleared");
	}
	sb->s_zmap[block/8192]->b_dirt = 1;/*置相应块的位图块所缓冲块为已修改 */
}

int new_block(int dev)/* 向设备申请一个数据块,并在内存中申请一缓冲块与该块对应以备后用 */
{
	struct buffer_head * bh;
	struct super_block * sb;
	int i,j;

	if (!(sb = get_super(dev))) /* 找到设备的超级块 */
		panic("trying to get new block from nonexistant device");
	j = 8192; /* 在设备的逻辑位图上搜索第一个0值比特位 */
	for (i=0 ; i<8 ; i++)
		if (bh=sb->s_zmap[i])
			if ((j=find_first_zero(bh->b_data))<8192)
				break;
	if (i>=8 || !bh || j>=8192)
		return 0;
	if (set_bit(j,bh->b_data))/* 置位该块对应的位图中的比特位 */
		panic("new_block: bit already set");
	bh->b_dirt = 1;/*置位缓冲区中的位图块为已修改 */
	j += i*8192 + sb->s_firstdatazone-1;/* 数据块号转实际块号 */
	if (j >= sb->s_nzones)
		return 0;
	if (!(bh=getblk(dev,j))) /* 申请一缓冲块 */
		panic("new_block: cannot get block");
	if (bh->b_count != 1) 
		panic("new block: count is != 1");
	clear_block(bh->b_data); /* 清缓冲块数据 */
	bh->b_uptodate = 1;
	bh->b_dirt = 1;
	brelse(bh); /* 释放该缓冲块（此时内存中有一干净，未锁定，未被引用的缓冲块与设备上的块相对应） */
	return j;
}

void free_inode(struct m_inode * inode)/* 释放i节点 */
{
	struct super_block * sb;
	struct buffer_head * bh;

	if (!inode)
		return;
	if (!inode->i_dev) {/* 若i节点上的设备号字段为0，说明该i节点没有使用，于是用0清空i节点所占内存区 */
		memset(inode,0,sizeof(*inode));
		return;
	}
	if (inode->i_count>1) {
		printk("trying to free inode with count=%d\n",inode->i_count);
		panic("free_inode");
	}
	if (inode->i_nlinks)
		panic("trying to free inode with links");
	if (!(sb = get_super(inode->i_dev)))
		panic("trying to free inode on nonexistent device");
	if (inode->i_num < 1 || inode->i_num > sb->s_ninodes)/* i_num（i节点号） */
		panic("trying to free inode 0 or nonexistant inode");
	if (!(bh=sb->s_imap[inode->i_num>>13]))
		panic("nonexistent imap in superblock");
	if (clear_bit(inode->i_num&8191,bh->b_data))/*复位i节点位图中对应的比特位 */
		printk("free_inode: bit already cleared.\n\r");
	bh->b_dirt = 1;
	memset(inode,0,sizeof(*inode));/* 清除i节点结构（内存中） */
}

struct m_inode * new_inode(int dev)/* 新建i节点（在内存i节点表中获取一空闲项，从i节点位图中找一个空闲i节点） */
{
	struct m_inode * inode;
	struct super_block * sb;
	struct buffer_head * bh;
	int i,j;

	if (!(inode=get_empty_inode()))/* 从内存i节点表中获取一个空闲i节点项(inode.c,194) */
		return NULL;
	if (!(sb = get_super(dev)))/* 找到设备的超级块 */
		panic("new_inode with unknown device");
	j = 8192;
	for (i=0 ; i<8 ; i++)/* 在i节点位图中找一空闲i节点 */
		if (bh=sb->s_imap[i])
			if ((j=find_first_zero(bh->b_data))<8192)
				break;
	if (!bh || j >= 8192 || j+i*8192 > sb->s_ninodes) {
		iput(inode);/* 若未在i节点位图中找到空闲位，则放回刚才在内存i节点表中申请的i节点结构 */
		return NULL;
	}
	if (set_bit(j,bh->b_data))//* 置位i节点位图中对应的比特位 */
		panic("new_inode: bit already set");
	bh->b_dirt = 1;
	inode->i_count=1;/* 引用计数 */
	inode->i_nlinks=1;/* 文件目录项链接数 */
	inode->i_dev=dev;
	inode->i_uid=current->euid;
	inode->i_gid=current->egid;
	inode->i_dirt=1;
	inode->i_num = j + i*8192;
	inode->i_mtime = inode->i_atime = inode->i_ctime = CURRENT_TIME;
	return inode;
}