聊聊文件系统的原理

// 这里是以 ext4本地文件系统为例
const struct inode_operations ext4_dir_inode_operations = {
    // 文件创建函数
    .create = ext4_create,
    // 查找函数
    .lookup = ext4_lookup,
    /****** 省略其他的定义的函数*****/
};
<p>const struct file_operations ext4_file_operations = {
.llseek = ext4_llseek,
// 读函数
.read_iter = ext4_file_read_iter,
// 写函数
.write_iter = ext4_file_write_iter,
.unlocked_ioctl = ext4_ioctl,</p><h1>ifdef CONFIG_COMPAT</h1><pre class="brush:php;toolbar:false;"><code>.compat_ioctl = ext4_compat_ioctl,
#endif
.mmap = ext4_file_mmap,
.mmap_supported_flags = MAP_SYNC,
// open函数对应实际文件系统的函数
.open = ext4_file_open,
.release = ext4_release_file,
.fsync = ext4_sync_file,
.get_unmapped_area = thp_get_unmapped_area,
.splice_read = generic_file_splice_read,
.splice_write = iter_file_splice_write,
.fallocate = ext4_fallocate,

// write函数进入内核态的ksys_write
// fd是已经打开的文件描述符，buf是需要写入的数据，count是写入的长度
ssize_t ksys_write(unsigned int fd, const char </strong>user *buf, size_t count) {
// f 代表打开的文件和打开文件的flag
struct fd f = fdget_pos(fd);
ssize_t ret = -EBADF;
// 如果打开的文件为空，则退出
if (f.file) {
// 获取文件读写的位置
loff_t pos = file_pos_read(f.file);
// 进入vfs_write的函数，接着处理文件写操作
ret = vfs_write(f.file, buf, count, &pos);
if (ret >= 0)
file_pos_write(f.file, pos);
fdput_pos(f);
}
return ret;
}</p><p>// vfs_write包装了ext4_file_write_iter 函数
ssize_t vfs_write(struct file <em>file, const char __user </em>buf, size_t count, loff_t *pos) {
ssize_t ret;
// 判断文件是否写入，如果不能则返回错误码
if (!(file->f_mode & FMODE_WRITE))
return -EBADF;
if (!(file->f_mode & FMODE_CAN_WRITE))
return -EINVAL;
if (unlikely(!access_ok(VERIFY_READ, buf, count)))
return -EFAULT;
// 检查文件系统可写区域以及锁检查
ret = rw_verify_area(WRITE, file, pos, count);
if (!ret) {
if (count > MAX_RW_COUNT)
count =  MAX_RW_COUNT;
file_start_write(file);
// 调用ext4_file_write_iter 开始写数据
ret = __vfs_write(file, buf, count, pos);
if (ret > 0) {
fsnotify_modify(file);
add_wchar(current, ret);
}
inc_syscw(current);
file_end_write(file);
}
return ret;
}</p><p>// <strong>vfs_write是包装函数
ssize_t </strong>vfs_write(struct file <em>file, const char __user </em>p, size_t count, loff_t *pos) {
if (file->f_op->write)
return file->f_op->write(file, p, count, pos);
else if (file->f_op->write_iter)
return new_sync_write(file, p, count, pos);
else
return -EINVAL;
}</p><p>// 把用户需要写入的数据封装为struct iovec,然后把这个iovec和fd对应的struct file，传递给时间文件系统的函数，进行文件写入。
static ssize_t new_sync_write(struct file <em>filp, const char __user </em>buf, size_t len, loff_t <em>ppos) {
struct iovec iov = { .iov_base = (void __user </em>)buf, .iov_len = len };
struct kiocb kiocb;
struct iov_iter iter;
ssize_t ret;</p><pre class="brush:php;toolbar:false;"><code>init_sync_kiocb(&kiocb, filp);
kiocb.ki_pos = *ppos;
iov_iter_init(&iter, WRITE, &iov, 1, len);
ret = call_write_iter(filp, &kiocb, &iter);
BUG_ON(ret == -EIOCBQUEUED);
if (ret > 0)
    *ppos = kiocb.ki_pos;
return ret;