static struct fdtable *alloc_fdtable(int nr)
{
 struct fdtable *fdt = NULL;
 int nfds = 0;
   fd_set *new_openset = NULL, *new_execset = NULL;
 struct file **new_fds;

 fdt = kmalloc(sizeof(*fdt), GFP_KERNEL);
 if (!fdt)
    goto out;
 memset(fdt, 0, sizeof(*fdt));

 nfds = __FD_SETSIZE;  //#define __FD_SETSIZE 1024
　　　　        //  #define PAGE_SHIFT  12
　　　　        //  #define PAGE_SIZE  (1UL << PAGE_SHIFT)
   /* Expand to the max in easy steps */
   do {
  if (nfds < (PAGE_SIZE * 8))//dfds = 1024
   nfds = PAGE_SIZE * 8;
  else {
   nfds = nfds * 2;
   if (nfds > NR_OPEN)
    nfds = NR_OPEN;
  }
 } while (nfds <= nr);//第一次expand时，nr应该等于32

   new_openset = alloc_fdset(nfds);//分配打开文件位图
   new_execset = alloc_fdset(nfds);
   if (!new_openset || !new_execset)
    goto out;
 fdt->open_fds = new_openset;
 fdt->close_on_exec = new_execset;
 fdt->max_fdset = nfds;//更新max_fdset值，此时这个值为32k

 nfds = NR_OPEN_DEFAULT;//nfds = 32
 /*
  * Expand to the max in easy steps, and keep expanding it until
  * we have enough for the requested fd array size.
  */
 do {
#if NR_OPEN_DEFAULT < 256
  if (nfds < 256)
   nfds = 256;//nfds = 256（32->256->1024）
   //无法超过1024，因为在最开始的就进行了检查，一定要小于current->signal->rlim[RLIMIT_NOFILE].rlim_cur）
  else
#endif
  if (nfds < (PAGE_SIZE / sizeof(struct file *)))
   nfds = PAGE_SIZE / sizeof(struct file *);
  else {
   nfds = nfds * 2;
   if (nfds > NR_OPEN)
    nfds = NR_OPEN;
    }
 } while (nfds <= nr);
 new_fds = alloc_fd_array(nfds);//分配文件描述符数组
 if (!new_fds)
  goto out;
 fdt->fd = new_fds;
 fdt->max_fds = nfds;//更新max_fds
 fdt->free_files = NULL;
 return fdt;
out:
   if (new_openset)
    free_fdset(new_openset, nfds);
   if (new_execset)
    free_fdset(new_execset, nfds);
 kfree(fdt);
 return NULL;
}