MAP_FIXED标志的疑惑

unsigned long
1371 get_unmapped_area_prot(struct file *file, unsigned long addr, unsigned long len,
     /* */
1372 unsigned long pgoff, unsigned long flags, int exec)
1373 {
1374 unsigned long ret;
1375
1376 if (!(flags & MAP_FIXED)) {
1377 unsigned long (*get_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
1378
1379 if (exec && current->mm->get_unmapped_exec_area)
1380 get_area = current->mm->get_unmapped_exec_area;
1381 else
1382 get_area = current->mm->get_unmapped_area;
1383
1384 if (file && file->f_op && file->f_op->get_unmapped_area)
1385 get_area = file->f_op->get_unmapped_area;
1386 addr = get_area(file, addr, len, pgoff, flags);
1387 if (IS_ERR_VALUE(addr))
1388 return addr;
1389 }
1390
1391 if (addr > TASK_SIZE - len)
1392 return -ENOMEM;
1393 if (addr & ~PAGE_MASK)
1394 return -EINVAL;
1395 if (file && is_file_hugepages(file)) {
1396 /*
1397 * Check if the given range is hugepage aligned, and
1398 * can be made suitable for hugepages.
1399 */
1400 ret = prepare_hugepage_range(addr, len, pgoff);
1401 } else {
1402 /*
1403 * Ensure that a normal request is not falling in a
1404 * reserved hugepage range. For some archs like IA-64,
1405 * there is a separate region for hugepages.
1406 */
1407 ret = is_hugepage_only_range(current->mm, addr, len);
1408 }
1409 if (ret)
1410 return -EINVAL;
1411 return addr;
1412 }