带有MAP_POPULATE的Linux mmap(),手册页似乎给出了错误的信息

发布时间：2020-12-14 01:01:34 所属栏目：Linux 来源：网络整理

导读：在我看来,MAP_POPULATE不仅仅适用于私有映射,与手册页所说的相反： MAP_POPULATE (since Linux 2.5.46) Populate (prefault) page tables for a mapping. For a file mapping,this causes read-ahead on the file. Later accesses to the mapping will not b

在我看来,MAP_POPULATE不仅仅适用于私有映射,与手册页所说的相反：

MAP_POPULATE (since Linux 2.5.46) Populate (prefault) page tables for
a mapping. For a file mapping,this causes read-ahead on the file.
Later accesses to the mapping will not be blocked by page faults.
MAP_POPULATE is only supported for private mappings since Linux
2.6.23.

基于我对mmap.c的Linux内核源的粗略检查,看起来MAP_POPULATE与versions going back as far as 2.6.34中的MAP_PRIVATE完全没有关系.

以下是3.14内核实现中mmap.c的最新代码的摘录：

...
1364         addr = mmap_region(file,addr,len,vm_flags,pgoff);
1365         if (!IS_ERR_VALUE(addr) &&
1366             ((vm_flags & VM_LOCKED) ||
1367              (flags & (MAP_POPULATE | MAP_NONBLOCK)) == MAP_POPULATE))
1368                 *populate = len;
1369         return addr;
...

我的观察错了吗？

解决方法

自您提出问题后,措辞已更新：

Before: MAP_POPULATE is only supported for private mappings since Linux 2.6.23.
After:  MAP_POPULATE is supported for private mappings only since Linux 2.6.23.

换句话说,您可以将MAP_POPULATE用于私有映射,但至少需要Linux 2.6.23.

希望这能澄清它！

（编辑：李大同）

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