带有 MAP_POPULATE 的 Linux mmap()，手册页似乎提供了错误的信息

Question

在我看来，MAP_POPULATE 不仅仅用于私有(private)映射，这与手册页所说的相矛盾:

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 内核源代码的粗略检查，看起来 MAP_POPULATE 与 MAP_PRIVATE 在 versions going back as far as 2.6.34 中完全没有关系.

这是来自 mmap.c 的最新代码的摘录从 3.14 内核实现开始:

...
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;
...

我的观察有误吗？

Answer 1

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，您可以使用 MAP_POPULATE 进行私有(private)映射，但至少需要 Linux 2.6.23。

希望这能澄清它!