Why are the PAGE_SIZE and the physical page frame size different in x86_64 kernel?

[Hao Lee]

Hi,

I'm confused about the meaning of the PAGE_SIZE macro in x86_64
kernel. Its value is 4KB and I once thought it was the size of a
physical page frame, but now I think I'm wrong. I find that most of
the physical address space is mapped by PMD entries, which means the
actual physical page frame size is 2MB. Could someone explain why the
PAGE_SIZE is not 2MB in x86_64 kernel? Thanks!

Regards,
Hao Lee