Great question—since you’re deep into Understanding the Linux Kernel, let’s connect the book’s 32-bit page table explanation directly to how 64-bit kernels handle 32-bit applications in compatibility mode.

First, let’s anchor to the detail you pulled from the book:

对于无Physical Address Extension的32位架构，两级分页已足够。Linux通过将Page Upper Directory和Page Middle Directory字段设为零位来简化，但保留它们在指针序列中的位置，以便相同代码可在32位和64位架构上运行。

This unified page table framework is exactly what enables 64-bit kernels to seamlessly run 32-bit apps without rewriting core memory management code. Here’s the breakdown:

• Reuse the 4-level page table hierarchy: 64-bit x86 kernels natively use a 4-level structure (PGD → PUD → PMD → PT). Even though 32-bit non-PAE apps only need two levels (Page Directory → Page Table), Linux doesn’t split its codebase for compatibility. Instead, it maps 32-bit virtual addresses into the 64-bit structure.
• Zero out unused directory fields: For a 32-bit app’s virtual address, the bits that would normally map to the PUD (Page Upper Directory) and PMD (Page Middle Directory) in a 64-bit address are hardcoded to 0. When the kernel traverses the page table, these levels act as no-ops—they point to a single entry that forwards straight to the relevant Page Directory, effectively collapsing the 4-level structure into the 2-level one 32-bit apps need.
• Maintain code consistency: By keeping PUD and PMD in the pointer sequence (even when they’re zeroed), the same page table traversal logic works for both 64-bit native apps and 32-bit compatibility mode apps. No conditional branches or separate code paths are required based on the app’s bitness.
• Preserve address space isolation: The 64-bit kernel still creates separate page tables for each 32-bit app, just like it does for native 64-bit processes. The zeroed PUD/PMD fields simply constrain the app’s virtual address space to the 32-bit range, while the kernel itself uses the full 64-bit address space for its own operations.

Put simply, Linux’s forward-thinking choice to retain a 4-level page table structure (even when parts are unused) is the secret sauce that makes 32-bit app compatibility on 64-bit kernels so straightforward.

内容的提问来源于stack exchange，提问作者idlmn89