Paging in Linux

4 level paging scheme
  • 32 bit arch
    • 2 paging levels (no PAE)
      • Doesn’t use Page Upper Directory and Page Middle Directory initialized to all 0s
      • Keep in mind base address is retained for Page Uppder/Middle Directories to make sure the code can execute on 64 bit systems without crashing
        • This is done by setting both tables to have one entry only and mapping them to the Page Global Directory as usual
    • 3 paging levels (using PAE)
      • Page Global directory ==PDPT (Page Directory Pointer Table)
      • Page Upper directory not used
      • Page Middle directory ==Page Directory
      • Page Table remains the same
  • 64 bit arch
    • 4 paging levels
      • Page Global Directory->Page Upper Directory->Page Middle Directory->Page Table
  • Each process has its own Page Global Directory and Page Tables
    • When switching processes cr3 is loaded with the correct address of the Page Global Directory of the new process so proper memory reads may be performed

Why 4 level scheme for 64 bit?

  1. Each process gets private physical address space protecting against addressing errors
  2. Distinguish pages (groups of data) from page frames (physical addresses in main memory)
    • Allows for a page of data stored in a page frame to be saved to disk and reloaded into another page frame (allows for virtual memory (ch17?))

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