The Linux kernel makes extensive usage of the paging features of x86 to allow fast process switches with small data fragmentation.
There are also however some features that the Linux kernel might not use, either because they are only for backwards compatibility, or because the Linux devs didn't feel it was worth it yet.
The Linux Kernel reserves two zones of virtual memory:
  • one for kernel memory
  • one for programs
The exact split is configured by CONFIG_VMSPLIT_.... By default:
  • on 32-bit:
    • the bottom 3/4 is program space: 00000000 to BFFFFFFF
    • the top 1/4 is kernel memory: C0000000 to FFFFFFFF, like this:
      ------------------ FFFFFFFF
      ------------------ C0000000
      ------------------ BFFFFFFF
      ------------------ 00000000
  • on 64-bit: currently only 48-bits are actually used, split into two equally sized disjoint spaces. The Linux kernel just assigns:
    • the bottom part to processes 00000000 00000000 to 008FFFFF FFFFFFFF
    • the top part to the kernel: FFFF8000 00000000 to FFFFFFFF FFFFFFFF, like this:
      ------------------ FFFFFFFF
      ------------------ C0000000
      (not addressable)
      ------------------ BFFFFFFF
      ------------------ 00000000
Kernel memory is also paged.
For each process, the virtual address space looks like this:
------------------ 2^32 - 1
Stack (grows down)
v v v v v v v v v


------------------ Maximum stack size.




brk (grows up)

------------------- 0
The kernel maintains a list of pages that belong to each process, and synchronizes that with the paging.
If the program accesses memory that does not belong to it, the kernel handles a page-fault, and decides what to do:
  • if it is above the maximum stack size, allocate those pages to the process
  • otherwise, send a SIGSEGV to the process, which usually kills it
When an ELF file is loaded by the kernel to start a program with the exec system call, the kernel automatically registers text, data, BSS and stack for the program.
The brk and mmap areas can be modified by request of the program through the brk and mmap system calls. But the kernel can also deny the program those areas if there is not enough memory.
brk and mmap can be used to implement malloc, or the so called "heap".
mmap is also used to load dynamically loaded libraries into the program's memory so that it can access and run it.
Calculating exact addresses Things are complicated by:
Besides a missing page, a very common source of page faults is copy-on-write (COW).
Page tables have extra flags that allow the OS to mark a page a read-only.
Those page faults only happen when a process tries to write to the page, and not read from it.
When Linux forks a process:
  • instead of copying all the pages, which is unnecessarily costly, it makes the page tables of the two process point to the same physical address.
  • it marks those linear addresses as read-only
  • whenever one of the processes tries to write to a page, the makes a copy of the physical memory, and updates the pages of the two process to point to the two different physical addresses
In v4.2, look under arch/x86/:
  • include/asm/pgtable*
  • include/asm/page*
  • mm/pgtable*
  • mm/page*
There seems to be no structs defined to represent the pages, only macros: include/asm/page_types.h is specially interesting. Excerpt:
#define _PAGE_BIT_PRESENT   0   /* is present */
#define _PAGE_BIT_RW        1   /* writeable */
#define _PAGE_BIT_USER      2   /* userspace addressable */
#define _PAGE_BIT_PWT       3   /* page write through */
arch/x86/include/uapi/asm/processor-flags.h defines CR0, and in particular the PG bit position:
#define X86_CR0_PG_BIT      31 /* Paging */

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