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Implenment VM features using page fault

  • lazy allocation

  • copy on write fork

  • demand paging

  • memory mapped files

Virtual Memory benefits

  • Isolation

  • level of indirection

    • trampline page
    • guard page

Information needed

  • the faulting VA -> stval register

  • type pf fault -> scause register

    • load
    • store
    • pc

  • the VA of Instruction that cause fault -> sepc register | trampframe?

lazy allocation

  • sbrk()原本应为进程改变堆的大小，分配内存
    sbrk

  • sbrk()系统调用基本不做事情，只p->size + n

  • 当要使用这一部分内存时，触发Page Fault

    • allocate 1 page
    • zero page
    • map the page
    • restart instruction

zero fill on demand

  • 初始化的page VA共用一个PA，即映射到同一个物理地址上(Read Only)
    zero

  • 当要写这些page时，触发Page Fault

    • copy
    • upgrade
    • restart instruction

copy on write fork

  • fork()时child使用parent的内存空间，即子进程映射到父进程的物理空间上(Read Only)
    cow

  • 当要写子进程，触发Page Fault

    • copy page
    • map
    • restart instruction
    • usrret

demand page

  • 当执行exec()系统调用时，load text data segment，eagarly pagetable，但是只在VA中分配，PTE不分配
    demand1

  • 当要执行程序时，触发Page Fault

    • read block/page from file into memory
    • map men into pgtbl
    • restart instruction

  • 当内存耗尽时
    demand2

    • evict a page -> file
    • use the just free page
    • restart intruction

  • 驱逐什么page? LRU and non-dirty

  • 使用PTE中自定义的RW位实现LRU和dirty
    PTE

memory mapped files

  • 当执行ld，sd指令时
    mmap

  • mmap(VA, len, protection, flags, fd, offset)映射PA到VA

  • unmap(VA, len) write back dirty block

Summary for Page Table

  • page tables + traps/page fault -> powerful elegent VM features