MIT 6.S081 Lab pgtbl

Compulsory exercises

Preparation

• reading
  

• To start the lab, switch to the pgtbl branch:

git fetch
git checkout pgtbl
make clean

Speed up system calls (easy)

• getpid()作为系统调用，每次调用时需要跳入内核并跳出，为加速pid的获取，ugetpid()函数可以通过分配虚拟地址将pid存入其中，从而实现不跳转不trap获取pid

• 修改内核函数实现ugetpid()
  

• map one read-only page at USYSCALL in proc_pagetable()

proc.c proc_pagetable()

if(mappages(pagetable, USYSCALL, PGSIZE,
            (uint64)(p->usyscall), PTE_R | PTE_U) < 0){
  uvmfree(pagetable, 0);
  return 0;
}

• allocate and initialize the page in allocproc()

proc.c allocproc()

// Allocate a usyscall page before creating a new pagetable
if((p->usyscall = (struct usyscall *)kalloc()) == 0){
  freeproc(p);
  release(&p->lock);
  return 0;
}

• free the page in freeproc()

proc.c freeproc()

if(p->usyscall) {
  kfree((void*)p->usyscall);
  p->usyscall = 0;
}

• umap USYSCALL in proc_freepagetable()

proc.c proc_freepagetable()

uvmunmap(pagetable, USYSCALL, 1, 0);

Print a page table (easy)

• RISC-V架构中的三级页表需要通过递归打印

• 添加内核函数vmprint()
  

• prototype for vmprint in defs.h

defs.h //vm.c

void            vmprint(pagetable_t);

• put vmprint() in vm.c

vm.c

// help function
void vmprinthelp(pagetable_t pagetable, int level) {
    char *dot;
    if (level == 2) dot = "..";
    if (level == 1) dot = ".. ..";
    if (level == 0) dot = ".. .. ..";
    for (int i = 0; i < 512; i++) {
        pte_t pte = pagetable[i];
        if (pte & PTE_V) {
            uint64 child = PTE2PA(pte);
            printf("%s%d: pte %p pa %p\n", dot, i, pte, child);
            if (level != 0) {
               vmprinthelp((pagetable_t)child, level - 1);               
            }
        }
    }
}

void
vmprint(pagetable_t pagetable)
{
    printf("page table %p\n", pagetable);
    vmprinthelp(pagetable, 2);
}

Detecting which pages have been accessed (hard)

• 探测页表中的页是否被访问过

• 添加pgaccess系统调用
  

• implementing sys_pgaccess() sysproc.c

sysproc.c

#ifdef LAB_PGTBL
int
sys_pgaccess(void)
{
  // lab pgtbl: your code here.
  uint64 firstpte;
  int num;
  uint64 buf;
  if (argaddr(0, &firstpte) < 0)
    return -1;
  if (argint(1, &num) < 0)
    return -1;
  if (argaddr(2, &buf) < 0)
    return -1;
  
  pagetable_t pagetable = myproc()->pagetable;
  pgaccess(pagetable, firstpte, num, buf);

  return 0;
}
#endif

• define PTE_A, the access bit, in riscv.h 参考下图PTE_A的位置
  

riscv.h

#define PTE_A (1L << 6) // 1 -> Accessed

• add pgaccess() in vm.c

  • 注意第一个page对应mask最低有效位
  • 注意探测一个带有PTE_A的page过后要消除PTE_A

vm.c

uint64
pgaccess(pagetable_t pagetable, uint64 firstpte, int num, uint64 buf) {
  uint64 mask = 0;
  uint64 limit = 64;

  for (uint64 i = 0; i < num && i < limit; i++) {
    pte_t *pte_ptr;
    pte_ptr = walk(pagetable, firstpte + i * (uint64)PGSIZE, 0);
    if ((pte_ptr != 0) && (*pte_ptr & PTE_A)) {
        mask = mask | (1L << i);
        *pte_ptr = *pte_ptr & (~PTE_A);
    }
  }

  copyout(pagetable, buf, (char *)&mask, 8);
  return 0;    
}

Optional challenge exercises

• Use super-pages to reduce the number of PTEs in page tables.

• Unmap the first page of a user process so that dereferencing a null pointer will result in a fault. You will have to start the user text segment at, for example, 4096, instead of 0.

• Add a system call that reports dirty pages (modified pages) using PTE_D.