Writing a program to count process memory pages in xv6

Question

I'm trying to write a system call that returns the number of memory pages the current process is using but I have no idea where to start and which variables I should look at. I saw two variables sz and pgdir in proc.h. But I don't know what each of them represents exactly.

Answer 1

Looking at proc.c, you have all you want to understand the memory management:

// Grow current process's memory by n bytes.
// Return 0 on success, -1 on failure.
int
growproc(int n)
{
  uint sz;
  struct proc *curproc = myproc();
  sz = curproc->sz;
    if((sz = allocuvm(curproc->pgdir, sz, sz + n)) == 0)
      return -1;
  curproc->sz = sz;
  switchuvm(curproc);
  return 0;
}

growproc is used to increase the process memory by n bytes. This function is used by the sbrk syscall, itself called by malloc.

From this, we assert that sz from struct proc { is actually the process memory size.

Reading allocuvm from vm.c, you can see two macros:

• PGROUNDUP(size) which transform a memory size to a memory size that is rounded to next page size,
• PGSIZE which is the page size.

So, the number of pages actually used by a process is (PGROUNDUP(proc)->sz)/PGSIZE.