Re: [PATCH v4] mm: introduce reference pages

On Sat, Jun 19, 2021 at 02:20:02AM -0700, Peter Collingbourne wrote:

  #include <stdio.h>
  #include <stdlib.h>
  #include <string.h>
  #include <sys/mman.h>
  #include <unistd.h>

  constexpr unsigned char pattern_byte = 0xaa;

  #define PAGE_SIZE 4096

  _Alignas(PAGE_SIZE) static unsigned char pattern[PAGE_SIZE];

  int main(int argc, char **argv) {
    if (argc < 3)
      return 1;
    bool use_refpage = argc > 3;
    size_t mmap_size = atoi(argv[1]);
    size_t touch_size = atoi(argv[2]);

    int refpage_fd;
    if (use_refpage) {
      memset(pattern, pattern_byte, PAGE_SIZE);
      refpage_fd = syscall(448, pattern, 0);
    }
    for (unsigned i = 0; i != 1000; ++i) {
      char *p;
      if (use_refpage) {
        p = (char *)mmap(0, mmap_size, PROT_READ | PROT_WRITE, MAP_PRIVATE,
                         refpage_fd, 0);
      } else {
        p = (char *)mmap(0, mmap_size, PROT_READ | PROT_WRITE,
                         MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
        memset(p, pattern_byte, mmap_size);
      }
      for (unsigned j = 0; j < touch_size; j += PAGE_SIZE)
        p[j] = 0;
      munmap(p, mmap_size);
    }
  }

I don't like the inteface. It is tied to PAGE_SIZE and this doesn't seem
to be very future looking. How would it work with THPs?

Maybe we should cosider passing down a filling pattern to kernel and let
kernel allocate appropriate page size on read page fault? The pattern has
to be power of 2 and limited in lenght.

-- 
 Kirill A. Shutemov