Re: [PATCH v4] mm: introduce reference pages

["Kirill A. Shutemov" <kirill@shutemov.name>]

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