ksm: take dirty bit as reference to avoid volatile pages scanning

Compared to the first version, this patch set addresses the problem of dirty bit updating of virtual machines, by adding two mmu_notifier interfaces. So it can now track the volatile working set inside KVM guest OS.

V1 log:
Currently, ksm uses page checksum to detect volatile pages. Izik Eidus suggested that we could use pte dirty bit to optimize. This patch series adds this new logic.

Preliminary benchmarks show that the scan speed is improved by up to 16 times on volatile transparent huge pages and up to 8 times on volatile regular pages.

Following is the test program to show this top speed up (you need to make ksmd takes about more than 90% of the cpu and watch the ksm/full_scans).

  #include <stdio.h>
  #include <stdlib.h>
  #include <errno.h>
  #include <string.h>
  #include <unistd.h>
  #include <sys/mman.h>
  
  #define MADV_MERGEABLE   12
  
  
  #define SIZE (2000*1024*1024)
  #define PAGE_SIZE 4096
  
  int main(int argc, char **argv)
  {
        unsigned char *p;
        int j;
        int ret;

          p = mmap(NULL, SIZE, PROT_WRITE|PROT_READ,
                   MAP_PRIVATE|MAP_ANONYMOUS, 0, 0);
        
        if (p == MAP_FAILED) {
                printf("mmap error\n");
                return 0;
        }
      
          ret = madvise(p, SIZE, MADV_MERGEABLE);
      
          if (ret==-1) {
                  printf("madvise failed \n");
                  return 0;
          }
  
        
        memset(p, 1, SIZE);
  
        while (1) {
                for (j=0; j<SIZE; j+=PAGE_SIZE) {
                        *((long*)(p+j+PAGE_SIZE-4)) = random();
                }
        }
  
        return 0;

}

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