ksm: take dirty bit as reference to avoid volatile pages

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;
  }

Hi,

One small note:
When kvm will use ksm on intel cpu with extended page tables support, the cpu won`t track dirty bit, therefore the calc_hash() logic should be used in such cases (untill intel will fadd this support in their cpus)...

Moreover I think that even though that AMD nested page tables does update dirty bit, you still need to sync it with the host page table using mmu notifiers ?

(Not that on regular application use case of ksm any of this should be an issue)

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Hmm, I will consider these two issues in the next version. Thanks for input!

Nai