gistfile1.txt

#define _GNU_SOURCE
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <sys/types.h>

int main(int argc, char **argv)
{
	char *p;
	int fd;

	/* Create a file and populate first page of page cache */
	fd = open("/tmp", O_TMPFILE | O_RDWR, S_IRUSR | S_IWUSR);
	write(fd, "1", 1);

	/* Create a *read-only* *private* mapping of the file */ 
	p = mmap(NULL, 4096, PROT_READ, MAP_PRIVATE, fd, 0);

	/*
	 * Since the mapping is read-only, mlock() will populate the mapping
	 * with PTEs pointing to page cache without triggering COW.
	 */
	mlock(p, 4096);

	/* 
	 * Mapping became read-write, but it's still populated with PTEs
	 * pointing to page cache.
	 */
	mprotect(p, 4096, PROT_READ | PROT_WRITE);

	/* Trigger COW: major fault in mlock()ed VMA. */
	*p = 1;

	return 0;
}