pmemTest.c

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

#define PAGE_SHIFT 12
#define PAGEMAP_LENGTH 8

typedef unsigned long ul;

// Function that returns page frame number of the virtual address that is passed
// in as 'addr'.
ul get_pfn(ul addr) {
	// https://shanetully.com/2014/12/translating-virtual-addresses-to-physcial-addresses-in-user-space/
	// https://www.kernel.org/doc/Documentation/vm/pagemap.txt
	FILE *pfd = fopen("/proc/self/pagemap", "rb");
	if (pfd == NULL) {
		printf("Error opening pagemanp = %s\n", strerror(errno));
	}

	ul pfn = 0, i = 1;
	ul offset = (addr / getpagesize()) * PAGEMAP_LENGTH;

	if(fseek(pfd, offset, SEEK_SET) != 0) {
		fprintf(stderr, "Failed to seek pagemap to proper location\n");
		exit(1);
	}

	fread(&pfn, PAGEMAP_LENGTH, 1, pfd);
	fclose(pfd);

	/*
	 * Bits 0-54  page frame number (PFN) if present
	 * Bits 0-4   swap type if swapped
	 * Bits 5-54  swap offset if swapped
	 * Bit  55    pte is soft-dirty (see Documentation/vm/soft-dirty.txt)
	 * Bit  56    page exclusively mapped (since 4.2)
	 * Bits 57-60 zero
	 * Bit  61    page is file-page or shared-anon (since 3.5)
	 * Bit  62    page swapped
	 * Bit  63    page present
	 */

	printf("page present:%lu page swapped:%lu page exclusively mapped:%lu"\
			" file page or shared-anon:%lu pfn:%lu\n", (pfn>>63)&i, (pfn>>62)&i,\
			(pfn>>56)&i, (pfn>>61)&i,(pfn&((i<<55) - i)));

	// return page frame number (bits 0 to 54)
	return (pfn&((i<<55) - i));
}

int open_memory(void) {
	int fd = open("/dev/mem", O_RDWR);

	if(fd == -1) {
		fprintf(stderr, "Error opening /dev/mem: %s\n", strerror(errno));
		exit(1);
	}

	return fd;
}

void seek_memory(int fd, unsigned long offset) {
	unsigned pos = lseek(fd, offset, SEEK_SET);

	if(pos == -1) {
		fprintf(stderr, "Failed to seek /dev/mem: %s\n", strerror(errno));
		exit(1);
	}
}

int main() {
	//int pmem_fd = open("/mnt/ext4-pmem0/testflush", O_RDWR|O_SYNC);
	int pmem_fd = open("/dev/dax0.0", O_RDWR|O_SYNC);
	if (pmem_fd < 0) {
		printf("unable to open file, err=%s\n", strerror(errno));
		return -1;
	}

	char* pmemaddr = mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED, pmem_fd, 0);
	if (pmemaddr == MAP_FAILED) {
		printf("mmap failed with error=%s\n", strerror(errno));
		return -1;
	}
	close(pmem_fd);
	printf("pmemaddr is %p\n", pmemaddr);

	// write 0x88 0x77 0x66 .. into first 8 bytes of mapped memory
	for(int i=0;i<8;i++) {
		*(pmemaddr+i) = (0x11*(i+1));
	}

	// get page frame number of mapped virtual address
	ul pfn = get_pfn((ul)(pmemaddr));

	// shift pfn by PAGE_SHIFT to get offset in /dev/mem where the data we wrote
	// should be.
	uint64_t offset = (pfn << PAGE_SHIFT);

	int mem_fd = open_memory();
	seek_memory(mem_fd, offset);

	ul tmp;
	if(read(mem_fd, &tmp, 8) == -1) {
		fprintf(stderr, "Read failed: %s\n", strerror(errno));
	} else {
		printf("read from device: %lx\n", tmp);
	}

	if (munmap(pmemaddr, 4096) != 0) {
		printf("munmap failed %s\n", strerror(errno));
	}

	return 0;
}