remap_page_range(), mmap(), and /proc

[Ray Strode <halfline@hawaii.rr.com>]

There is some code written by someone I know that works on some kernels, 
but not
others.  In particular, it doesn't work on 2.4.18-3 from redhat 7.3, but 
it does work
on later versions of 2.4.18 from redhat and from 2.4.19.  I'm wondering 
if the code is
wrong, but just happens to work with certain kernel versions.  Also, if 
that is the
case, I would like to know what is wrong.  The code is for a kernel 
module that tries
to share a variable between kernelspace and userspace via 
remap_page_range()/mmap()
and the /proc filesystem.  The variable gets incremented in kernelspace 
each time the mouse
is moved.  Depending on the kernel, the variable in userspace may or may 
not get updated.
The code is small, so i'm just attaching it.

I would appreciate any help at all, even if no one knows exactly why 
it's behaving this way.

Thanks,
Ray Strode

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

int
main(int argc, char *argv[])
	{
	int fd, sc;
	int *ptr;

	fd = open("/proc/seqcnt", O_RDWR);
	if (fd < 0)
		{
		perror("open");
		return -1;
		}
	ptr = mmap(NULL, sizeof(int), PROT_READ|PROT_WRITE,
		   MAP_SHARED/*|MAP_ANONYMOUS*/, fd, 0);
	if (ptr == MAP_FAILED)
		{
		perror("mmap");
		return -1;
		}
	printf("mmap ptr = %p\n", ptr);

	/* Validate result. */
	printf("mmap original = %d\n", *ptr);
	*ptr = argc > 1 ? atoi(argv[1]) : 1234;
	printf("mmap modified = %d\n", *ptr);
	*ptr = 0;
	printf("mmap cleared = %d\n", *ptr);

	read(fd, &sc, sizeof(sc));
	printf("io original = %d\n", sc);
	sc = argc > 1 ? atoi(argv[1]) : 1234;
	write(fd, &sc, sizeof(sc));
	read(fd, &sc, sizeof(sc));
	printf("io modified = %d\n", sc);
	printf("mmap test = %d\n", *ptr);
	sc = 0;
	write(fd, &sc, sizeof(sc));
	read(fd, &sc, sizeof(sc));
	printf("io cleared = %d\n", sc);

	/* Allow something to happen. */
	printf("sleep...\n");
	sleep(5);
	msync(ptr, sizeof(int), MS_SYNC|MS_INVALIDATE);
	printf("mmap final = %d\n", *ptr);
	read(fd, &sc, sizeof(sc));
	printf("io final = %d\n", sc);

	if (munmap(ptr, sizeof(int)) < 0)
		perror("munmap");
	close(fd);
	return 0;
	}