POSIX_FADV_DONTNEED troubles

test.c

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

#define BUFFER_SIZE 8192

int
main(int argc, char **argv)
{
        char *mapped;
        char *buffer;
        int fd;
        int rc;
        char pagevector[1];

        /* get some clean data into the page cache */
        buffer = malloc(BUFFER_SIZE);
        assert(buffer != NULL);
        fd = open("my_file", O_RDWR | O_CREAT, 0664);
        assert(fd >= 0);
        memset(buffer, 0, BUFFER_SIZE);
        rc = write(fd, buffer, BUFFER_SIZE);
        assert(rc == BUFFER_SIZE);
        rc = fsync(fd);
        assert(rc == 0);
        rc = lseek(fd, 0, SEEK_SET);
        assert(rc == 0);
        rc = read(fd, buffer, BUFFER_SIZE);
        assert(rc == BUFFER_SIZE);
        mapped = mmap(NULL, BUFFER_SIZE, PROT_READ, MAP_FILE | MAP_PRIVATE, fd, 0);
        assert(mapped != NULL);
        rc = mincore(mapped, 4096, pagevector);
        assert(rc == 0);
        assert(pagevector[0] & MINCORE_INCORE);

        /* kick it out of the page cache */
        rc = posix_fadvise(fd, 0, 8192, POSIX_FADV_DONTNEED);
        assert(rc == 0);
        rc = mincore(mapped, 4096, pagevector);
        assert(rc == 0);
        assert(!(pagevector[0] & MINCORE_INCORE)); /* <-- fails.  why? */
        return 0;
}