if you mmap an shm_open created fd, then ftruncate+mmap again, do you lose the memory you've been writing to?

shm_test.c

#include <stdbool.h>
#include <stdio.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <sys/mman.h>

void print_region(char *ptr, size_t len);

// mark a few locations in a pointer
void mark(char *ptr, size_t len) {
    int i;
    // do not mark position 0
    for (i=1; i<len; i++) {
        if (!(i%17)) {
            ptr[i] = 1;
        }
        if (!(i%23)) {
            ptr[i] = 2;
        }
    }
}

// check for the marks from mark
bool check(char *ptr, size_t len) {
    int i;
    for (i=1; i<len; i++) {
        if (!(i%17) && (i%23)) {
            if (ptr[i] != 1) {
                return false;
            }
        } else if (!(i%23)) {
            if (ptr[i] != 2) {
                return false;
            }
        } else if (ptr[i] != 0) {
            return false;
        }
    }
    return true;
}

int main(int argc, char **argv) {
    int i;
    int fd, ok, max;
    char *ptr;
    
    shm_unlink("/test");

    // create new shm
    fd = shm_open("/test", O_CREAT|O_RDWR|O_TRUNC, 0);
    if (fd < 0) {
        perror("could not create shm_open fd");
        return;
    }

    // make shm 1k
    ok = ftruncate(fd, 1024);

    if (ok < 0) {
        perror("initial ftruncate");
        goto done;
    }

    ptr = (char *)mmap(NULL, 1024, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
    if (ptr == MAP_FAILED) {
        printf("fd: %d\n", fd);
        perror("map failed");
        goto done;
    }

    mark(ptr, 1024);
    if (!check(ptr, 1024)) {
        printf("region failed check");
        goto done;
    }

    max = 1024*1024;
    printf("checking from size=1024 to size=%d\n", max);
    for (i=2; i<=max/1024; i*=2) {
        int size=1024*i;

        ok = ftruncate(fd, size);
        if (ok < 0) {
            perror("second ftruncate");
            goto done;
        }

        ptr = (char *)mmap(NULL, size, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);

        if (!check(ptr, size/2)) {
            printf("region failed check;  mmap overwrote?");
            goto done;
        }

        if (check(ptr, size)) {
            printf("newly mapped region unexpectedly passed check");
            goto done;
        }

        mark(ptr, size);
        if (!check(ptr, size)) {
            printf("region failed check");
            goto done;
        }
    }

done:
    shm_unlink("/test");
}

then what‘s the concluson under linux.