Jann Hornfun Linux fact: because MAP_SHARED|MAP_ANONYMOUS is actually a file-backed mapping under the hood, unmapping part of such a mapping does not discard the data stored in that part:
$ cat mremap.c#define _GNU_SOURCE
#include <err.h>
#include <stdio.h>
#include <sys/mman.h>
int main(void) {
char *p = mmap(NULL, 0x2000, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS, -1, 0);
if (p == MAP_FAILED) err(1, "mmap");
p[0x1000] = 'X';
if (munmap(p+0x1000, 0x1000)) err(1, "munmap");
// that 'X' we just wrote... is it gone?
// nope, let's bring it back!
p = mremap(p, 0x1000, 0x2000, MREMAP_MAYMOVE);
if (p == MAP_FAILED) err(1, "mremap");
printf("p[0x1000]='%c'\n", p[0x1000]);
}
$ gcc -o mremap mremap.c
$ ./mremap
p[0x1000]='X'
$
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