mmap MAP_NORESERVE of 2^42 bytes consumes 16Gb of actual RAM

This bug has been copied automatically from: https://bugs.launchpad.net/qemu/+bug/1898011
Reported by 'Kostya Serebryany' on 2020-10-01 :

Run this program:

#include <sys/mman.h>
#include <stdio.h>
int main() {
        for (int i = 30; i <= 44; i++) {
                fprintf(stderr, "trying 2**%d\n", i);
                mmap((void*)0x600000000000,1ULL << i,
                        PROT_NONE,
MAP_PRIVATE|MAP_ANONYMOUS|MAP_FIXED|MAP_NORESERVE,-1,0);
        }
}

(tried qemu-x86_64 and qemu-aarch64, 4.2.1 and trunk/5.1.50)

On each iteration qemu will consume 2x more physical RAM,
e.g. when mapping 2^42 it will have RSS of 16Gb.

On normal linux it works w/o consuming much RAM, due to MAP_NORESERVE.

Also: qemu -strace prints 0 instead of the correct size starting from
size=2^32
and prints -2147483648 for size=2^31.

mmap(0x0000600000000000,1073741824,PROT_NONE,MAP_PRIVATE|MAP_ANONYMOUS|MAP_FIXED|MAP_NORESERVE,-1,0)
= 0x0000600000000000

mmap(0x0000600000000000,-2147483648,PROT_NONE,MAP_PRIVATE|MAP_ANONYMOUS|MAP_FIXED|MAP_NORESERVE,-1,0)
= 0x0000600000000000

mmap(0x0000600000000000,0,PROT_NONE,MAP_PRIVATE|MAP_ANONYMOUS|MAP_FIXED|MAP_NORESERVE,-1,0)
= 0x0000600000000000