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