A simple program to illustrate the effect of MAP_NORESERVE in mmap.

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#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>

static void meminfo() {
    system("cat /proc/meminfo | grep Committed_AS");
}

int main() {
    // to flush meminfo output
    setbuf(stdout, NULL);

    size_t one_g = 1024*1024*1024; // 1G
    size_t size = 5 * one_g;

    {
        puts("Without MAP_NORESERVE (i.e. with-reserve)");
        puts("Before:");
        meminfo();

        void* ptr = mmap(NULL, size,
                         PROT_READ | PROT_WRITE,
                         MAP_PRIVATE | MAP_ANONYMOUS,
                         -1, 0);
        if (ptr == MAP_FAILED) {
            perror("mmap");
            return 1;
        }

        puts("After:");
        meminfo();

        munmap(ptr, size);
    }

    puts("");

    {
        puts("With MAP_NORESERVE");
        puts("Before:");
        meminfo();

        void* ptr = mmap(NULL, size,
                         PROT_READ | PROT_WRITE,
                         MAP_PRIVATE | MAP_ANONYMOUS | MAP_NORESERVE,
                         -1, 0);
        if (ptr == MAP_FAILED) {
            perror("mmap");
            return 1;
        }

        puts("After:");
        meminfo();

        munmap(ptr, size);
    }


    return 0;
}

Output on my box:

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Without MAP_NORESERVE (i.e. with-reserve)
Before:
Committed_AS:   14575316 kB
After:
Committed_AS:   19818196 kB

With MAP_NORESERVE
Before:
Committed_AS:   14575448 kB
After:
Committed_AS:   14575316 kB

(The negative naming, NORESERVE, is super confusing, but all in all accounting happens when we reserve.)

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