问mremap实际上是做什么的？

EN

void *new_address = mremap(old_address, old_size, old_size, 
    MREMAP_MAYMOVE | MREMAP_FIXED | MREMAP_DONTUNMAP, new_address);

/* The purpose of this program is to glue together physically allocated pieces of 
 * memory into a contiguous range of virtual addresses. */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <signal.h>
#include <stdint.h>
#include <time.h>
#define __USE_GNU
#include <sys/mman.h>

/* Initializes numberOfSegments pieces of page-aligned memory of size 
 * physicalSize, where the ith segment is initialized to (char) i. */
void ** allocatePhysicalMem(int numberOfSegments, size_t physicalSize)
{
    void **physicalMems = malloc(numberOfSegments * sizeof(void *));

    for (int i = 0; i < numberOfSegments; i++) {
        physicalMems[i] = mmap(NULL, physicalSize, PROT_READ | PROT_WRITE,
                MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
        memset(physicalMems[i], (char) i, physicalSize);
    }

    return physicalMems;
}

void cleanup(void **physicalMems, size_t physicalSize, int numberOfSegments)
{
    for (int i = 0; i < numberOfSegments; i++) {
        munmap(physicalMems[i], physicalSize);
    }

    free(physicalMems);
}

int main()
{
    int numberOfSegments = 4;
    size_t physicalSize = 4096;
    size_t virtualSize = numberOfSegments * physicalSize;

    void *virtualMem = mmap(NULL, virtualSize, PROT_READ | PROT_WRITE,
                    MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
    void **physicalMems = allocatePhysicalMem(numberOfSegments, physicalSize);
    
    for (int i = 0; i < numberOfSegments; i++) {
        void *segment = (void *)((uintptr_t) virtualMem + i * physicalSize);
        segment = mremap(physicalMems[i], physicalSize, physicalSize, 
                MREMAP_MAYMOVE | MREMAP_FIXED | MREMAP_DONTUNMAP, segment);
        if (segment == MAP_FAILED) {
            perror("remapping failed");
            exit(EXIT_FAILURE);
        }
    }

    /* Expected output: a bunch of 0s, a bunch of 1s, ... */
    int stride = 2048;
    for (size_t i = 0; i < virtualSize; i += stride) {
        printf("%d", (int) ((char *)virtualMem)[i]);
    }
    printf("\n");

    for (int i = 0; i < numberOfSegments; i++) {
        memset(physicalMems[i], 0, physicalSize);
    }

    /* Expected output: a bunch of 0s */
    for (size_t i = 0; i < virtualSize; i += stride) {
        printf("%d", (int) ((char *)virtualMem)[i]);
    }
    printf("\n");

    cleanup(physicalMems, physicalSize, numberOfSegments);
    return EXIT_SUCCESS;
}

segment = mremap(physicalMems[i], physicalSize, physicalSize, 
                MREMAP_MAYMOVE | MREMAP_FIXED | MREMAP_DONTUNMAP, segment);

physicalMems[i] = mremap(physicalMems[i], physicalSize, physicalSize, 
                MREMAP_MAYMOVE | MREMAP_FIXED, segment);