Mmaping Memory Range Larger Than the Total Size of Physical Memory and Swap

In Linux, mmap() is a system call that is used to map a portion of a file or device into the memory of a process. This can be useful for a variety of purposes, such as memory-mapped I/O, shared memory, and virtual memory management. However, when mapping a large range of memory that is larger than the total size of physical memory and swap, the mmap() call may fail. In this post, we will explore how to make mmap() a large range of memory which is larger than the total size of physical memory and swap successfully.

The Problem with Large Memory Mappings

When a process uses mmap() to map a large range of memory that is larger than the total size of physical memory and swap, the operating system needs to reserve swap space for the mapping. However, if there is not enough swap space available, the mmap() call will fail.

Using the MAP_NORESERVE Flag

One solution to this problem is to add the MAP_NORESERVE flag to the mmap() call to make it not reserve any swap space for the mapping. This flag tells the operating system to not reserve any swap space for the mapping, which can allow the mmap() call to succeed even if there is not enough swap space available.

To use the MAP_NORESERVE flag with the mmap() call, you can modify the flags parameter to include the MAP_NORESERVE flag. Here’s an example code snippet that demonstrates how to use the MAP_NORESERVE flag with the mmap() call:

#include <sys/mman.h>

void* addr = (void*)0x4000000000; // Map at address 4TB
size_t len = 0x10000000000; // Map 1TB
int prot = PROT_READ | PROT_WRITE;
int flags = MAP_ANONYMOUS | MAP_PRIVATE | MAP_NORESERVE;
int fd = -1;
off_t offset = 0;

void* mapped_addr = mmap(addr, len, prot, flags, fd, offset);

In this example, the MAP_NORESERVE flag is included in the flags parameter, along with the MAP_ANONYMOUS and MAP_PRIVATE flags. The MAP_ANONYMOUS flag specifies that the mapping should not be backed by a file or device, and the MAP_PRIVATE flag specifies that the mapping should not be shared with other processes.

By adding the MAP_NORESERVE flag to the mmap() call, you can make it not reserve any swap space for the mapping, which can allow the mmap() call to succeed even if there is not enough swap space available.