Linux mapping virtual memory range to existing virtual memory range?
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25-09-2019 - |
문제
In Linux, is there a way (in user space) to map a virtual address range to the physical pages that back an existing virtual address range? The mmap() function only allows one to map files or "new" physical pages. I need to be able to do something like this:
int* addr1 = malloc(SIZE);
int* addr2 = 0x60000; // Assume nothing is allocated here
fancy_map_function(addr1, addr2, SIZE);
assert(*addr1 == *addr2); // Should succeed
assert(addr1 != addr2); // Should succeed
해결책
I was curious so I tested the shared memory idea suggested in question comments, and it seems to work:
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/mman.h>
#include <assert.h>
#define SIZE 256
int main (int argc, char ** argv) {
int fd;
int *addr1, *addr2;
fd = shm_open("/example_shm", O_RDWR | O_CREAT, 0777);
ftruncate( fd, SIZE);
addr1 = mmap(0, SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
addr2 = mmap(0, SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
printf("addr1 = %p addr2 = %p\n", addr1, addr2);
*addr1 = 0x12345678;
assert(*addr1 == *addr2); // Should succeed
assert(addr1 != addr2); // Should succeed
return 0;
}
(Obviously real code will want to check the return value of the syscalls for errors and clean up after itself)
다른 팁
If you have the fd for the file mapped at addr1
, you can simply mmap
it again at addr2
.
Otherwise, the Linux-specific remap_file_pages
can modify the virtual address ⇆ file offset translation within a single VMA, with page-sized granularity, including mapping the same file offset to multiple addresses.
Open /proc/self/mem
and mmap
the range of virtual addresses you need from it.