Question

I have a call to mmap() which I try to map 64MB using MAP_ANONYMOUS as follows:

void *block = mmap(0, 67108864, PROT_READ, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
if (block == MAP_FAILED)
    exit(1);

I understand that to actually own the memory, I need to hit that block of memory. I want to add some sort of 0’s or empty strings to actually own the memory. How would I do that? I tried the following, but that obviously segfaults (I know why it does):

char *temp = block;
for (int i = 0; i < 67108864; i++) {
    *temp = '0';
    temp++;
}

How would I actually gain ownership of that block by assigning something in that block?

Thanks!

Answer 1

Your process already owns the memory, but what I think you want is to make it resident. That is, you want the kernel to allocate physical memory for the mmaped region.

The kernel allocates a virtual memory area (VMA) for the process, but this just specifies a valid region and doesn’t actually allocate physical pages (or frames as they are also sometimes called). To make the kernel allocate entries in the page table, all you need to do is force a page fault.

The easiest way to force a page fault is to touch the memory just like you’re doing. Though, because your page size is almost certainly 4096 bytes, you really only need to read one byte every 4096 bytes thereby reducing the amount of work you actually need to do.

Finally, because you are setting the pages PROT_READ, you will actually want to read from each page rather than try to write.