Is it possible to allocate a certain sector of RAM under Linux?

Question

I have recently gotten a faulty RAM and despite already finding out this I would like to try a much easier concept - write a program that would allocate faulty regions of RAM and never release them. It might not work well if they get allocated before the program runs, but it'd be much easier to reboot on failure than to build a kernel with patches.

So the question is:

1. How to write a program that would allocate given sectors (or pages containing given sectors)
2. and (if possible) report if it was successful.

Answer 1

Check out BadRAM: it seems to do exactly what you want.

Answer 2

This will problematic. To understand why, you have to understand the relation between physical and virtual memory.

On any modern Operating System, programs will get a very large address space for themselves, with the remainder of the address space being used for the OS itself. Other programs are simply invisible: there's no address at which they're found. How is this possible? Simple: processes use virtual addresses. A virtual address does not correspond directly to physical RAM. Instead, there's an address translation table, managed by the OS. When your process runs, the table only contains mappings for RAM that's allocated to you.

Now, that implies that the OS decides what physical RAM is allocated to your program. It can (and will) change that at runtimke. For instance, swapping is implemented using the same mechanism. When swapping out, a page of RAM is written to disk, and its mapping deleted from the translation table. When you try to use the virtual address, the OS detects the missing mapping, restores the page from disk to RAM, and puts back a mapping. It's unlikely that you get back the same page of physical RAM, but the virtual address doesn't change during the whole swap-out/swap-in. So, even if you happened to allocate a page of bad memory, you couldn't keep it. Programs don't own RAM, they own a virtual address space.

Now, Linux does offer some specific kernel functions that allocate memory in a slightly different way, but it seems that you want to bypass the kernel entirely. You can find a much more detailed description in http://lwn.net/images/pdf/LDD3/ch08.pdf

Answer 3

Well, it's not an answer on how to write a program, but it fixes the issue whitout compiling a kernel:

Use memmap or mem parameters: http://gquigs.blogspot.com/2009/01/bad-memory-howto.html

I will edit this answer when I get it running and give details.