So, I need to take a memory dump of my Ubuntu machine. I tried using dd, but it turns out that for kernel 2.6 (mine is 2.6.38- /dev/mem is disabled. I have literally tried everything that I could find on the internet. So if someone could give me clear instructions that would be great. I have tried changing the Kconfig.debug ad setting STRICT_DEVMEM=y. That did not work. So I added the lines config NONPROMISC_DEVMEM
bool "Disable Promiscuous dev/mem"
default n
---help---....
If in doubt, say Y.
That also did work.
I also looked at some solutions which said that changes need to be made to other (arch/x86/mm/init_32.c, init_64.c, drivers/char/mem.c). But I do not see those files.

I don't know if I'm doing anything wrong.
Can someone please let me know if there's anything else I need to do? Also, should I rebuild the kernel once I make changes to that file?

If you are wanting to change the kernel configuration, then yes, you will need to at least recompile the modules that use those configurations. The solutions that suggest changing kernel files require that you get the kernel source package first, make the changes, then recompile the kernel.

Alternatively, you would insert your own kernel module that can access ram without restrictions (eg fmem).

In fact it should be the other way round. If you set CONFIG_STRICT_DEVMEM = y, you'll definitely not be allowed to access certain parts in the physical address space. As far as I know, CONFIG_STRICT_DEVMEM is enabled by defualt ( that is set to 'y' ) in Ubuntu, so you should in fact set it to 'n' and then try
Very surprising! I've Kernel 2.6.37.1 and mem.c is very much there. Please check your sources again.

Also check for functions like range_is_allowed()(drivers/char/mem.c) and devmem_is_allowed()(arch/x86/mm/init.c) in your source and you can learn why your /dev/mem is being blocked!