Linux kernel code segment memory page modifications

Question

I'm trying to implement a "Semantic based memory sharing model" for Xen. As a part of my project, i'm trying to share kernel code pages across VMs. I' ve assumed that the code segments of linux kernels with similar version are 100% identical. But when i carry out some experiments using Virtual Machines running Debian Squeeze, i have found 3 memory pages are different in kernel code segment.

So my question is that, does the linux kernel modifies its code pages at runtime?

Answer 1

Yes, it can - for example, spinlocks can be dynamically patched out of the code if the kernel sees at runtime that it is running on a uniprocessor system. I do not know of an exhaustive list of such cases, you will need to inspect the code.

See the LWN article on SMP Alternatives for more information on one system that does runtime patching within the kernel.