Kernel Samepage Merging (KSM)

Kernel Samepage Merging (KSM) is an optional memory deduplication feature offered by the Linux kernel, which is enabled by default in Proxmox VE. KSM works by scanning a range of physical memory pages for identical content, and identifying the virtual pages that are mapped to them. If identical pages are found, the corresponding virtual pages are re-mapped so that they all point to the same physical page, and the old pages are freed. The virtual pages are marked as "copy-on-write", so that any writes to them will be written to a new area of memory, leaving the shared physical page intact.

Implications of KSM

KSM can optimize memory usage in virtualization environments, as multiple VMs running similar operating systems or workloads could potentially share a lot of common memory pages.

However, while KSM can reduce memory usage, it also comes with some security risks, as it can expose VMs to side-channel attacks. Research has shown that it is possible to infer information about a running VM via a second VM on the same host, by exploiting certain characteristics of KSM.

Thus, if you are using Proxmox VE to provide hosting services, you should consider disabling KSM, in order to provide your users with additional security. Furthermore, you should check your country’s regulations, as disabling KSM may be a legal requirement.

Disabling KSM

To see if KSM is active, you can check the output of:

# systemctl status ksmtuned

If it is, it can be disabled immediately with:

# systemctl disable --now ksmtuned

Finally, to unmerge all the currently merged pages, run:

# echo 2 > /sys/kernel/mm/ksm/run