[[kernel_samepage_merging]]
Kernel Samepage Merging (KSM)
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Kernel Samepage Merging (KSM) is an optional memory deduplication feature
offered by the Linux kernel, which is enabled by default in {pve}. 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 {pve} 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:

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# systemctl status ksmtuned
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If it is, it can be disabled immediately with:

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# systemctl disable --now ksmtuned
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Finally, to unmerge all the currently merged pages, run:

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