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55 lines
1.8 KiB
Plaintext
55 lines
1.8 KiB
Plaintext
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[[kernel_samepage_merging]]
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Kernel Samepage Merging (KSM)
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-----------------------------
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ifdef::wiki[]
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:pve-toplevel:
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endif::wiki[]
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Kernel Samepage Merging (KSM) is an optional memory deduplication feature
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offered by the Linux kernel, which is enabled by default in {pve}. KSM
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works by scanning a range of physical memory pages for identical content, and
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identifying the virtual pages that are mapped to them. If identical pages are
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found, the corresponding virtual pages are re-mapped so that they all point to
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the same physical page, and the old pages are freed. The virtual pages are
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marked as "copy-on-write", so that any writes to them will be written to a new
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area of memory, leaving the shared physical page intact.
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Implications of KSM
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~~~~~~~~~~~~~~~~~~~
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KSM can optimize memory usage in virtualization environments, as multiple VMs
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running similar operating systems or workloads could potentially share a lot of
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common memory pages.
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However, while KSM can reduce memory usage, it also comes with some security
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risks, as it can expose VMs to side-channel attacks. Research has shown that it
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is possible to infer information about a running VM via a second VM on the same
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host, by exploiting certain characteristics of KSM.
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Thus, if you are using {pve} to provide hosting services, you should consider
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disabling KSM, in order to provide your users with additional security.
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Furthermore, you should check your country's regulations, as disabling KSM may
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be a legal requirement.
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Disabling KSM
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~~~~~~~~~~~~~
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To see if KSM is active, you can check the output of:
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----
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# systemctl status ksmtuned
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----
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If it is, it can be disabled immediately with:
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----
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# systemctl disable --now ksmtuned
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----
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Finally, to unmerge all the currently merged pages, run:
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----
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# echo 2 > /sys/kernel/mm/ksm/run
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----
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