Re: [RFC PATCH 0/2] SKSM: Synchronous Kernel Samepage Merging

[David Hildenbrand <david@redhat.com>]

On 28.02.25 15:59, Sean Christopherson wrote:
> On Fri, Feb 28, 2025, David Hildenbrand wrote:
>> On 28.02.25 06:17, Linus Torvalds wrote:
>>> On Thu, 27 Feb 2025 at 19:03, Mathieu Desnoyers
>>> <mathieu.desnoyers@efficios.com> wrote:
>>>>
>>>> I'd be fine with SKSM replacing KSM entirely. However, I don't
>>>> think we should try to re-implement the existing KSM userspace ABIs
>>>> over SKSM.
>>>
>>> No, absolutely. The only point (for me) for your new synchronous one
>>> would be if it replaced the kernel thread async scanning, which would
>>> make the old user space interface basically pointless.
>>>
>>> But I don't actually know who uses KSM right now. My reaction really
>>> comes from a "it's not nice code in the kernel", not from any actual
>>> knowledge of the users.
>>>
>>> Maybe it works really well in some cloud VM environment, and we're
>>> stuck with it forever.
>>
>> Exactly that; and besides the VM use-case, lately people stated using it in
>> the context of interpreters (IIRC inside Meta) quite successfully as well.
> 
> Does Red Hat (or any other KVM supporters) actually recommend using KSM for VMs
> in cloud environments?

Private clouds yes, that's where it is most commonly used for. I would 
assume that nobody for

For example, there is some older documentation here:

https://docs.redhat.com/en/documentation/red_hat_enterprise_linux/6/html/virtualization_administration_guide/chap-ksm#chap-KSM

which touches on the security aspects:

"The page deduplication technology (used also by the KSM implementation) 
may introduce side channels that could potentially be used to leak 
information across multiple guests. In case this is a concern, KSM can 
be disabled on a per-guest basis."

> 
> The security implications of scanning guest memory and having co-tenant VMs share
> mappings (should) make it a complete non-starter for any scenario where VMs and/or
> their workloads are owned by third parties.

Jep.

> 
> I can imagine there might be first-party use cases, but I would expect many/most
> of those to be able to explicitly share mappings, which would provide far, far
> better power and performance characteristics.

Note that KSM can be very efficient when you have multiple VMs running 
the same kernel,executable,libraries etc. If my memory doesn't trick me, 
that's precisely for what it was originally invented, and how it is 
getting used today in the context of VMs.

For example, QEMU will mark all guest memory is mergeable using MADV, to 
limit the deduplicaton to guest RAM only.

-- 
Cheers,

David / dhildenb