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

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Message-ID: <78ebaad9-c7ba-423a-a824-c2b1a499aea6@efficios.com>
Date: Fri, 28 Feb 2025 10:01:35 -0500
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: David Hildenbrand <david@...hat.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org,
 Matthew Wilcox <willy@...radead.org>, Olivier Dion <odion@...icios.com>,
 linux-mm@...ck.org
Subject: Re: [RFC PATCH 0/2] SKSM: Synchronous Kernel Samepage Merging

On 2025-02-28 08:59, David Hildenbrand wrote:
> On 28.02.25 06:17, Linus Torvalds wrote:
>> On Thu, 27 Feb 2025 at 19:03, Mathieu Desnoyers
>> <mathieu.desnoyers@...icios.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.
> 

I suspect that SKSM is a better fit for JIT and code patching than KSM,
because user-space knows better when a set of pages is going to become
invariant for a long time and thus benefit from merging. This removes
the background scanning from the picture.

Does the interpreter use-case require background scanning, or does
it know when a set of pages are meant to become invariant for a long
time ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com