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

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Message-ID: <cc1dec8c-8323-4c67-913f-5d8fb55ce715@efficios.com>
Date: Fri, 28 Feb 2025 09:59:00 -0500
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: 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 00:17, 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.
> 

For the VM use-case, I wonder if we could just add a userfaultfd
"COW" event that would notify userspace when a COW happens ?

This would allow userspace to replace ksmd by tracking the age of
those anonymous pages, and issue madvise MADV_MERGE on them to
write-protect+merge them when it is deemed useful.

With both a new userfaultfd COW event and madvise MADV_MERGE,
is there anything else that is fundamentally missing to move
all the scanning complexity of KSM to userspace for the VM
deduplication use-case ?

Thanks,

Mathieu

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