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

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Message-ID: <a70158c3-d683-42d6-8af5-c800d51039e4@redhat.com>
Date: Fri, 28 Feb 2025 16:34:50 +0100
From: David Hildenbrand <david@...hat.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
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 28.02.25 03:51, Linus Torvalds wrote:
> On Thu, 27 Feb 2025 at 18:31, Mathieu Desnoyers
> <mathieu.desnoyers@...icios.com> wrote:
>>
>> This series introduces SKSM, a new page deduplication ABI,
>> aiming to fix the limitations inherent to the KSM ABI.
> 
> So I'm not interested in seeing *another* KSM version.
> 
> Because I absolutely do *NOT* want a new chapter in the saga of SLUB
> vs SLAB vs SLOB.
> 
> However, if the feeling is that this can *replace* the current horror
> that is KSM, I'm a lot more interested. I suspect our current KSM
> model has largely been a failure, and this might be "good enough".

Maybe it would be comparable to khugepaged vs. MADV_COLLAPSE?

Many/most use cases just leave THP scanning+collapsing to khugepaged; 
selected ones might "know better" what to do, so they effectively 
disable khugepaged, and manually collapse THPs using MADV_COLLAPSE.

If it would be similar to that, it would not be completely different KSM 
version, just a different way to trigger merging: background scanning 
vs. user-space triggered ("synchronous").

I could see use cases for such a synchronous interface, but I doubt it 
could replace the background scanning that is actively getting used for 
existing use cases; I have similar thoughts about khugepaged vs. 
MADV_COLLAPSE.

-- 
Cheers,

David / dhildenb