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

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Message-ID: <331ed532-eb98-4665-9d1b-b6b8bf46e396@efficios.com>
Date: Fri, 28 Feb 2025 16:38:02 -0500
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: David Hildenbrand <david@...hat.com>,
 Sean Christopherson <seanjc@...gle.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
 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 10:10, David Hildenbrand wrote:
[...]
> For example, QEMU will mark all guest memory is mergeable using MADV, to 
> limit the deduplicaton to guest RAM only.
> 

On a related note, I think the madvise(2) documentation is inaccurate.

It states:

        MADV_MERGEABLE (since Linux 2.6.32)
               Enable  Kernel Samepage Merging (KSM) for the pages in the range
               specified by addr and length. [...]

AFAIU, based on code review of ksm_madvise(), this is not strictly true.

The KSM implementation enables KSM for pages in the entire vma containing the range.
So if it so happens that two mmap areas with identical protection flags are merged,
both will be considered mergeable by KSM as soon as at least one page from any of
those areas is made mergeable.

This does not appear to be an issue in qemu because guard pages with different
protection are placed between distinct mappings, which should prevent combining
the vmas.

Thanks,

Mathieu

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