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

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Message-ID: <adc90638-79a7-4015-accc-0932611cc697@redhat.com>
Date: Fri, 28 Feb 2025 22:45:57 +0100
From: David Hildenbrand <david@...hat.com>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.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 28.02.25 22:38, Mathieu Desnoyers wrote:
> 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.

I *think* it does what is documented. In madvise_vma_behavior(), 
ksm_madvise() will update "new_flags".

Then we call madvise_update_vma() to split the VMA if required and set 
new_flags only on the split VMA. The handling is similar to other MADV 
operations that end up modifying vm_flags.

If I am missing something and this is indeed broken, we should 
definitely write a selftest for it and fix it.

-- 
Cheers,

David / dhildenb