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

[Mathieu Desnoyers]

On 2025-02-28 16:45, David Hildenbrand wrote:
> 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.

You are correct, I missed that part. Thanks for the clarification!

Mathieu



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