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

[David Hildenbrand]

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