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

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

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