Re: [RFC] mm: restrict zero-page remapping to underused THP splits

[Usama Arif]

On 11/05/2026 14:44, David Hildenbrand (Arm) wrote:
> On 5/11/26 15:42, David Hildenbrand (Arm) wrote:
>> On 5/11/26 15:10, Usama Arif wrote:
>>>
>>>
>>> On 11/05/2026 07:36, David Hildenbrand (Arm) wrote:
>>>>
>>>>
>>>>
>>>> Hi!
>>>>
>>>>
>>>> Yes, but that's what the users ask for: if there is a chance to deduplicate
>>>> memory, it shall be deduplicated asap.
>>>>
>>>>
>>>> Right, the probability is low, and it would change existing semantics, breaking
>>>> existing users.
>>>>
>>>> In addition, we would have to add large folio support for KSM, which I rather
>>>> would avoid.
>>>>
>>>>
>>>> Right, but that's what you get with KSM: bad performance if there is a chance to
>>>> deduplicate :)
>>>>
>>>> (and bad performance from scanning overhead)
>>>>
>>>>
>>>> It's not just the zero page, but really any page content. The zero page is
>>>> currently only "special" after we added conditional support to deduplicate to
>>>> the shared zeropage in KSM. The shrinker doesn't help for any other page content
>>>> besides zero-filled.
>>>>
>>>> Further, the shrinker is something system-wide, whereby KSM is usually only
>>>> enabled for selected VMAs (with some exceptions nowadays).
>>>>
>>>> Also note that KSM deduplicates independent of the folio size: not just THPs,
>>>> but really any (large) folio. Yes, it splits large folios, but that's really
>>>> just to keep the T in THP.
>>>>
>>>>
>>>> Right, and it makes KSM more THP aware. Which is something I would avoid right now.
>>>>
>>>>
>>>> That would change the semantics where, for example, where we expect that memory
>>>> was deduplicated after a KSM run.
>>>>
>>>> VMs (where KSM is usually employed) are expected to be mostly backed by THPs:
>>>> except where we can deduplicate memory. Skipping THPs would essentially break
>>>> the main use case for KSM :)
>>>>
>>>> Does that make sense?
>>>>
>>>
>>> Yes, all of above makes sense. But I feel like this means someone should not
>>> set THP policy to always and enable KSM together. 
>>
>> IIRC, QEMU will, as default, set MADV_HUGEPAGE and MADV_MERGEABLE :)

That is interesting... :)

>>
>> (KSM itself later has to be enabled manually on a system level)
> 
> Digging around, RHEL documents that one might want to consider disabling THPs
> for performance:
> 
> "As KSM can reduce the occurence of transparent huge pages, you may want to
> disable it before enabling THP." [1]
> 
> But that doesn't mean that some people are using that combination.
> 
> In the end "some THPs" is better than "no THPs" :)
> 
> [1]
> https://docs.redhat.com/en/documentation/red_hat_enterprise_linux/6/html/virtualization_tuning_and_optimization_guide/sect-virtualization_tuning_optimization_guide-memory-huge_pages
> 

Thanks for sharing this and the link, its very useful.