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

[David Hildenbrand (Arm)]

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 :)
> 
> (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

-- 
Cheers,

David