Re: [PATCH v1] mm/huge_memory: fix shrinking of all-zero THPs with max_ptes_none default

David Hildenbrand
2025-09-05T15:36:09Z

On 05.09.25 17:30, Lorenzo Stoakes wrote:
> On Fri, Sep 05, 2025 at 04:11:37PM +0200, David Hildenbrand wrote:
>> We added an early exit in thp_underused(), probably to avoid scanning
>> pages when there is no chance for success.
>>
>> However, assume we have max_ptes_none = 511 (default).
>>
>> Nothing should stop us from freeing all pages part of a THP that
>
> Freeing 'all pages which are part of a THP' rather?

I'm German, I don't know what I'm doing. :D

>
>> is completely zero (512) and khugepaged will for sure not try to
>
> that is -> that are?

the THP is zero?

>
>> instantiate a THP in that case (512 shared zeropages).
>
> But if you write faulted they're not the zero page? And how are they shared? I
> mean be being dumb here.

The shrinker will replace zeroed pages by the shared zeropages.

>
>>
>> This can just trivially happen if someone writes a single 0 byte into a
>> PMD area, or of course, when data ends up being zero later.
>>
>> So let's remove that early exit.
>>
>> Do we want to CC stable? Hm, not sure. Probably not urgent.
>
> Surely this is worth having?

Alrighty, let me cc stable.

>
>>
>> Note that, as default, the THP shrinker is active
>> (/sys/kernel/mm/transparent_hugepage/shrink_underused = 1), and all
>> THPs are added to the deferred split lists. However, with the
>> max_ptes_none default we would never scan them. We would not do that. If
>
> Nit but 'we would not do that' is kinda duplicative here :)

Yeah, fixed it already while rewriting: this was meant to be "would now".

Thanks!

--
Cheers

David / dhildenb