Re: [RFC 1/2] mm: thp: allocate PTE page tables lazily at split time

[Usama Arif <usama.arif@linux.dev>]

On 11/02/2026 13:25, David Hildenbrand (Arm) wrote:
> CCing ppc folks
> 
> On 2/11/26 13:49, Usama Arif wrote:
>> When the kernel creates a PMD-level THP mapping for anonymous pages,
>> it pre-allocates a PTE page table and deposits it via
>> pgtable_trans_huge_deposit(). This deposited table is withdrawn during
>> PMD split or zap. The rationale was that split must not fail—if the
>> kernel decides to split a THP, it needs a PTE table to populate.
>>
>> However, every anon THP wastes 4KB (one page table page) that sits
>> unused in the deposit list for the lifetime of the mapping. On systems
>> with many THPs, this adds up to significant memory waste. The original
>> rationale is also not an issue. It is ok for split to fail, and if the
>> kernel can't find an order 0 allocation for split, there are much bigger
>> problems. On large servers where you can easily have 100s of GBs of THPs,
>> the memory usage for these tables is 200M per 100G. This memory could be
>> used for any other usecase, which include allocating the pagetables
>> required during split.
>>
>> This patch removes the pre-deposit for anonymous pages on architectures
>> where arch_needs_pgtable_deposit() returns false (every arch apart from
>> powerpc, and only when radix hash tables are not enabled) and allocates
>> the PTE table lazily—only when a split actually occurs. The split path
>> is modified to accept a caller-provided page table.
>>
>> PowerPC exception:
>>
>> It would have been great if we can completely remove the pagetable
>> deposit code and this commit would mostly have been a code cleanup patch,
>> unfortunately PowerPC has hash MMU, it stores hash slot information in
>> the deposited page table and pre-deposit is necessary. All deposit/
>> withdraw paths are guarded by arch_needs_pgtable_deposit(), so PowerPC
>> behavior is unchanged with this patch. On a better note,
>> arch_needs_pgtable_deposit will always evaluate to false at compile time
>> on non PowerPC architectures and the pre-deposit code will not be
>> compiled in.
> 
> Is there a way to remove this? It's always been a confusing hack, now it's unpleasant to have around :)


I spent some time researching this (I havent worked with PowerPC before)
as I really wanted to get rid of all the pre-deposit code. I cant really see a
way without removing PMD THP support. I was going to CC the PowerPC maintainers
but I see that you already did!

> 
> In particular, seeing that radix__pgtable_trans_huge_deposit() just 1:1 copied generic pgtable_trans_huge_deposit() hurts my belly.
> 
> 
> IIUC, hash is mostly used on legacy power systems, radix on newer ones.
> 

Yes that is what I found as well.

> So one obvious solution: remove PMD THP support for hash MMUs along with all this hacky deposit code.
> 

I would be happy with that!

> 
> the "vma_is_anonymous(vma) && !arch_needs_pgtable_deposit()" and similar checks need to be wrapped in a reasonable helper and likely this all needs to get cleaned up further.

Ack. The code will definitely look a lot lot cleaner and wont have much of this if we decide to remove
PMD THP support for hash MMU.

> 
> The implementation if the generic pgtable_trans_huge_deposit and the radix handlers etc must be removed. If any code would trigger them it would be a bug.
> 
> If we have to keep this around, pgtable_trans_huge_deposit() should likely get renamed to arch_pgtable_trans_huge_deposit() etc, as there will not be generic support for it.
> 

Ack.