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

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

On 12/02/2026 12:13, Ritesh Harjani (IBM) wrote:
> "David Hildenbrand (Arm)" <david@kernel.org> writes:
> 
>> CCing ppc folks
>>
> 
> Thanks David!
> 
>> 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 This deposited table is withdrawn during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 :)
>>
> 
> Hash MMU on PowerPC works fundamentally different than other MMUs
> (unlike Radix MMU on PowerPC). So yes, it requires few tricks to fit
> into the Linux's multi-level SW page table model. ;) 
> 
> 
>> In particular, seeing that radix__pgtable_trans_huge_deposit() just 1:1 
>> copied generic pgtable_trans_huge_deposit() hurts my belly.
>>
> 
> On PowerPC, pgtable_t can be a pte fragment. 
> 
> typedef pte_t *pgtable_t;
> 
> That means a single page can be shared among other PTE page tables. So, we
> cannot use page->lru which the generic implementation uses. I guess due
> to this, there is a slight change in implementation of
> radix__pgtable_trans_huge_deposit(). 
> 
> Doing a grep search, I think that's the same for sparc and s390 as well.
> 
>>
>> IIUC, hash is mostly used on legacy power systems, radix on newer ones.
>>
>> So one obvious solution: remove PMD THP support for hash MMUs along with 
>> all this hacky deposit code.
>>
> 
> Unfortunately, please no. There are real customers using Hash MMU on
> Power9 and even on older generations and this would mean breaking Hash
> PMD THP support for them. 
> 
> 

Thanks for confirming! I will keep the pagetable deposit for powerpc
in the next revision.
I will rename pgtable_trans_huge_deposit to arch_pgtable_trans_huge_deposit
and move it to arch/powerpc. It will an empty function for the rest of the
architectures.

>>
>> 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.
>>
>> 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.
>>
> 
> Sure, I think after this patch series, the radix__pgtable_trans_huge_deposit() 
> will mostly be a dead code anyways. I will spend some time going
> through this series and will also give it a test on powerpc HW (with
> both Hash and Radix MMU).
> 
> I guess, we should also look at removing pgtable_trans_huge_deposit() and
> pgtable_trans_huge_withdraw() implementations from s390 and sparc, since
> those too will be dead code after this.
> 
> 
>> 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.
>>
> 
> Sure. That make sense since PowerPC Hash MMU will still need this.
> 
> -ritesh