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

[Ritesh Harjani (IBM) <ritesh.list@gmail.com>]

"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 This deposited table is withdrawn during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. 


>
> 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