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

[David Hildenbrand (Arm)]

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 This deposited table is withdrawn during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 :)

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.

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


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.

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.

-- 
Cheers,

David