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From: Usama Arif In-Reply-To: <875x82ma6q.ritesh.list@gmail.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 12/02/2026 12:13, Ritesh Harjani (IBM) wrote: > "David Hildenbrand (Arm)" 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 This deposited table is withdrawn during 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