From: Usama Arif <usama.arif@linux•dev>
To: "Ritesh Harjani (IBM)" <ritesh.list@gmail•com>,
"David Hildenbrand (Arm)" <david@kernel•org>,
Andrew Morton <akpm@linux-foundation•org>,
lorenzo.stoakes@oracle•com, willy@infradead•org,
linux-mm@kvack•org
Cc: fvdl@google•com, hannes@cmpxchg•org, riel@surriel•com,
shakeel.butt@linux•dev, kas@kernel•org, baohua@kernel•org,
dev.jain@arm•com, baolin.wang@linux•alibaba.com,
npache@redhat•com, Liam.Howlett@oracle•com, ryan.roberts@arm•com,
vbabka@suse•cz, lance.yang@linux•dev,
linux-kernel@vger•kernel.org, kernel-team@meta•com,
Madhavan Srinivasan <maddy@linux•ibm.com>,
Michael Ellerman <mpe@ellerman•id.au>,
linuxppc-dev@lists•ozlabs.org
Subject: Re: [RFC 1/2] mm: thp: allocate PTE page tables lazily at split time
Date: Thu, 12 Feb 2026 15:25:33 +0000 [thread overview]
Message-ID: <93908945-e0a8-429c-b119-eff63ebb2479@linux.dev> (raw)
In-Reply-To: <875x82ma6q.ritesh.list@gmail.com>
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 This deposited table is withdrawn during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
next prev parent reply other threads:[~2026-02-12 17:01 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20260211125507.4175026-1-usama.arif@linux.dev>
[not found] ` <20260211125507.4175026-2-usama.arif@linux.dev>
2026-02-11 13:25 ` [RFC 1/2] mm: thp: allocate PTE page tables lazily at split time David Hildenbrand (Arm)
2026-02-11 13:38 ` Usama Arif
2026-02-12 12:13 ` Ritesh Harjani
2026-02-12 15:25 ` Usama Arif [this message]
2026-02-12 15:39 ` David Hildenbrand (Arm)
2026-02-12 16:46 ` Ritesh Harjani
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