From: Usama Arif <usama.arif@linux•dev>
To: "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: Wed, 11 Feb 2026 13:38:17 +0000 [thread overview]
Message-ID: <735c0277-2155-47f5-a650-b4c8a0a2e6fc@linux.dev> (raw)
In-Reply-To: <13ab56cb-7fdb-4ee4-9170-f9f4fa4b6e37@kernel.org>
On 11/02/2026 13:25, David Hildenbrand (Arm) wrote:
> 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 This deposited table is withdrawn during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 :)
I spent some time researching this (I havent worked with PowerPC before)
as I really wanted to get rid of all the pre-deposit code. I cant really see a
way without removing PMD THP support. I was going to CC the PowerPC maintainers
but I see that you already did!
>
> 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.
>
Yes that is what I found as well.
> So one obvious solution: remove PMD THP support for hash MMUs along with all this hacky deposit code.
>
I would be happy with that!
>
> 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.
Ack. The code will definitely look a lot lot cleaner and wont have much of this if we decide to remove
PMD THP support for hash MMU.
>
> 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.
>
Ack.
next prev parent reply other threads:[~2026-02-11 21:39 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 [this message]
2026-02-12 12:13 ` Ritesh Harjani
2026-02-12 15:25 ` Usama Arif
2026-02-12 15:39 ` David Hildenbrand (Arm)
2026-02-12 16:46 ` Ritesh Harjani
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