From: "David Hildenbrand (Arm)" <david@kernel.org>
To: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: linux-arm-kernel@lists.infradead.org,
Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will@kernel.org>,
Ryan Roberts <ryan.roberts@arm.com>,
Mark Rutland <mark.rutland@arm.com>,
Lorenzo Stoakes <ljs@kernel.org>,
Andrew Morton <akpm@linux-foundation.org>,
Mike Rapoport <rppt@kernel.org>,
Linu Cherian <linu.cherian@arm.com>,
linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH V2 06/14] arm64/mm: Enable fixmap with 5 level page table
Date: Thu, 17 Sep 2026 20:28:10 +0200 [thread overview]
Message-ID: <efdec0bc-cab0-451f-ad70-ac24c1f69fdd@kernel.org> (raw)
In-Reply-To: <pi6o4hjdaz2tndzlx64qvzbgxhuyh3kyewpfetiqnnetbtpzpa@prrxusgc3lpm>
On 9/17/26 08:18, Anshuman Khandual wrote:
> On Wed, Sep 16, 2026 at 04:30:05PM +0200, David Hildenbrand (Arm) wrote:
>> On 9/7/26 05:50, Anshuman Khandual wrote:
>>> FEAT_D128 halves PTRS_PER_PXX thus shrinking the VA range coverage
>>> for each page table level. Hence in order to preserve all existing
>>> VA range configurations, some geometry now need to become 5-level.
>>>
>>
>> Hmm, wasn't this 5-levels before, but other code allocated+initialized the p4d
>> table for us?
>
> But fixmap was never 5 level earlier.
I mean the page table hierarchy. It's confusing.
>
>>
>> I mean, I see how we are filling in another level, but I don't see how we would
>> reconfigure something to use 5 page table levels?
>
> IIUC existing configurations which never required 5 levels would not need
> this reconfiguration even now.
We only map a handful of MB into a single leaf page table. The only reason why
we have to install more page tables into upper levels is because they were not
instantiated by other code.
>
>>
>>> Since fixmap is used to build and manipulate page tables early on
>>> during boot the mapping must also gain that additional level which
>>> was not required earlier.
>>
>> I am no expert ... but I am confused :)
>>
>> We do have a loop only in early_fixmap_init_pmd(). So we cannot possibly span
>> more than a single ... PTE table?
>>
>>>
>>> Enable fixmap with 5 level page table when required. This creates table
>>> entries at the PGD level. Add a fallback stub for pgd_page_paddr() when
>>> (PGTBALE_LEVELS <= 4) which helps in intercepting any unintended usage.
>>
>>
>> Again, I am very confused. Is it really just "As the page table layout changes,
>> we might no longer have a p4d table pre-populated for the range our fixmap falls
>> into. So if there is no p4d table yet, populate a static one just like we do for
>> the other levels."
>
> Right.
Best to rewrite the patch description along these lines.
--
Cheers,
David
next prev parent reply other threads:[~2026-09-17 18:28 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-09-07 3:50 [PATCH V2 00/14] arm64/mm: Enable 128 bit page table entries Anshuman Khandual
2026-09-07 3:50 ` [PATCH V2 01/14] mm: Add read-write accessors for vm_page_prot Anshuman Khandual
2026-09-16 13:23 ` David Hildenbrand (Arm)
2026-09-16 13:41 ` Lorenzo Stoakes (ARM)
2026-09-17 4:02 ` Anshuman Khandual
2026-09-07 3:50 ` [PATCH V2 02/14] arm64/mm: Route all pgtable reads via ptval_get() Anshuman Khandual
2026-09-16 13:35 ` David Hildenbrand (Arm)
2026-09-16 13:38 ` David Hildenbrand (Arm)
2026-09-17 4:26 ` Anshuman Khandual
2026-09-07 3:50 ` [PATCH V2 03/14] arm64/mm: Route all pgtable writes via ptval_set() Anshuman Khandual
2026-09-16 13:36 ` David Hildenbrand (Arm)
2026-09-07 3:50 ` [PATCH V2 04/14] arm64/mm: Route all pgtable atomics to central helpers Anshuman Khandual
2026-09-16 13:39 ` David Hildenbrand (Arm)
2026-09-17 4:18 ` Anshuman Khandual
2026-09-07 3:50 ` [PATCH V2 05/14] arm64/mm: Override read-write accessors for vm_page_prot Anshuman Khandual
2026-09-16 13:39 ` David Hildenbrand (Arm)
2026-09-17 4:09 ` Anshuman Khandual
2026-09-07 3:50 ` [PATCH V2 06/14] arm64/mm: Enable fixmap with 5 level page table Anshuman Khandual
2026-09-16 14:30 ` David Hildenbrand (Arm)
2026-09-17 6:18 ` Anshuman Khandual
2026-09-17 18:28 ` David Hildenbrand (Arm) [this message]
2026-09-07 3:50 ` [PATCH V2 07/14] arm64/mm: Enable pgtable geometry for FEAT_D128 Anshuman Khandual
2026-09-07 3:50 ` [PATCH V2 08/14] arm64/mm: Enable pgtable descriptor " Anshuman Khandual
2026-09-07 3:50 ` [PATCH V2 09/14] arm64/mm: Enable 128 bit translation Anshuman Khandual
2026-09-07 3:50 ` [PATCH V2 10/14] arm64/mm: Add an abstraction level for tlbi_op Anshuman Khandual
2026-09-07 3:50 ` [PATCH V2 11/14] arm64/mm: Enable TLBIP instruction based TLB flush Anshuman Khandual
2026-09-07 3:50 ` [PATCH V2 12/14] arm64/mm: Relax KASAN shadow alignment for FEAT_D128 Anshuman Khandual
2026-09-07 3:50 ` [PATCH V2 13/14] arm64/mm: Make ARM64_D128 selectable Anshuman Khandual
2026-09-07 3:50 ` [PATCH V2 14/14] arm64/configs: Add d128.config Anshuman Khandual
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