linux-kernel - Re: [PATCH v2 1/3] arm64/mm: Refactor PMD_PRESENT_INVALID and PTE_PROT

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Date: Tue, 30 Apr 2024 13:37:53 +0200
From: David Hildenbrand <david@...hat.com>
To: Catalin Marinas <catalin.marinas@....com>,
 Ryan Roberts <ryan.roberts@....com>
Cc: Will Deacon <will@...nel.org>, Joey Gouly <joey.gouly@....com>,
 Ard Biesheuvel <ardb@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Anshuman Khandual <anshuman.khandual@....com>, Peter Xu <peterx@...hat.com>,
 Mike Rapoport <rppt@...ux.ibm.com>, Shivansh Vij <shivanshvij@...look.com>,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] arm64/mm: Refactor PMD_PRESENT_INVALID and
 PTE_PROT_NONE bits

On 30.04.24 13:11, Catalin Marinas wrote:
> On Mon, Apr 29, 2024 at 06:15:45PM +0100, Ryan Roberts wrote:
>> On 29/04/2024 17:20, Catalin Marinas wrote:
>>> On Mon, Apr 29, 2024 at 03:02:05PM +0100, Ryan Roberts wrote:
>>>> diff --git a/arch/arm64/include/asm/pgtable-prot.h b/arch/arm64/include/asm/pgtable-prot.h
>>>> index dd9ee67d1d87..de62e6881154 100644
>>>> --- a/arch/arm64/include/asm/pgtable-prot.h
>>>> +++ b/arch/arm64/include/asm/pgtable-prot.h
>>>> @@ -18,14 +18,7 @@
>>>>   #define PTE_DIRTY		(_AT(pteval_t, 1) << 55)
>>>>   #define PTE_SPECIAL		(_AT(pteval_t, 1) << 56)
>>>>   #define PTE_DEVMAP		(_AT(pteval_t, 1) << 57)
>>>> -#define PTE_PROT_NONE		(_AT(pteval_t, 1) << 58) /* only when !PTE_VALID */
>>>> -
>>>> -/*
>>>> - * This bit indicates that the entry is present i.e. pmd_page()
>>>> - * still points to a valid huge page in memory even if the pmd
>>>> - * has been invalidated.
>>>> - */
>>>> -#define PMD_PRESENT_INVALID	(_AT(pteval_t, 1) << 59) /* only when !PMD_SECT_VALID */
>>>> +#define PTE_INVALID		(_AT(pteval_t, 1) << 59) /* only when !PTE_VALID */
>>>
>>> Nitpick - I prefer the PTE_PRESENT_INVALID name as it makes it clearer
>>> it's a present pte. We already have PTE_VALID, calling it PTE_INVALID
>>> looks like a negation only.
>>
>> Meh, for me the pte can only be valid or invalid if it is present. So it's
>> implicit. And if you have PTE_PRESENT_INVALID you should also have
>> PTE_PRESENT_VALID.
>>
>> We also have pte_mkinvalid(), which is core-mm-defined. In your scheme, surely
>> it should be pte_mkpresent_invalid()?
>>
>> But you're the boss, I'll change this to PTE_PRESENT_INVALID. :-(
> 
> TBH, I don't have a strong opinion but best to avoid the bikeshedding.
> I'll leave the decision to you ;). It would match the pmd_mkinvalid()
> core code. But if you drop 'present' make sure you add a comment above
> that it's meant for present ptes.

FWIW, I was confused by

present = valid | invalid

Something like

present = present_valid | present_invalid

would be more obvious at least to me ;)

-- 
Cheers,

David / dhildenb