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 13:53:32 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: David Hildenbrand <david@...hat.com>,
 Catalin Marinas <catalin.marinas@....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 30/04/2024 12:37, David Hildenbrand wrote:
> 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

OK fair enough.

> 
> Something like
> 
> present = present_valid | present_invalid

I don't want to change pte_valid() to pte_present_valid(); that would also be a
fair bit of churn.

I'll take Catalin's suggestion and make this PTE_PRESENT_INVALID and
pte_present_invalid(). And obviously leave pmd_mkinvalid() as it is.
(Conversation in the other thread has concluded that it's ok to invalidate a
non-present pmd afterall).

> 
> would be more obvious at least to me ;)
>