linux-kernel - Re: [PATCH V2 1/3] arm64/mm: Drop __HAVE_ARCH_HUGE_PTEP

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Date:   Wed, 6 May 2020 15:51:42 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Will Deacon <will@...nel.org>
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        Catalin Marinas <catalin.marinas@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2 1/3] arm64/mm: Drop __HAVE_ARCH_HUGE_PTEP_GET



On 05/06/2020 01:13 PM, Will Deacon wrote:
> On Wed, May 06, 2020 at 11:42:12AM +0530, Anshuman Khandual wrote:
>> Platform specific huge_ptep_get() is required only when fetching the huge
>> PTE involves more than just dereferencing the page table pointer. This is
>> not the case on arm64 platform. Hence huge_ptep_pte() can be dropped along
>> with it's __HAVE_ARCH_HUGE_PTEP_GET subscription.
>>
>> Cc: Catalin Marinas <catalin.marinas@....com>
>> Cc: Will Deacon <will@...nel.org>
>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> Cc: linux-arm-kernel@...ts.infradead.org
>> Cc: linux-mm@...ck.org
>> Cc: linux-kernel@...r.kernel.org
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
>> ---
>>  arch/arm64/include/asm/hugetlb.h | 6 ------
>>  1 file changed, 6 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/hugetlb.h b/arch/arm64/include/asm/hugetlb.h
>> index 2eb6c234d594..b88878ddc88b 100644
>> --- a/arch/arm64/include/asm/hugetlb.h
>> +++ b/arch/arm64/include/asm/hugetlb.h
>> @@ -17,12 +17,6 @@
>>  extern bool arch_hugetlb_migration_supported(struct hstate *h);
>>  #endif
>>  
>> -#define __HAVE_ARCH_HUGE_PTEP_GET
>> -static inline pte_t huge_ptep_get(pte_t *ptep)
>> -{
>> -	return READ_ONCE(*ptep);
>> -}
> 
> Hmm, I'm nervous about dropping the READ_ONCE() here. We added them after
> running into page-table issues with THP [1] and it's really important to
> use them consistently to avoid hitting that again.
> 
> If the generic code used READ_ONCE(), I'd be happy to switch to it.

Sure, will add READ_ONCE() in the generic huge_ptep_get(). AFAICS it should
not cause any problem for other platforms (but let me know otherwise).

> 
> Will
> 
> [1] https://lore.kernel.org/lkml/1506527369-19535-1-git-send-email-will.deacon@arm.com/
>