linux-kernel - Re: [PATCH 3/7] mm: Use ptep_get() for accessing PTE entries

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Message-ID: <357931ba-d059-453b-a91d-1bed7fbe5914@arm.com>
Date: Mon, 16 Sep 2024 08:46:06 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: Ryan Roberts <ryan.roberts@....com>, linux-mm@...ck.org
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 David Hildenbrand <david@...hat.com>, "Mike Rapoport (IBM)"
 <rppt@...nel.org>, Arnd Bergmann <arnd@...db.de>, x86@...nel.org,
 linux-m68k@...ts.linux-m68k.org, linux-fsdevel@...r.kernel.org,
 kasan-dev@...glegroups.com, linux-kernel@...r.kernel.org,
 linux-perf-users@...r.kernel.org
Subject: Re: [PATCH 3/7] mm: Use ptep_get() for accessing PTE entries



On 9/13/24 15:57, Ryan Roberts wrote:
> On 13/09/2024 09:44, Anshuman Khandual wrote:
>> Convert PTE accesses via ptep_get() helper that defaults as READ_ONCE() but
>> also provides the platform an opportunity to override when required.
>>
>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> Cc: David Hildenbrand <david@...hat.com>
>> Cc: Ryan Roberts <ryan.roberts@....com>
>> Cc: "Mike Rapoport (IBM)" <rppt@...nel.org>
>> Cc: linux-mm@...ck.org
>> Cc: linux-kernel@...r.kernel.org
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
>> ---
>>  include/linux/pgtable.h | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
>> index 2a6a3cccfc36..05e6995c1b93 100644
>> --- a/include/linux/pgtable.h
>> +++ b/include/linux/pgtable.h
>> @@ -1060,7 +1060,7 @@ static inline int pgd_same(pgd_t pgd_a, pgd_t pgd_b)
>>   */
>>  #define set_pte_safe(ptep, pte) \
>>  ({ \
>> -	WARN_ON_ONCE(pte_present(*ptep) && !pte_same(*ptep, pte)); \
>> +	WARN_ON_ONCE(pte_present(ptep_get(ptep)) && !pte_same(ptep_get(ptep), pte)); \
> 
> Suggest reading once into a temporary so that the pte can't change between the 2
> gets. In practice, it's not likely to be a huge problem for this instance since
> its under the PTL so can only be racing with HW update of access and dirty. But
> good practice IMHO:
> 
>     pte_t __old = ptep_get(ptep); \
>     WARN_ON_ONCE(pte_present(__old) && !pte_same(__old, pte)); \

Sure, will change as suggested.

> 
> Thanks,
> Ryan
> 
>>  	set_pte(ptep, pte); \
>>  })
>>  
>