linux-kernel - Re: [PATCH v2 3/7] mm: Add batched versions of ptep_modify_prot

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Message-ID: <d4516c2c-2468-43c7-a270-d068392460e5@arm.com>
Date: Tue, 29 Apr 2025 14:31:12 +0530
From: Dev Jain <dev.jain@....com>
To: Anshuman Khandual <anshuman.khandual@....com>, akpm@...ux-foundation.org
Cc: ryan.roberts@....com, david@...hat.com, willy@...radead.org,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org, catalin.marinas@....com,
 will@...nel.org, Liam.Howlett@...cle.com, lorenzo.stoakes@...cle.com,
 vbabka@...e.cz, jannh@...gle.com, peterx@...hat.com, joey.gouly@....com,
 ioworker0@...il.com, baohua@...nel.org, kevin.brodsky@....com,
 quic_zhenhuah@...cinc.com, christophe.leroy@...roup.eu,
 yangyicong@...ilicon.com, linux-arm-kernel@...ts.infradead.org,
 namit@...are.com, hughd@...gle.com, yang@...amperecomputing.com,
 ziy@...dia.com
Subject: Re: [PATCH v2 3/7] mm: Add batched versions of
 ptep_modify_prot_start/commit



On 29/04/25 2:09 pm, Anshuman Khandual wrote:
> 
> 
> On 4/29/25 10:53, Dev Jain wrote:
>> Batch ptep_modify_prot_start/commit in preparation for optimizing mprotect.
>> Architecture can override these helpers.
>>
>> Signed-off-by: Dev Jain <dev.jain@....com>
>> ---
>>   include/linux/pgtable.h | 38 ++++++++++++++++++++++++++++++++++++++
>>   1 file changed, 38 insertions(+)
>>
>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
>> index b50447ef1c92..ed287289335f 100644
>> --- a/include/linux/pgtable.h
>> +++ b/include/linux/pgtable.h
>> @@ -891,6 +891,44 @@ static inline void wrprotect_ptes(struct mm_struct *mm, unsigned long addr,
>>   }
>>   #endif
>>   
>> +/* See the comment for ptep_modify_prot_start */
>> +#ifndef modify_prot_start_ptes
>> +static inline pte_t modify_prot_start_ptes(struct vm_area_struct *vma,
>> +		unsigned long addr, pte_t *ptep, unsigned int nr)
>> +{
>> +	pte_t pte, tmp_pte;
>> +
>> +	pte = ptep_modify_prot_start(vma, addr, ptep);
>> +	while (--nr) {
>> +		ptep++;
>> +		addr += PAGE_SIZE;
>> +		tmp_pte = ptep_modify_prot_start(vma, addr, ptep);
>> +		if (pte_dirty(tmp_pte))
>> +			pte = pte_mkdirty(pte);
>> +		if (pte_young(tmp_pte))
>> +			pte = pte_mkyoung(pte);
>> +	}
>> +	return pte;
>> +}
>> +#endif
>> +
>> +/* See the comment for ptep_modify_prot_commit */
>> +#ifndef modify_prot_commit_ptes
>> +static inline void modify_prot_commit_ptes(struct vm_area_struct *vma, unsigned long addr,
>> +		pte_t *ptep, pte_t old_pte, pte_t pte, unsigned int nr)
>> +{
>> +	for (;;) {
>> +		ptep_modify_prot_commit(vma, addr, ptep, old_pte, pte);
>> +		if (--nr == 0)
>> +			break;
>> +		ptep++;
>> +		addr += PAGE_SIZE;
>> +		old_pte = pte_next_pfn(old_pte);
>> +		pte = pte_next_pfn(pte);
>> +	}
>> +}
>> +#endif
> 
> Is there any particular reason why the first loop here is while { }
> based where as the second one is a for { } loop ?

modify_prot_start_ptes follows the pattern of get_and_clear_full_ptes. 
modify_prot_commit_ptes follows the pattern of wrprotect_ptes.

> 
>> +
>>   /*
>>    * On some architectures hardware does not set page access bit when accessing
>>    * memory page, it is responsibility of software setting this bit. It brings
> 
> These helpers should be added with at least a single caller using them.