linux-kernel - Re: [PATCH v2 1/7] mm: Refactor code in mprotect

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Message-ID: <b780d1b8-d6f0-42f1-aa5c-c6e016bbd9ae@arm.com>
Date: Tue, 29 Apr 2025 12:24:37 +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 1/7] mm: Refactor code in mprotect



On 29/04/25 12:11 pm, Anshuman Khandual wrote:
> 
> 
> On 4/29/25 10:53, Dev Jain wrote:
>> Reduce indentation in change_pte_range() by refactoring some of the code
>> into a new function. No functional change.
>>
>> Signed-off-by: Dev Jain <dev.jain@....com>
>> ---
>>   mm/mprotect.c | 116 +++++++++++++++++++++++++++++---------------------
>>   1 file changed, 68 insertions(+), 48 deletions(-)
>>
>> diff --git a/mm/mprotect.c b/mm/mprotect.c
>> index 88608d0dc2c2..70f59aa8c2a8 100644
>> --- a/mm/mprotect.c
>> +++ b/mm/mprotect.c
>> @@ -83,6 +83,71 @@ bool can_change_pte_writable(struct vm_area_struct *vma, unsigned long addr,
>>   	return pte_dirty(pte);
>>   }
>>   
>> +
>> +
>> +static bool prot_numa_skip(struct vm_area_struct *vma, struct folio *folio,
>> +		int target_node)
>> +{
>> +	bool toptier;
>> +	int nid;
>> +
>> +	/* Also skip shared copy-on-write pages */
>> +	if (is_cow_mapping(vma->vm_flags) &&
>> +	    (folio_maybe_dma_pinned(folio) ||
>> +	     folio_maybe_mapped_shared(folio)))
>> +		return true;
>> +
>> +	/*
>> +	 * While migration can move some dirty pages,
>> +	 * it cannot move them all from MIGRATE_ASYNC
>> +	 * context.
>> +	 */
>> +	if (folio_is_file_lru(folio) &&
>> +	    folio_test_dirty(folio))
>> +		return true;
>> +
>> +	/*
>> +	 * Don't mess with PTEs if page is already on the node
>> +	 * a single-threaded process is running on.
>> +	 */
>> +	nid = folio_nid(folio);
>> +	if (target_node == nid)
>> +		return true;
>> +	toptier = node_is_toptier(nid);
>> +
>> +	/*
>> +	 * Skip scanning top tier node if normal numa
>> +	 * balancing is disabled
>> +	 */
>> +	if (!(sysctl_numa_balancing_mode & NUMA_BALANCING_NORMAL) &&
>> +	    toptier)
>> +		return true;
>> +	return false;
>> +}
>> +
>> +static bool prot_numa_avoid_fault(struct vm_area_struct *vma,
>> +		unsigned long addr, pte_t oldpte, int target_node)
>> +{
>> +	struct folio *folio;
>> +	int ret;
>> +
>> +	/* Avoid TLB flush if possible */
>> +	if (pte_protnone(oldpte))
>> +		return true;
>> +
>> +	folio = vm_normal_folio(vma, addr, oldpte);
>> +	if (!folio || folio_is_zone_device(folio) ||
>> +	    folio_test_ksm(folio))
>> +		return true;
>> +	ret = prot_numa_skip(vma, folio, target_node);
> 
> What purpose does it solve to create additional helper prot_numa_skip().
> IOW - why cannot all of these be inside prot_numa_avoid_fault() itself.

That is in preparation for patch 2, so that I can skip a PTE batch.

> 
>> +	if (ret)
>> +		return ret;
>> +	if (folio_use_access_time(folio))
>> +		folio_xchg_access_time(folio,
>> +			jiffies_to_msecs(jiffies));
>> +	return false;
>> +}
>> +
>>   static long change_pte_range(struct mmu_gather *tlb,
>>   		struct vm_area_struct *vma, pmd_t *pmd, unsigned long addr,
>>   		unsigned long end, pgprot_t newprot, unsigned long cp_flags)
>> @@ -116,56 +181,11 @@ static long change_pte_range(struct mmu_gather *tlb,
>>   			 * Avoid trapping faults against the zero or KSM
>>   			 * pages. See similar comment in change_huge_pmd.
>>   			 */
>> -			if (prot_numa) {
>> -				struct folio *folio;
>> -				int nid;
>> -				bool toptier;
>> -
>> -				/* Avoid TLB flush if possible */
>> -				if (pte_protnone(oldpte))
>> -					continue;
>> -
>> -				folio = vm_normal_folio(vma, addr, oldpte);
>> -				if (!folio || folio_is_zone_device(folio) ||
>> -				    folio_test_ksm(folio))
>> -					continue;
>> -
>> -				/* Also skip shared copy-on-write pages */
>> -				if (is_cow_mapping(vma->vm_flags) &&
>> -				    (folio_maybe_dma_pinned(folio) ||
>> -				     folio_maybe_mapped_shared(folio)))
>> -					continue;
>> -
>> -				/*
>> -				 * While migration can move some dirty pages,
>> -				 * it cannot move them all from MIGRATE_ASYNC
>> -				 * context.
>> -				 */
>> -				if (folio_is_file_lru(folio) &&
>> -				    folio_test_dirty(folio))
>> +			if (prot_numa &&
>> +			    prot_numa_avoid_fault(vma, addr,
>> +						  oldpte, target_node))
>>   					continue;
>>   
>> -				/*
>> -				 * Don't mess with PTEs if page is already on the node
>> -				 * a single-threaded process is running on.
>> -				 */
>> -				nid = folio_nid(folio);
>> -				if (target_node == nid)
>> -					continue;
>> -				toptier = node_is_toptier(nid);
>> -
>> -				/*
>> -				 * Skip scanning top tier node if normal numa
>> -				 * balancing is disabled
>> -				 */
>> -				if (!(sysctl_numa_balancing_mode & NUMA_BALANCING_NORMAL) &&
>> -				    toptier)
>> -					continue;
>> -				if (folio_use_access_time(folio))
>> -					folio_xchg_access_time(folio,
>> -						jiffies_to_msecs(jiffies));
>> -			}
>> -
>>   			oldpte = ptep_modify_prot_start(vma, addr, pte);
>>   			ptent = pte_modify(oldpte, newprot);
>>