linux-kernel - Re: [PATCH v2] mm: add access/dirty bit on numa page fault

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Date:   Fri, 18 Mar 2022 09:01:32 +0800
From:   maobibo <maobibo@...ngson.cn>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Anshuman Khandual <anshuman.khandual@....com>
Subject: Re: [PATCH v2] mm: add access/dirty bit on numa page fault



On 03/17/2022 08:23 PM, Matthew Wilcox wrote:
> On Thu, Mar 17, 2022 at 02:50:33AM -0400, Bibo Mao wrote:
>> On platforms like x86/arm which supports hw page walking, access
>> and dirty bit is set by hw, however on some platforms without
>> such hw functions, access and dirty bit is set by software in
>> next trap.
>>
>> During numa page fault, dirty bit can be added for old pte if
>> fail to migrate on write fault. And if it succeeds to migrate,
>> access bit can be added for migrated new pte, also dirty bit
>> can be added for write fault.
> 
> Is this a correctness problem, in which case this will need to be
> backported, or is this a performance problem, in which case can you
> share some numbers?
It is only performance issue, and there is no obvious performance
improvement for general workloads on my hand, but I do not test
it on microbenchmark.

> 
>> Signed-off-by: Bibo Mao <maobibo@...ngson.cn>
>> ---
>>  mm/memory.c | 21 ++++++++++++++++++++-
>>  1 file changed, 20 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index c125c4969913..65813bec9c06 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -4404,6 +4404,22 @@ static vm_fault_t do_numa_page(struct vm_fault *vmf)
>>  	if (migrate_misplaced_page(page, vma, target_nid)) {
>>  		page_nid = target_nid;
>>  		flags |= TNF_MIGRATED;
>> +
>> +		/*
>> +		 * update pte entry with access bit, and dirty bit for
>> +		 * write fault
>> +		 */
>> +		spin_lock(vmf->ptl);
>> +		pte = *vmf->pte;
>> +		pte = pte_mkyoung(pte);
>> +		if (was_writable) {
>> +			pte = pte_mkwrite(pte);
>> +			if (vmf->flags & FAULT_FLAG_WRITE)
>> +				pte = pte_mkdirty(pte);
>> +		}
>> +		set_pte_at(vma->vm_mm, vmf->address, vmf->pte, pte);
>> +		update_mmu_cache(vma, vmf->address, vmf->pte);
>> +		pte_unmap_unlock(vmf->pte, vmf->ptl);
>>  	} else {
>>  		flags |= TNF_MIGRATE_FAIL;
>>  		vmf->pte = pte_offset_map(vmf->pmd, vmf->address);
>> @@ -4427,8 +4443,11 @@ static vm_fault_t do_numa_page(struct vm_fault *vmf)
>>  	old_pte = ptep_modify_prot_start(vma, vmf->address, vmf->pte);
>>  	pte = pte_modify(old_pte, vma->vm_page_prot);
>>  	pte = pte_mkyoung(pte);
>> -	if (was_writable)
>> +	if (was_writable) {
>>  		pte = pte_mkwrite(pte);
>> +		if (vmf->flags & FAULT_FLAG_WRITE)
>> +			pte = pte_mkdirty(pte);
>> +	}
>>  	ptep_modify_prot_commit(vma, vmf->address, vmf->pte, old_pte, pte);
>>  	update_mmu_cache(vma, vmf->address, vmf->pte);
>>  	pte_unmap_unlock(vmf->pte, vmf->ptl);
>> -- 
>> 2.31.1
>>
>>