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

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Date:   Wed, 16 Mar 2022 15:03:28 +0800
From:   maobibo <maobibo@...ngson.cn>
To:     Anshuman Khandual <anshuman.khandual@....com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: add access/dirty bit on numa page fault



On 03/16/2022 02:43 PM, Anshuman Khandual wrote:
> 
> 
> On 3/16/22 06:38, Bibo Mao wrote:
>> 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.
> 
> The current code does not set the access and dirty bits when ever
> applicable i.e on FAULT_FLAG_WRITE, on the pte (old if migration
> fails, new if migration succeeds) ? Did not this cause any problem
> earlier ? I am wondering how this might have gone unnoticed.

On arm/x86 platform hw will set access/dirty bits automatically,
however on MIPS platform access/dirty bits are set by software in next
page fault, it is relatively easier to watch on MIPS platform.

regards
bibo,mao

> 
>>
>> 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);