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

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Message-ID: <93302a47-9fda-25c7-4212-41b8dd027696@arm.com>
Date:   Wed, 16 Mar 2022 12:13:07 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Bibo Mao <maobibo@...ngson.cn>,
        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 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.

> 
> 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);