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

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Message-ID: <YjMoMVWXoJH9cmuf@casper.infradead.org>
Date:   Thu, 17 Mar 2022 12:23:13 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Bibo Mao <maobibo@...ngson.cn>
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 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?

> 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
> 
>