linux-kernel - Re: [PATCH v5 2/2] mm: Allocate THP on hugezeropage wp-fault

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Message-ID: <7d21314c-9c53-4aa0-9a27-b07d21da429e@huawei.com>
Date: Tue, 24 Sep 2024 21:13:20 +0800
From: Kefeng Wang <wangkefeng.wang@...wei.com>
To: Dev Jain <dev.jain@....com>, <akpm@...ux-foundation.org>,
	<david@...hat.com>, <willy@...radead.org>, <kirill.shutemov@...ux.intel.com>
CC: <ryan.roberts@....com>, <anshuman.khandual@....com>,
	<catalin.marinas@....com>, <cl@...two.org>, <vbabka@...e.cz>,
	<mhocko@...e.com>, <apopple@...dia.com>, <dave.hansen@...ux.intel.com>,
	<will@...nel.org>, <baohua@...nel.org>, <jack@...e.cz>,
	<mark.rutland@....com>, <hughd@...gle.com>, <aneesh.kumar@...nel.org>,
	<yang@...amperecomputing.com>, <peterx@...hat.com>, <ioworker0@...il.com>,
	<jglisse@...gle.com>, <ziy@...dia.com>,
	<linux-arm-kernel@...ts.infradead.org>, <linux-kernel@...r.kernel.org>,
	<linux-mm@...ck.org>
Subject: Re: [PATCH v5 2/2] mm: Allocate THP on hugezeropage wp-fault



On 2024/9/24 18:16, Dev Jain wrote:
> Introduce do_huge_zero_wp_pmd() to handle wp-fault on a hugezeropage and
> replace it with a PMD-mapped THP. Remember to flush TLB entry
> corresponding to the hugezeropage. In case of failure, fallback
> to splitting the PMD.
> 
> Signed-off-by: Dev Jain <dev.jain@....com>
> ---
>   mm/huge_memory.c | 44 +++++++++++++++++++++++++++++++++++++++++++-
>   1 file changed, 43 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index bdbf67c18f6c..fbb195bc2038 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1790,6 +1790,41 @@ void huge_pmd_set_accessed(struct vm_fault *vmf)
>   	spin_unlock(vmf->ptl);
>   }
>   
> +static vm_fault_t do_huge_zero_wp_pmd(struct vm_fault *vmf)
> +{
> +	unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
> +	struct vm_area_struct *vma = vmf->vma;
> +	struct mmu_notifier_range range;
> +	struct folio *folio;
> +	vm_fault_t ret = 0;
> +
> +	folio = vma_alloc_anon_folio_pmd(vma, vmf->address);
> +	if (unlikely(!folio)) {
> +		ret = VM_FAULT_FALLBACK;

I'd like to return VM_FAULT_FALLBACK if you re-post.

Reviewed-by: Kefeng Wang <wangkefeng.wang@...wei.com>

> +		goto out;
> +	}
> +
> +	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma->vm_mm, haddr,
> +				haddr + HPAGE_PMD_SIZE);
> +	mmu_notifier_invalidate_range_start(&range);
> +	vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd);
> +	if (unlikely(!pmd_same(pmdp_get(vmf->pmd), vmf->orig_pmd)))
> +		goto release;
> +	ret = check_stable_address_space(vma->vm_mm);
> +	if (ret)
> +		goto release;
> +	(void)pmdp_huge_clear_flush(vma, haddr, vmf->pmd);
> +	map_anon_folio_pmd(folio, vmf->pmd, vma, haddr);
> +	goto unlock;
> +release:
> +	folio_put(folio);
> +unlock:
> +	spin_unlock(vmf->ptl);
> +	mmu_notifier_invalidate_range_end(&range);
> +out:
> +	return ret;
> +}
> +
>   vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf)
>   {
>   	const bool unshare = vmf->flags & FAULT_FLAG_UNSHARE;
> @@ -1802,8 +1837,15 @@ vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf)
>   	vmf->ptl = pmd_lockptr(vma->vm_mm, vmf->pmd);
>   	VM_BUG_ON_VMA(!vma->anon_vma, vma);
>   
> -	if (is_huge_zero_pmd(orig_pmd))
> +	if (is_huge_zero_pmd(orig_pmd)) {
> +		vm_fault_t ret = do_huge_zero_wp_pmd(vmf);
> +
> +		if (!(ret & VM_FAULT_FALLBACK))
> +			return ret;
> +
> +		/* Fallback to splitting PMD if THP cannot be allocated */
>   		goto fallback;
> +	}
>   
>   	spin_lock(vmf->ptl);
>