linux-kernel - Re: [RFC PATCH 11/26] hugetlb: add hugetlb_walk

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Message-ID: <ac95fce2-b095-62ff-7f87-d803572e39f0@nutanix.com>
Date:   Mon, 27 Jun 2022 18:37:34 +0530
From:   "manish.mishra" <manish.mishra@...anix.com>
To:     James Houghton <jthoughton@...gle.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Peter Xu <peterx@...hat.com>
Cc:     David Hildenbrand <david@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Jue Wang <juew@...gle.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 11/26] hugetlb: add hugetlb_walk_to to do PT walks


On 24/06/22 11:06 pm, James Houghton wrote:
> This adds it for architectures that use GENERAL_HUGETLB, including x86.
>
> Signed-off-by: James Houghton <jthoughton@...gle.com>
> ---
>   include/linux/hugetlb.h |  2 ++
>   mm/hugetlb.c            | 45 +++++++++++++++++++++++++++++++++++++++++
>   2 files changed, 47 insertions(+)
>
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index e7a6b944d0cc..605aa19d8572 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -258,6 +258,8 @@ pte_t *huge_pte_alloc(struct mm_struct *mm, struct vm_area_struct *vma,
>   			unsigned long addr, unsigned long sz);
>   pte_t *huge_pte_offset(struct mm_struct *mm,
>   		       unsigned long addr, unsigned long sz);
> +int hugetlb_walk_to(struct mm_struct *mm, struct hugetlb_pte *hpte,
> +		    unsigned long addr, unsigned long sz, bool stop_at_none);
>   int huge_pmd_unshare(struct mm_struct *mm, struct vm_area_struct *vma,
>   				unsigned long *addr, pte_t *ptep);
>   void adjust_range_if_pmd_sharing_possible(struct vm_area_struct *vma,
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 557b0afdb503..3ec2a921ee6f 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -6981,6 +6981,51 @@ pte_t *huge_pte_offset(struct mm_struct *mm,
>   	return (pte_t *)pmd;
>   }


not strong feeling but this name looks confusing to me as it does

not only walk over page-tables but can also alloc.

> +int hugetlb_walk_to(struct mm_struct *mm, struct hugetlb_pte *hpte,
> +		    unsigned long addr, unsigned long sz, bool stop_at_none)
> +{
> +	pte_t *ptep;
> +
> +	if (!hpte->ptep) {
> +		pgd_t *pgd = pgd_offset(mm, addr);
> +
> +		if (!pgd)
> +			return -ENOMEM;
> +		ptep = (pte_t *)p4d_alloc(mm, pgd, addr);
> +		if (!ptep)
> +			return -ENOMEM;
> +		hugetlb_pte_populate(hpte, ptep, P4D_SHIFT);
> +	}
> +
> +	while (hugetlb_pte_size(hpte) > sz &&
> +			!hugetlb_pte_present_leaf(hpte) &&
> +			!(stop_at_none && hugetlb_pte_none(hpte))) {

Should this ordering of if-else condition be in reverse, i mean it will look

more natural and possibly less condition checks as we go from top to bottom.

> +		if (hpte->shift == PMD_SHIFT) {
> +			ptep = pte_alloc_map(mm, (pmd_t *)hpte->ptep, addr);
> +			if (!ptep)
> +				return -ENOMEM;
> +			hpte->shift = PAGE_SHIFT;
> +			hpte->ptep = ptep;
> +		} else if (hpte->shift == PUD_SHIFT) {
> +			ptep = (pte_t *)pmd_alloc(mm, (pud_t *)hpte->ptep,
> +						  addr);
> +			if (!ptep)
> +				return -ENOMEM;
> +			hpte->shift = PMD_SHIFT;
> +			hpte->ptep = ptep;
> +		} else if (hpte->shift == P4D_SHIFT) {
> +			ptep = (pte_t *)pud_alloc(mm, (p4d_t *)hpte->ptep,
> +						  addr);
> +			if (!ptep)
> +				return -ENOMEM;
> +			hpte->shift = PUD_SHIFT;
> +			hpte->ptep = ptep;
> +		} else
> +			BUG();
> +	}
> +	return 0;
> +}
> +
>   #endif /* CONFIG_ARCH_WANT_GENERAL_HUGETLB */
>   
>   #ifdef CONFIG_HUGETLB_HIGH_GRANULARITY_MAPPING