linux-kernel - Re: [PATCH v2 10/10] mm/hugetlb: Document why page_vma_mapped

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Message-ID: <6a970de6-e3da-f57d-14fd-55f65ddcb27d@nvidia.com>
Date:   Wed, 7 Dec 2022 16:16:11 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Peter Xu <peterx@...hat.com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>
CC:     Jann Horn <jannh@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        James Houghton <jthoughton@...gle.com>,
        Rik van Riel <riel@...riel.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Nadav Amit <nadav.amit@...il.com>,
        "Mike Kravetz" <mike.kravetz@...cle.com>,
        David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Muchun Song <songmuchun@...edance.com>
Subject: Re: [PATCH v2 10/10] mm/hugetlb: Document why page_vma_mapped_walk()
 is safe to walk

On 12/7/22 12:31, Peter Xu wrote:
> Taking vma lock here is not needed for now because all potential hugetlb
> walkers here should have i_mmap_rwsem held.  Document the fact.
> 
> Reviewed-by: Mike Kravetz <mike.kravetz@...cle.com>
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
>   mm/page_vma_mapped.c | 10 ++++++++--
>   1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> index e97b2e23bd28..2e59a0419d22 100644
> --- a/mm/page_vma_mapped.c
> +++ b/mm/page_vma_mapped.c
> @@ -168,8 +168,14 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>   		/* The only possible mapping was handled on last iteration */
>   		if (pvmw->pte)
>   			return not_found(pvmw);
> -
> -		/* when pud is not present, pte will be NULL */
> +		/*
> +		 * NOTE: we don't need explicit lock here to walk the
> +		 * hugetlb pgtable because either (1) potential callers of
> +		 * hugetlb pvmw currently holds i_mmap_rwsem, or (2) the
> +		 * caller will not walk a hugetlb vma (e.g. ksm or uprobe).
> +		 * When one day this rule breaks, one will get a warning
> +		 * in hugetlb_walk(), and then we'll figure out what to do.
> +		 */

Confused. Is this documentation actually intended to refer to hugetlb_walk()
itself, or just this call site? If the former, then let's move it over
to be right before hugetlb_walk().

>   		pvmw->pte = hugetlb_walk(vma, pvmw->address, size);
>   		if (!pvmw->pte)
>   			return false;

thanks,
-- 
John Hubbard
NVIDIA