linux-kernel - Re: [PATCH 08/10] mm/hugetlb: Make walk_hugetlb

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Date:   Mon, 5 Dec 2022 15:33:57 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        James Houghton <jthoughton@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Rik van Riel <riel@...riel.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Muchun Song <songmuchun@...edance.com>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH 08/10] mm/hugetlb: Make walk_hugetlb_range() safe to pmd
 unshare

On 11/29/22 14:35, Peter Xu wrote:
> Since walk_hugetlb_range() walks the pgtable, it needs the vma lock
> to make sure the pgtable page will not be freed concurrently.
> 
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
>  mm/pagewalk.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/pagewalk.c b/mm/pagewalk.c
> index 7f1c9b274906..d98564a7be57 100644
> --- a/mm/pagewalk.c
> +++ b/mm/pagewalk.c
> @@ -302,6 +302,7 @@ static int walk_hugetlb_range(unsigned long addr, unsigned long end,
>  	const struct mm_walk_ops *ops = walk->ops;
>  	int err = 0;
>  
> +	hugetlb_vma_lock_read(vma);
>  	do {
>  		next = hugetlb_entry_end(h, addr, end);
>  		pte = huge_pte_offset(walk->mm, addr & hmask, sz);

For each found pte, we will be calling mm_walk_ops->hugetlb_entry() with
the vma_lock held.  I looked into the various hugetlb_entry routines, and
I am not sure about hmm_vma_walk_hugetlb_entry.  It seems like it could
possibly call hmm_vma_fault -> handle_mm_fault -> hugetlb_fault.  If this
can happen, then we may have an issue as hugetlb_fault will also need to
acquire the vma_lock in read mode.

I do not know the hmm code well enough to know if this may be an actual
issue?
-- 
Mike Kravetz