Re: [PATCH v3 8/9] mm/hugetlb: Make walk_hugetlb_range() safe to pmd unshare

From: John Hubbard <jhubbard@nvidia.com>
To: Peter Xu <peterx@redhat.com>, <linux-mm@kvack.org>,
	<linux-kernel@vger.kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Miaohe Lin <linmiaohe@huawei.com>,
	David Hildenbrand <david@redhat.com>,
	Nadav Amit <nadav.amit@gmail.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Jann Horn <jannh@google.com>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	James Houghton <jthoughton@google.com>,
	Rik van Riel <riel@surriel.com>,
	Muchun Song <songmuchun@bytedance.com>
Subject: Re: [PATCH v3 8/9] mm/hugetlb: Make walk_hugetlb_range() safe to pmd unshare
Date: Fri, 9 Dec 2022 11:58:50 -0800	[thread overview]
Message-ID: <bbe8baf3-45b5-e843-2dcd-e7f0de035066@nvidia.com> (raw)
In-Reply-To: <20221209170100.973970-9-peterx@redhat.com>

On 12/9/22 09:00, 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.
> 
> Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>   include/linux/pagewalk.h | 11 ++++++++++-
>   mm/hmm.c                 | 15 ++++++++++++++-
>   mm/pagewalk.c            |  2 ++
>   3 files changed, 26 insertions(+), 2 deletions(-)

Reviewed-by: John Hubbard <jhubbard@nvidia.com>

thanks,
-- 
John Hubbard
NVIDIA

> 
> diff --git a/include/linux/pagewalk.h b/include/linux/pagewalk.h
> index 959f52e5867d..27a6df448ee5 100644
> --- a/include/linux/pagewalk.h
> +++ b/include/linux/pagewalk.h
> @@ -21,7 +21,16 @@ struct mm_walk;
>    *			depth is -1 if not known, 0:PGD, 1:P4D, 2:PUD, 3:PMD.
>    *			Any folded depths (where PTRS_PER_P?D is equal to 1)
>    *			are skipped.
> - * @hugetlb_entry:	if set, called for each hugetlb entry
> + * @hugetlb_entry:	if set, called for each hugetlb entry. This hook
> + *			function is called with the vma lock held, in order to
> + *			protect against a concurrent freeing of the pte_t* or
> + *			the ptl. In some cases, the hook function needs to drop
> + *			and retake the vma lock in order to avoid deadlocks
> + *			while calling other functions. In such cases the hook
> + *			function must either refrain from accessing the pte or
> + *			ptl after dropping the vma lock, or else revalidate
> + *			those items after re-acquiring the vma lock and before
> + *			accessing them.
>    * @test_walk:		caller specific callback function to determine whether
>    *			we walk over the current vma or not. Returning 0 means
>    *			"do page table walk over the current vma", returning
> diff --git a/mm/hmm.c b/mm/hmm.c
> index 3850fb625dda..796de6866089 100644
> --- a/mm/hmm.c
> +++ b/mm/hmm.c
> @@ -493,8 +493,21 @@ static int hmm_vma_walk_hugetlb_entry(pte_t *pte, unsigned long hmask,
>   	required_fault =
>   		hmm_pte_need_fault(hmm_vma_walk, pfn_req_flags, cpu_flags);
>   	if (required_fault) {
> +		int ret;
> +
>   		spin_unlock(ptl);
> -		return hmm_vma_fault(addr, end, required_fault, walk);
> +		hugetlb_vma_unlock_read(vma);
> +		/*
> +		 * Avoid deadlock: drop the vma lock before calling
> +		 * hmm_vma_fault(), which will itself potentially take and
> +		 * drop the vma lock. This is also correct from a
> +		 * protection point of view, because there is no further
> +		 * use here of either pte or ptl after dropping the vma
> +		 * lock.
> +		 */
> +		ret = hmm_vma_fault(addr, end, required_fault, walk);
> +		hugetlb_vma_lock_read(vma);
> +		return ret;
>   	}
>   
>   	pfn = pte_pfn(entry) + ((start & ~hmask) >> PAGE_SHIFT);
> 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);
> @@ -314,6 +315,7 @@ static int walk_hugetlb_range(unsigned long addr, unsigned long end,
>   		if (err)
>   			break;
>   	} while (addr = next, addr != end);
> +	hugetlb_vma_unlock_read(vma);
>   
>   	return err;
>   }