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References: <20221030212929.335473-1-peterx@redhat.com> <20221030213043.335669-1-peterx@redhat.com>
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From:   James Houghton <jthoughton@google.com>
Date:   Wed, 2 Nov 2022 11:04:01 -0700
Message-ID: <CADrL8HUrCkjnBuD7=NJZ5gOBYWBbu=pa0sKp4FMnDH7OhtKvSA@mail.gmail.com>
Subject: Re: [PATCH RFC 09/10] mm/hugetlb: Make hugetlb_fault() RCU-safe
To:     Peter Xu <peterx@redhat.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Mike Kravetz <mike.kravetz@oracle.com>,
        David Hildenbrand <david@redhat.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Rik van Riel <riel@surriel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Muchun Song <songmuchun@bytedance.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Nadav Amit <nadav.amit@gmail.com>
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On Sun, Oct 30, 2022 at 2:30 PM Peter Xu <peterx@redhat.com> wrote:
>
> RCU makes sure the pte_t* won't go away from under us.  Please refer to the
> comment above huge_pte_offset() for more information.

Thanks for this series, Peter! :)

>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  mm/hugetlb.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 5dc87e4e6780..6d336d286394 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -5822,6 +5822,8 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
>         int need_wait_lock = 0;
>         unsigned long haddr = address & huge_page_mask(h);
>
> +       /* For huge_pte_offset() */
> +       rcu_read_lock();
>         ptep = huge_pte_offset(mm, haddr, huge_page_size(h));
>         if (ptep) {
>                 /*
> @@ -5830,13 +5832,15 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
>                  * not actually modifying content here.
>                  */
>                 entry = huge_ptep_get(ptep);
> +               rcu_read_unlock();
>                 if (unlikely(is_hugetlb_entry_migration(entry))) {
>                         migration_entry_wait_huge(vma, ptep);

ptep is used here (and we dereference it in
`__migration_entry_wait_huge`), so this looks unsafe to me. A simple
way to fix this would be to move the migration entry check after the
huge_pte_alloc call.

- James

>                         return 0;
>                 } else if (unlikely(is_hugetlb_entry_hwpoisoned(entry)))
>                         return VM_FAULT_HWPOISON_LARGE |
>                                 VM_FAULT_SET_HINDEX(hstate_index(h));
> -       }
> +       } else
> +               rcu_read_unlock();
>
>         /*
>          * Serialize hugepage allocation and instantiation, so that we don't
> --
> 2.37.3
>