linux-kernel - Re: [PATCH v2] mm/hugetlb: fix a addressing exception caused by huge_pte

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Date:   Sat, 22 Feb 2020 00:23:44 -0500
From:   Qian Cai <cai@....pw>
To:     "Longpeng(Mike)" <longpeng2@...wei.com>
Cc:     akpm@...ux-foundation.org, mike.kravetz@...cle.com,
        kirill.shutemov@...ux.intel.com, linux-kernel@...r.kernel.org,
        arei.gonglei@...wei.com, weidong.huang@...wei.com,
        weifuqiang@...wei.com, kvm@...r.kernel.org, linux-mm@...ck.org,
        Matthew Wilcox <willy@...radead.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH v2] mm/hugetlb: fix a addressing exception caused by huge_pte_offset()



> On Feb 21, 2020, at 10:34 PM, Longpeng(Mike) <longpeng2@...wei.com> wrote:
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index dd8737a..90daf37 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -4910,28 +4910,30 @@ pte_t *huge_pte_offset(struct mm_struct *mm,
> {
>    pgd_t *pgd;
>    p4d_t *p4d;
> -    pud_t *pud;
> -    pmd_t *pmd;
> +    pud_t *pud, pud_entry;
> +    pmd_t *pmd, pmd_entry;
> 
>    pgd = pgd_offset(mm, addr);
> -    if (!pgd_present(*pgd))
> +    if (!pgd_present(READ_ONCE(*pgd)))
>        return NULL;
>    p4d = p4d_offset(pgd, addr);
> -    if (!p4d_present(*p4d))
> +    if (!p4d_present(READ_ONCE(*p4d)))
>        return NULL;

What’s the point of READ_ONCE() on those two places?