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

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Message-ID: <20200222170222.GJ24185@bombadil.infradead.org>
Date:   Sat, 22 Feb 2020 09:02:22 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     "Longpeng (Mike)" <longpeng2@...wei.com>
Cc:     Qian Cai <cai@....pw>, 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,
        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 Sat, Feb 22, 2020 at 02:33:10PM +0800, Longpeng (Mike) wrote:
> 在 2020/2/22 13:23, Qian Cai 写道:
> >> 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?
> > 
> As explained in the commit messages, it's for safe(e.g. avoid the compilier
> mischief). You can also find the same usage in the ARM64's huge_pte_offset() in
> arch/arm64/mm/hugetlbpage.c

I rather agree with Qian; if we need something like READ_ONCE() here,
why don't we always need it as part of pgd_present()?  It seems like an
unnecessary burden for every user.