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Subject: Re: [PATCH v2] mm/hugetlb: fix a addressing exception caused by
 huge_pte_offset()
To:     Qian Cai <cai@lca.pw>
CC:     <akpm@linux-foundation.org>, <mike.kravetz@oracle.com>,
        <kirill.shutemov@linux.intel.com>, <linux-kernel@vger.kernel.org>,
        <arei.gonglei@huawei.com>, <weidong.huang@huawei.com>,
        <weifuqiang@huawei.com>, <kvm@vger.kernel.org>,
        <linux-mm@kvack.org>, "Matthew Wilcox" <willy@infradead.org>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        <stable@vger.kernel.org>
References: <C4ED630A-FAD8-4998-A0A3-9C36F3303379@lca.pw>
From:   "Longpeng (Mike)" <longpeng2@huawei.com>
Message-ID: <f274b368-6fdb-2ae3-160e-fd8b105b9ac4@huawei.com>
Date:   Sat, 22 Feb 2020 14:33:10 +0800
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在 2020/2/22 13:23, Qian Cai 写道:
> 
> 
>> On Feb 21, 2020, at 10:34 PM, Longpeng(Mike) <longpeng2@huawei.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

-- 
Regards,
Longpeng(Mike)