Re: [PATCH v2] mm/hugetlb: fix a addressing exception caused by huge_pte_offset()

[Matthew Wilcox]

On Sat, Feb 22, 2020 at 02:33:10PM +0800, Longpeng (Mike) wrote:
> å 2020/2/22 13:23, Qian Cai åé:
> >> On Feb 21, 2020, at 10:34 PM, Longpeng(Mike) <longpeng2@xxxxxxxxxx> 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;
> > 
> > Whatâs 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.