linux-kernel - Re: [PATCH v2 1/3] arm64: hugetlb: Fix huge_pte_offset to return poisoned page table entries

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Message-ID: <87h8zr24qb.fsf@e105922-lin.cambridge.arm.com>
Date:   Wed, 07 Jun 2017 16:32:28 +0100
From:   Punit Agrawal <punit.agrawal@....com>
To:     Will Deacon <will.deacon@....com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        David Woods <dwoods@...lanox.com>, steve.capper@....com,
        tbaicar@...eaurora.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, manoj.iyer@...onical.com
Subject: Re: [PATCH v2 1/3] arm64: hugetlb: Fix huge_pte_offset to return poisoned page table entries

Will Deacon <will.deacon@....com> writes:

> On Wed, Jun 07, 2017 at 03:30:37PM +0100, Catalin Marinas wrote:
>> On Wed, Jun 07, 2017 at 02:47:32PM +0100, Will Deacon wrote:
>> > On Wed, May 17, 2017 at 04:23:34PM +0100, Punit Agrawal wrote:
>> > > --- a/arch/arm64/mm/hugetlbpage.c
>> > > +++ b/arch/arm64/mm/hugetlbpage.c
>> > > @@ -136,36 +136,27 @@ pte_t *huge_pte_offset(struct mm_struct *mm, unsigned long addr)
>> > >  {
>> > >  	pgd_t *pgd;
>> > >  	pud_t *pud;
>> > > -	pmd_t *pmd = NULL;
>> > > -	pte_t *pte = NULL;
>> > > +	pmd_t *pmd;
>> > >  
>> > >  	pgd = pgd_offset(mm, addr);
>> > >  	pr_debug("%s: addr:0x%lx pgd:%p\n", __func__, addr, pgd);
>> > >  	if (!pgd_present(*pgd))
>> > >  		return NULL;
>> > > +
>> > >  	pud = pud_offset(pgd, addr);
>> > > -	if (!pud_present(*pud))
>> > > +	if (pud_none(*pud))
>> > >  		return NULL;
>> > 
>> > Do you actually need this special case?
>> > 
>> > > -
>> > > -	if (pud_huge(*pud))
>> > > +	/* swap or huge page */
>> > > +	if (!pud_present(*pud) || pud_huge(*pud))
>> > 
>> > ... couldn't you just add a '|| pud_none(*pud)' in here?
>> > 

I think an earlier version took this approach but...

>> > >  		return (pte_t *)pud;
>> 
>> But then you no longer return NULL if *pud == 0.
>
> Does that actually matter? The bits of hugetlb code I looked at will
> deferenced the returned pud and handle the huge_pte_none case correctly.

For hugetlb fault handling (hugetlb_fault()), returning NULL vs pointer
to the pud/pmd results in different behaviour. If we return the pud when
pud_none(), then we lose the resulting hugepage size check we get from
huge_pte_alloc().

>
> Will