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: <20170607145715.GI30263@arm.com>
Date:   Wed, 7 Jun 2017 15:57:16 +0100
From:   Will Deacon <will.deacon@....com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Punit Agrawal <punit.agrawal@....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

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?
> > 
> > >  		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.

Will