linux-kernel - Re: [PATCH 1/2] arm64: hugetlb: remove the wrong pmd check in find_num

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Date:   Fri, 4 Nov 2016 09:48:14 -0600
From:   Catalin Marinas <catalin.marinas@....com>
To:     Huang Shijie <shijie.huang@....com>
Cc:     dwoods@...lanox.com, steve.capper@....com, will.deacon@....com,
        linux-kernel@...r.kernel.org, kaly.xin@....com,
        akpm@...ux-foundation.org, nd@....com,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 1/2] arm64: hugetlb: remove the wrong pmd check in
 find_num_contig()

On Fri, Nov 04, 2016 at 10:52:17AM +0800, Huang Shijie wrote:
> On Thu, Nov 03, 2016 at 06:16:16PM -0600, Catalin Marinas wrote:
> > On Thu, Nov 03, 2016 at 10:27:38AM +0800, Huang Shijie wrote:
> > > diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c
> > > index 2e49bd2..4811ef1 100644
> > > --- a/arch/arm64/mm/hugetlbpage.c
> > > +++ b/arch/arm64/mm/hugetlbpage.c
> > > @@ -61,10 +61,6 @@ static int find_num_contig(struct mm_struct *mm, unsigned long addr,
> > >  		return 1;
> > >  	}
> > >  	pmd = pmd_offset(pud, addr);
> > > -	if (!pmd_present(*pmd)) {
> > > -		VM_BUG_ON(!pmd_present(*pmd));
> > > -		return 1;
> > > -	}
> > >  	if ((pte_t *)pmd == ptep) {
> > >  		*pgsize = PMD_SIZE;
> > >  		return CONT_PMDS;
> > 
> > BTW, for the !pud_present() and !pgd_present() cases, shouldn't
> > find_num_contig() actually return 0? These are more likely real bugs, so
> > no point in setting the huge pte.
> 
> The kernel will not call the find_num_contig() if the PGD/PUD are empty.
> Please see the code in the hugetlb_fault().
> 
>    ------------------------------------------------------
> 	ptep = huge_pte_offset(mm, address);
> 	if (ptep) {
> 	    ...............................
> 	} else {
> 		ptep = huge_pte_alloc(mm, address, huge_page_size(h));
> 		if (!ptep)
> 			return VM_FAULT_OOM;
> 	}
>    ------------------------------------------------------

Exactly. So what is the reason for returning 1 if !pgd_present()? Would
removing the checks entirely or adding BUG() be a better option?

-- 
Catalin