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

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Message-ID: <20161108022527.GB982@sha-win-210.asiapac.arm.com>
Date:   Tue, 8 Nov 2016 10:25:29 +0800
From:   Huang Shijie <shijie.huang@....com>
To:     Catalin Marinas <catalin.marinas@....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 09:48:14AM -0600, Catalin Marinas wrote:
> 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
I think the author was too cautious for returning 1 if !pgd_present().
:)
> removing the checks entirely or adding BUG() be a better option?
I will remove the checks in the next version.

Thanks
Huang Shijie