From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751762AbdFGPlG (ORCPT ); Wed, 7 Jun 2017 11:41:06 -0400 Received: from foss.arm.com ([217.140.101.70]:34372 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751072AbdFGPlE (ORCPT ); Wed, 7 Jun 2017 11:41:04 -0400 Date: Wed, 7 Jun 2017 16:41:11 +0100 From: Will Deacon To: Punit Agrawal Cc: Catalin Marinas , David Woods , steve.capper@arm.com, tbaicar@codeaurora.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, manoj.iyer@canonical.com Subject: Re: [PATCH v2 1/3] arm64: hugetlb: Fix huge_pte_offset to return poisoned page table entries Message-ID: <20170607154111.GA2669@arm.com> References: <20170517152336.6052-1-punit.agrawal@arm.com> <20170517152336.6052-2-punit.agrawal@arm.com> <20170607134731.GG30263@arm.com> <20170607143037.ovyo5pxywty2r6as@localhost> <20170607145715.GI30263@arm.com> <87h8zr24qb.fsf@e105922-lin.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87h8zr24qb.fsf@e105922-lin.cambridge.arm.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jun 07, 2017 at 04:32:28PM +0100, Punit Agrawal wrote: > Will Deacon 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(). Ok, so does that mean that many of the huge_pte_none checks in mm/hugetlb.c that operate on a huge_ptep_get of non-NULL output from huge_pte_offset are actually redundant? Will