From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751646AbdGZIup (ORCPT ); Wed, 26 Jul 2017 04:50:45 -0400 Received: from mx2.suse.de ([195.135.220.15]:34619 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1750813AbdGZIum (ORCPT ); Wed, 26 Jul 2017 04:50:42 -0400 Date: Wed, 26 Jul 2017 10:50:38 +0200 From: Michal Hocko To: Punit Agrawal Cc: Andrew Morton , Naoya Horiguchi , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, steve.capper@arm.com, will.deacon@arm.com, catalin.marinas@arm.com, kirill.shutemov@linux.intel.com, Mike Kravetz Subject: Re: [PATCH 1/1] mm/hugetlb: Make huge_pte_offset() consistent and document behaviour Message-ID: <20170726085038.GB2981@dhcp22.suse.cz> References: <20170725154114.24131-1-punit.agrawal@arm.com> <20170725154114.24131-2-punit.agrawal@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170725154114.24131-2-punit.agrawal@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 Tue 25-07-17 16:41:14, Punit Agrawal wrote: > When walking the page tables to resolve an address that points to > !p*d_present() entry, huge_pte_offset() returns inconsistent values > depending on the level of page table (PUD or PMD). > > It returns NULL in the case of a PUD entry while in the case of a PMD > entry, it returns a pointer to the page table entry. > > A similar inconsitency exists when handling swap entries - returns NULL > for a PUD entry while a pointer to the pte_t is retured for the PMD > entry. > > Update huge_pte_offset() to make the behaviour consistent - return NULL > in the case of p*d_none() and a pointer to the pte_t for hugepage or > swap entries. > > Document the behaviour to clarify the expected behaviour of this > function. This is to set clear semantics for architecture specific > implementations of huge_pte_offset(). hugetlb pte semantic is a disaster and I agree it could see some cleanup/clarifications but I am quite nervous to see a patchi like this. How do we check that nothing will get silently broken by this change? > Signed-off-by: Punit Agrawal > --- > mm/hugetlb.c | 22 +++++++++++++++++++--- > 1 file changed, 19 insertions(+), 3 deletions(-) > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c > index bc48ee783dd9..72dd1139a8e4 100644 > --- a/mm/hugetlb.c > +++ b/mm/hugetlb.c > @@ -4603,6 +4603,13 @@ pte_t *huge_pte_alloc(struct mm_struct *mm, > return pte; > } > > +/* > + * huge_pte_offset() - Walk the page table to resolve the hugepage > + * entry at address @addr > + * > + * Return: Pointer to page table or swap entry (PUD or PMD) for address @addr > + * or NULL if the entry is p*d_none(). > + */ > pte_t *huge_pte_offset(struct mm_struct *mm, > unsigned long addr, unsigned long sz) > { > @@ -4617,13 +4624,22 @@ pte_t *huge_pte_offset(struct mm_struct *mm, > p4d = p4d_offset(pgd, addr); > if (!p4d_present(*p4d)) > return NULL; > + > pud = pud_offset(p4d, addr); > - if (!pud_present(*pud)) > + if (pud_none(*pud)) > return NULL; > - if (pud_huge(*pud)) > + /* hugepage or swap? */ > + if (pud_huge(*pud) || !pud_present(*pud)) > return (pte_t *)pud; > + > pmd = pmd_offset(pud, addr); > - return (pte_t *) pmd; > + if (pmd_none(*pmd)) > + return NULL; > + /* hugepage or swap? */ > + if (pmd_huge(*pmd) || !pmd_present(*pmd)) > + return (pte_t *) pmd; > + > + return NULL; > } > > #endif /* CONFIG_ARCH_WANT_GENERAL_HUGETLB */ > -- > 2.11.0 > -- Michal Hocko SUSE Labs From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f69.google.com (mail-wm0-f69.google.com [74.125.82.69]) by kanga.kvack.org (Postfix) with ESMTP id 05F996B02F4 for ; Wed, 26 Jul 2017 04:50:43 -0400 (EDT) Received: by mail-wm0-f69.google.com with SMTP id h126so7552694wmf.10 for ; Wed, 26 Jul 2017 01:50:42 -0700 (PDT) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id q23si16801882wrc.56.2017.07.26.01.50.41 for (version=TLS1 cipher=AES128-SHA bits=128/128); Wed, 26 Jul 2017 01:50:41 -0700 (PDT) Date: Wed, 26 Jul 2017 10:50:38 +0200 From: Michal Hocko Subject: Re: [PATCH 1/1] mm/hugetlb: Make huge_pte_offset() consistent and document behaviour Message-ID: <20170726085038.GB2981@dhcp22.suse.cz> References: <20170725154114.24131-1-punit.agrawal@arm.com> <20170725154114.24131-2-punit.agrawal@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170725154114.24131-2-punit.agrawal@arm.com> Sender: owner-linux-mm@kvack.org List-ID: To: Punit Agrawal Cc: Andrew Morton , Naoya Horiguchi , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, steve.capper@arm.com, will.deacon@arm.com, catalin.marinas@arm.com, kirill.shutemov@linux.intel.com, Mike Kravetz On Tue 25-07-17 16:41:14, Punit Agrawal wrote: > When walking the page tables to resolve an address that points to > !p*d_present() entry, huge_pte_offset() returns inconsistent values > depending on the level of page table (PUD or PMD). > > It returns NULL in the case of a PUD entry while in the case of a PMD > entry, it returns a pointer to the page table entry. > > A similar inconsitency exists when handling swap entries - returns NULL > for a PUD entry while a pointer to the pte_t is retured for the PMD > entry. > > Update huge_pte_offset() to make the behaviour consistent - return NULL > in the case of p*d_none() and a pointer to the pte_t for hugepage or > swap entries. > > Document the behaviour to clarify the expected behaviour of this > function. This is to set clear semantics for architecture specific > implementations of huge_pte_offset(). hugetlb pte semantic is a disaster and I agree it could see some cleanup/clarifications but I am quite nervous to see a patchi like this. How do we check that nothing will get silently broken by this change? > Signed-off-by: Punit Agrawal > --- > mm/hugetlb.c | 22 +++++++++++++++++++--- > 1 file changed, 19 insertions(+), 3 deletions(-) > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c > index bc48ee783dd9..72dd1139a8e4 100644 > --- a/mm/hugetlb.c > +++ b/mm/hugetlb.c > @@ -4603,6 +4603,13 @@ pte_t *huge_pte_alloc(struct mm_struct *mm, > return pte; > } > > +/* > + * huge_pte_offset() - Walk the page table to resolve the hugepage > + * entry at address @addr > + * > + * Return: Pointer to page table or swap entry (PUD or PMD) for address @addr > + * or NULL if the entry is p*d_none(). > + */ > pte_t *huge_pte_offset(struct mm_struct *mm, > unsigned long addr, unsigned long sz) > { > @@ -4617,13 +4624,22 @@ pte_t *huge_pte_offset(struct mm_struct *mm, > p4d = p4d_offset(pgd, addr); > if (!p4d_present(*p4d)) > return NULL; > + > pud = pud_offset(p4d, addr); > - if (!pud_present(*pud)) > + if (pud_none(*pud)) > return NULL; > - if (pud_huge(*pud)) > + /* hugepage or swap? */ > + if (pud_huge(*pud) || !pud_present(*pud)) > return (pte_t *)pud; > + > pmd = pmd_offset(pud, addr); > - return (pte_t *) pmd; > + if (pmd_none(*pmd)) > + return NULL; > + /* hugepage or swap? */ > + if (pmd_huge(*pmd) || !pmd_present(*pmd)) > + return (pte_t *) pmd; > + > + return NULL; > } > > #endif /* CONFIG_ARCH_WANT_GENERAL_HUGETLB */ > -- > 2.11.0 > -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org