From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx0a-001b2d01.pphosted.com (mx0a-001b2d01.pphosted.com [148.163.156.1]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 427yKl69NZzF37v for ; Mon, 10 Sep 2018 16:08:59 +1000 (AEST) Received: from pps.filterd (m0098404.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.22/8.16.0.22) with SMTP id w8A68tlx131203 for ; Mon, 10 Sep 2018 02:08:57 -0400 Received: from e06smtp02.uk.ibm.com (e06smtp02.uk.ibm.com [195.75.94.98]) by mx0a-001b2d01.pphosted.com with ESMTP id 2mdech1fva-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Mon, 10 Sep 2018 02:08:57 -0400 Received: from localhost by e06smtp02.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Mon, 10 Sep 2018 07:08:52 +0100 From: "Aneesh Kumar K.V" To: Christophe Leroy , Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , npiggin@gmail.com, aneesh.kumar@linux.vnet.ibm.com Cc: linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH v1 00/17] ban the use of _PAGE_XXX flags outside platform specific code In-Reply-To: <1f4d6cc9-5d18-b171-02b0-d715629d594f@c-s.fr> References: <8736uneylc.fsf@linux.ibm.com> <1f4d6cc9-5d18-b171-02b0-d715629d594f@c-s.fr> Date: Mon, 10 Sep 2018 11:38:46 +0530 MIME-Version: 1.0 Content-Type: text/plain Message-Id: <8736uhsx3l.fsf@linux.ibm.com> List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Christophe Leroy writes: > On 09/06/2018 09:58 AM, Aneesh Kumar K.V wrote: >> Christophe Leroy writes: >> >>> Today flags like for instance _PAGE_RW or _PAGE_USER are used through >>> common parts of code. >>> Using those directly in common parts of code have proven to lead to >>> mistakes or misbehaviour, because their use is not always as trivial >>> as one could think. >>> >>> For instance, (flags & _PAGE_USER) == 0 isn't enough to tell >>> that a page is a kernel page, because some targets are using >>> _PAGE_PRIVILEDGED and not _PAGE_USER, so the test has to be >>> (flags & (_PAGE_USER | _PAGE_PRIVILEDGED)) == _PAGE_PRIVILEDGED >>> This has to (bad) consequences: >>> >>> - All targets must define every bit, even the unsupported ones, >>> leading to a lot of useless #define _PAGE_XXX 0 >>> - If someone forgets to take into account all possible _PAGE_XXX bits >>> for the case, we can get unexpected behaviour on some targets. >>> >>> This becomes even more complex when we come to using _PAGE_RW. >>> Testing (flags & _PAGE_RW) is not enough to test whether a page >>> if writable or not, because: >>> >>> - Some targets have _PAGE_RO instead, which has to be unset to tell >>> a page is writable >>> - Some targets have _PAGE_R and _PAGE_W, in which case >>> _PAGE_RW = _PAGE_R | _PAGE_W >>> - Even knowing whether a page is readable is not always trivial because: >>> - Some targets requires to check that _PAGE_R is set to ensure page >>> is readable >>> - Some targets requires to check that _PAGE_NA is not set >>> - Some targets requires to check that _PAGE_RO or _PAGE_RW is set >>> >>> Etc .... >>> >>> In order to work around all those issues and minimise the risks of errors, >>> this serie aims at removing all use of _PAGE_XXX flags from powerpc code >>> and always use pte_xxx() and pte_mkxxx() accessors instead. Those accessors >>> are then defined in target specific parts of the kernel code. >> >> The series is really good. It also helps in code readability. Few things >> i am not sure there is a way to reduce the overhead >> >> - access = _PAGE_EXEC; >> + access = pte_val(pte_mkexec(__pte(0))); >> >> Considering we have multiple big endian to little endian coversion there >> for book3s 64. > > Thanks for the review. > > For the above, I propose the following: > > diff --git a/arch/powerpc/mm/hash_utils_64.c > b/arch/powerpc/mm/hash_utils_64.c > index f23a89d8e4ce..904ac9c84ea5 100644 > --- a/arch/powerpc/mm/hash_utils_64.c > +++ b/arch/powerpc/mm/hash_utils_64.c > @@ -1482,7 +1482,7 @@ static bool should_hash_preload(struct mm_struct > *mm, unsigned long ea) > #endif > > void hash_preload(struct mm_struct *mm, unsigned long ea, > - unsigned long access, unsigned long trap) > + bool is_exec, unsigned long trap) > { > int hugepage_shift; > unsigned long vsid; > @@ -1490,6 +1490,7 @@ void hash_preload(struct mm_struct *mm, unsigned > long ea, > pte_t *ptep; > unsigned long flags; > int rc, ssize, update_flags = 0; > + unsigned long access = is_exec ? _PAGE_EXEC : 0; I guess it will be better if we do unsigned long access = _PAGE_PRESENT | _PAGE_READ if (is_exec) access |= _PAGE_EXEC. That will also bring it closer to __hash_page. I agree that we should always find _PAGE_PRESENT and _PAGE_READ set, because we just handled the page fault. -aneesh