From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751677AbdGZMeH (ORCPT ); Wed, 26 Jul 2017 08:34:07 -0400 Received: from mx2.suse.de ([195.135.220.15]:58728 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751558AbdGZMeF (ORCPT ); Wed, 26 Jul 2017 08:34:05 -0400 Date: Wed, 26 Jul 2017 14:33:57 +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: <20170726123357.GP2981@dhcp22.suse.cz> References: <20170725154114.24131-1-punit.agrawal@arm.com> <20170725154114.24131-2-punit.agrawal@arm.com> <20170726085038.GB2981@dhcp22.suse.cz> <20170726085325.GC2981@dhcp22.suse.cz> <87bmo7jt31.fsf@e105922-lin.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87bmo7jt31.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 26-07-17 13:11:46, Punit Agrawal wrote: > Hi Michal, > > Michal Hocko writes: > > > On Wed 26-07-17 10:50:38, Michal Hocko wrote: > >> 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? > > Glad I'm not the only one who finds the hugetlb semantics somewhat > confusing. :) This is a huge understatement. It is a source of nightmares. > I've been running tests from mce-test suite and libhugetlbfs for similar > changes we did on arm64. There could be assumptions that were not > exercised but I'm not sure how to check for all the possible usages. > > Do you have any other suggestions that can help improve confidence in > the patch? Unfortunatelly I don't. I just know there were many subtle assumptions all over the place so I am rather careful to not touch the code unless really necessary. That being said, I am not opposing your patch. -- Michal Hocko SUSE Labs From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f70.google.com (mail-wm0-f70.google.com [74.125.82.70]) by kanga.kvack.org (Postfix) with ESMTP id 113646B0313 for ; Wed, 26 Jul 2017 08:34:02 -0400 (EDT) Received: by mail-wm0-f70.google.com with SMTP id e204so7343272wma.2 for ; Wed, 26 Jul 2017 05:34:02 -0700 (PDT) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id 2si13463901wrh.288.2017.07.26.05.34.00 for (version=TLS1 cipher=AES128-SHA bits=128/128); Wed, 26 Jul 2017 05:34:00 -0700 (PDT) Date: Wed, 26 Jul 2017 14:33:57 +0200 From: Michal Hocko Subject: Re: [PATCH 1/1] mm/hugetlb: Make huge_pte_offset() consistent and document behaviour Message-ID: <20170726123357.GP2981@dhcp22.suse.cz> References: <20170725154114.24131-1-punit.agrawal@arm.com> <20170725154114.24131-2-punit.agrawal@arm.com> <20170726085038.GB2981@dhcp22.suse.cz> <20170726085325.GC2981@dhcp22.suse.cz> <87bmo7jt31.fsf@e105922-lin.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87bmo7jt31.fsf@e105922-lin.cambridge.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 Wed 26-07-17 13:11:46, Punit Agrawal wrote: > Hi Michal, > > Michal Hocko writes: > > > On Wed 26-07-17 10:50:38, Michal Hocko wrote: > >> 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? > > Glad I'm not the only one who finds the hugetlb semantics somewhat > confusing. :) This is a huge understatement. It is a source of nightmares. > I've been running tests from mce-test suite and libhugetlbfs for similar > changes we did on arm64. There could be assumptions that were not > exercised but I'm not sure how to check for all the possible usages. > > Do you have any other suggestions that can help improve confidence in > the patch? Unfortunatelly I don't. I just know there were many subtle assumptions all over the place so I am rather careful to not touch the code unless really necessary. That being said, I am not opposing your patch. -- 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