From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ot1-f71.google.com (mail-ot1-f71.google.com [209.85.210.71]) by kanga.kvack.org (Postfix) with ESMTP id 276106B0006 for ; Fri, 12 Oct 2018 04:02:45 -0400 (EDT) Received: by mail-ot1-f71.google.com with SMTP id j65-v6so8062613otc.5 for ; Fri, 12 Oct 2018 01:02:45 -0700 (PDT) Received: from foss.arm.com (usa-sjc-mx-foss1.foss.arm.com. [217.140.101.70]) by mx.google.com with ESMTP id w50si211717oti.317.2018.10.12.01.02.44 for ; Fri, 12 Oct 2018 01:02:44 -0700 (PDT) Subject: Re: [PATCH] mm/thp: Correctly differentiate between mapped THP and PMD migration entry References: <1539057538-27446-1-git-send-email-anshuman.khandual@arm.com> <20181009130421.bmus632ocurn275u@kshutemo-mobl1> <20181009131803.GH6248@arm.com> From: Anshuman Khandual Message-ID: Date: Fri, 12 Oct 2018 13:32:39 +0530 MIME-Version: 1.0 In-Reply-To: <20181009131803.GH6248@arm.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Will Deacon , "Kirill A. Shutemov" Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, kirill.shutemov@linux.intel.com, akpm@linux-foundation.org, mhocko@suse.com, zi.yan@cs.rutgers.edu On 10/09/2018 06:48 PM, Will Deacon wrote: > On Tue, Oct 09, 2018 at 04:04:21PM +0300, Kirill A. Shutemov wrote: >> On Tue, Oct 09, 2018 at 09:28:58AM +0530, Anshuman Khandual wrote: >>> A normal mapped THP page at PMD level should be correctly differentiated >>> from a PMD migration entry while walking the page table. A mapped THP would >>> additionally check positive for pmd_present() along with pmd_trans_huge() >>> as compared to a PMD migration entry. This just adds a new conditional test >>> differentiating the two while walking the page table. >>> >>> Fixes: 616b8371539a6 ("mm: thp: enable thp migration in generic path") >>> Signed-off-by: Anshuman Khandual >>> --- >>> On X86, pmd_trans_huge() and is_pmd_migration_entry() are always mutually >>> exclusive which makes the current conditional block work for both mapped >>> and migration entries. This is not same with arm64 where pmd_trans_huge() >>> returns positive for both mapped and migration entries. Could some one >>> please explain why pmd_trans_huge() has to return false for migration >>> entries which just install swap bits and its still a PMD ? >> >> I guess it's just a design choice. Any reason why arm64 cannot do the >> same? > > Anshuman, would it work to: > > #define pmd_trans_huge(pmd) (pmd_present(pmd) && !(pmd_val(pmd) & PMD_TABLE_BIT)) yeah this works but some how does not seem like the right thing to do but can be the very last option.