From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg1-f197.google.com (mail-pg1-f197.google.com [209.85.215.197]) by kanga.kvack.org (Postfix) with ESMTP id 16FFF6B0269 for ; Mon, 15 Oct 2018 04:32:55 -0400 (EDT) Received: by mail-pg1-f197.google.com with SMTP id u43-v6so6097054pgn.4 for ; Mon, 15 Oct 2018 01:32:55 -0700 (PDT) Received: from mail-sor-f65.google.com (mail-sor-f65.google.com. [209.85.220.65]) by mx.google.com with SMTPS id p72-v6sor440200pfk.73.2018.10.15.01.32.53 for (Google Transport Security); Mon, 15 Oct 2018 01:32:54 -0700 (PDT) From: "Aneesh Kumar K.V" 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> Message-ID: <4bf3951d-410f-fac4-dfb2-7dee5568e6ff@linux.ibm.com> Date: Mon, 15 Oct 2018 14:02:48 +0530 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Anshuman Khandual , 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 10/12/18 1:32 PM, Anshuman Khandual wrote: > > > 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. > There can be other code paths that makes that assumption. I ended up doing the below for pmd_trans_huge on ppc64. /* * Only returns true for a THP. False for pmd migration entry. * We also need to return true when we come across a pte that * in between a thp split. While splitting THP, we mark the pmd * invalid (pmdp_invalidate()) before we set it with pte page * address. A pmd_trans_huge() check against a pmd entry during that time * should return true. * We should not call this on a hugetlb entry. We should check for HugeTLB * entry using vma->vm_flags * The page table walk rule is explained in Documentation/vm/transhuge.rst */ static inline int pmd_trans_huge(pmd_t pmd) { if (!pmd_present(pmd)) return false; if (radix_enabled()) return radix__pmd_trans_huge(pmd); return hash__pmd_trans_huge(pmd); } -aneesh