From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ot1-f69.google.com (mail-ot1-f69.google.com [209.85.210.69]) by kanga.kvack.org (Postfix) with ESMTP id C14806B026A for ; Tue, 16 Oct 2018 09:16:55 -0400 (EDT) Received: by mail-ot1-f69.google.com with SMTP id d34so16799887otb.10 for ; Tue, 16 Oct 2018 06:16:55 -0700 (PDT) Received: from foss.arm.com (foss.arm.com. [217.140.101.70]) by mx.google.com with ESMTP id g12si4536896otl.218.2018.10.16.06.16.54 for ; Tue, 16 Oct 2018 06:16:54 -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> <4bf3951d-410f-fac4-dfb2-7dee5568e6ff@linux.ibm.com> From: Anshuman Khandual Message-ID: <7e11e5b0-fad5-01fb-6b01-66bbd50b6a2e@arm.com> Date: Tue, 16 Oct 2018 18:46:48 +0530 MIME-Version: 1.0 In-Reply-To: <4bf3951d-410f-fac4-dfb2-7dee5568e6ff@linux.ibm.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: owner-linux-mm@kvack.org List-ID: To: "Aneesh Kumar K.V" , 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 10/15/2018 02:02 PM, Aneesh Kumar K.V wrote: > 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)A A A A (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. > Yeah, did see that in one of the previous proposals. https://patchwork.kernel.org/patch/10544291/ But the existing semantics does not look right and makes vague assumptions. Zi Yan has already asked Andrea for his input in this regard on the next thread. So I guess while being here, its a good idea to revisit existing semantics and it's assumptions before fixing it in arch specific helpers. - Anshuman > /* > A * Only returns true for a THP. False for pmd migration entry. > A * We also need to return true when we come across a pte that > A * in between a thp split. While splitting THP, we mark the pmd > A * invalid (pmdp_invalidate()) before we set it with pte page > A * address. A pmd_trans_huge() check against a pmd entry during that time > A * should return true. > A * We should not call this on a hugetlb entry. We should check for HugeTLB > A * entry using vma->vm_flags > A * The page table walk rule is explained in Documentation/vm/transhuge.rst > A */ > static inline int pmd_trans_huge(pmd_t pmd) > { > A A A A if (!pmd_present(pmd)) > A A A A A A A return false; > > A A A A if (radix_enabled()) > A A A A A A A return radix__pmd_trans_huge(pmd); > A A A A return hash__pmd_trans_huge(pmd); > } > > -aneesh >