From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-oi1-f197.google.com (mail-oi1-f197.google.com [209.85.167.197]) by kanga.kvack.org (Postfix) with ESMTP id 67ED36B029D for ; Mon, 5 Nov 2018 19:35:16 -0500 (EST) Received: by mail-oi1-f197.google.com with SMTP id r68-v6so7699493oie.12 for ; Mon, 05 Nov 2018 16:35:16 -0800 (PST) Received: from foss.arm.com (usa-sjc-mx-foss1.foss.arm.com. [217.140.101.70]) by mx.google.com with ESMTP id d36si10115556otd.53.2018.11.05.16.35.15 for ; Mon, 05 Nov 2018 16:35:15 -0800 (PST) Date: Tue, 6 Nov 2018 00:35:10 +0000 From: Will Deacon Subject: Re: [PATCH] mm/thp: Correctly differentiate between mapped THP and PMD migration entry Message-ID: <20181106003509.GA27283@brain-police> References: <1539057538-27446-1-git-send-email-anshuman.khandual@arm.com> <7E8E6B14-D5C4-4A30-840D-A7AB046517FB@cs.rutgers.edu> <84509db4-13ce-fd53-e924-cc4288d493f7@arm.com> <1968F276-5D96-426B-823F-38F6A51FB465@cs.rutgers.edu> <5e0e772c-7eef-e75c-2921-e80d4fbe8324@arm.com> <2398C491-E1DA-4B3C-B60A-377A09A02F1A@cs.rutgers.edu> <20181017020930.GN30832@redhat.com> <9d9aaf03-617a-d383-7d59-8b98fdd3c1e7@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <9d9aaf03-617a-d383-7d59-8b98fdd3c1e7@arm.com> Sender: owner-linux-mm@kvack.org List-ID: To: Anshuman Khandual Cc: Andrea Arcangeli , Zi Yan , linux-mm@kvack.org, linux-kernel@vger.kernel.org, kirill.shutemov@linux.intel.com, akpm@linux-foundation.org, mhocko@suse.com, Naoya Horiguchi On Fri, Nov 02, 2018 at 11:45:00AM +0530, Anshuman Khandual wrote: > On 10/17/2018 07:39 AM, Andrea Arcangeli wrote: > > What we need to do during split is an invalidate of the huge TLB. > > There's no pmd_trans_splitting anymore, so we only clear the present > > bit in the PTE despite pmd_present still returns true (just like > > PROT_NONE, nothing new in this respect). pmd_present never meant the > > On arm64, the problem is that pmd_present() is tied with pte_present() which > checks for PTE_VALID (also PTE_PROT_NONE) but which gets cleared during PTE > invalidation. pmd_present() returns false just after the first step of PMD > splitting. So pmd_present() needs to be decoupled from PTE_VALID which is > same as PMD_SECT_VALID and instead should depend upon a pte bit which sticks > around like PAGE_PSE as in case of x86. I am working towards a solution. Could we not just go via a PROT_NONE mapping during the split, instead of having to allocate a new software bit to treat these invalid ptes as present? Will