From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-oi1-f200.google.com (mail-oi1-f200.google.com [209.85.167.200]) by kanga.kvack.org (Postfix) with ESMTP id 022AA6B02FC for ; Tue, 6 Nov 2018 04:51:59 -0500 (EST) Received: by mail-oi1-f200.google.com with SMTP id f62-v6so8528666oia.2 for ; Tue, 06 Nov 2018 01:51:58 -0800 (PST) Received: from foss.arm.com (foss.arm.com. [217.140.101.70]) by mx.google.com with ESMTP id v75-v6si9502472oia.127.2018.11.06.01.51.57 for ; Tue, 06 Nov 2018 01:51:57 -0800 (PST) From: Anshuman Khandual 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> <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> <20181106003509.GA27283@brain-police> Message-ID: <9370f2b9-0fcd-6bbb-fa29-568bbd9aba59@arm.com> Date: Tue, 6 Nov 2018 15:21:46 +0530 MIME-Version: 1.0 In-Reply-To: <20181106003509.GA27283@brain-police> 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 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 11/06/2018 06:05 AM, Will Deacon wrote: > 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? The problem might occur during page fault (i.e __handle_mm_fault). As discussed previously on this thread any potential PTE sticky bit would be used for both pmd_trans_huge() and pmd_present() wrappers to maintain existing semantics. At present, PMD state analysis during page fault has conditional block like this. if (pmd_trans_huge(orig_pmd) || pmd_devmap(orig_pmd)) { if (pmd_protnone(orig_pmd) && vma_is_accessible(vma)) return do_huge_pmd_numa_page(&vmf, orig_pmd); Using PROT_NONE for pmd_trans_huge() might force PMD page fault to go through NUMA fault handling all the time as both pmd_trans_huge() and pmd_protnone() will return true in that situation.