From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.1 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 7B6F9C433E3 for ; Tue, 18 Aug 2020 09:42:35 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 43DFD20639 for ; Tue, 18 Aug 2020 09:42:35 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 43DFD20639 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id BBDCB6B002F; Tue, 18 Aug 2020 05:42:34 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id B6DC46B0030; Tue, 18 Aug 2020 05:42:34 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id A852A6B0031; Tue, 18 Aug 2020 05:42:34 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0070.hostedemail.com [216.40.44.70]) by kanga.kvack.org (Postfix) with ESMTP id 936D86B002F for ; Tue, 18 Aug 2020 05:42:34 -0400 (EDT) Received: from smtpin13.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay03.hostedemail.com (Postfix) with ESMTP id 586DD824556B for ; Tue, 18 Aug 2020 09:42:34 +0000 (UTC) X-FDA: 77163199428.13.copy12_45160692701e Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin13.hostedemail.com (Postfix) with ESMTP id 2317F18140B60 for ; Tue, 18 Aug 2020 09:42:34 +0000 (UTC) X-HE-Tag: copy12_45160692701e X-Filterd-Recvd-Size: 4761 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by imf09.hostedemail.com (Postfix) with ESMTP for ; Tue, 18 Aug 2020 09:42:33 +0000 (UTC) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 9E0231FB; Tue, 18 Aug 2020 02:42:32 -0700 (PDT) Received: from [10.163.65.203] (unknown [10.163.65.203]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id CD78D3F6CF; Tue, 18 Aug 2020 02:42:29 -0700 (PDT) Subject: Re: [PATCH 1/2] arm64/mm: Change THP helpers to comply with generic MM semantics To: Jonathan Cameron Cc: linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, catalin.marinas@arm.com, will@kernel.org, akpm@linux-foundation.org, Mark Rutland , Marc Zyngier , Suzuki Poulose , linux-kernel@vger.kernel.org References: <1597655984-15428-1-git-send-email-anshuman.khandual@arm.com> <1597655984-15428-2-git-send-email-anshuman.khandual@arm.com> <20200818101301.000027ef@Huawei.com> From: Anshuman Khandual Message-ID: <8db455b6-8fe5-b552-119f-4abab0cc8501@arm.com> Date: Tue, 18 Aug 2020 15:11:58 +0530 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: <20200818101301.000027ef@Huawei.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 2317F18140B60 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam05 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 08/18/2020 02:43 PM, Jonathan Cameron wrote: > On Mon, 17 Aug 2020 14:49:43 +0530 > Anshuman Khandual wrote: > >> pmd_present() and pmd_trans_huge() are expected to behave in the following >> manner during various phases of a given PMD. It is derived from a previous >> detailed discussion on this topic [1] and present THP documentation [2]. >> >> pmd_present(pmd): >> >> - Returns true if pmd refers to system RAM with a valid pmd_page(pmd) >> - Returns false if pmd does not refer to system RAM - Invalid pmd_page(pmd) >> >> pmd_trans_huge(pmd): >> >> - Returns true if pmd refers to system RAM and is a trans huge mapping >> >> ------------------------------------------------------------------------- >> | PMD states | pmd_present | pmd_trans_huge | >> ------------------------------------------------------------------------- >> | Mapped | Yes | Yes | >> ------------------------------------------------------------------------- >> | Splitting | Yes | Yes | >> ------------------------------------------------------------------------- >> | Migration/Swap | No | No | >> ------------------------------------------------------------------------- >> >> The problem: >> >> PMD is first invalidated with pmdp_invalidate() before it's splitting. This >> invalidation clears PMD_SECT_VALID as below. >> >> PMD Split -> pmdp_invalidate() -> pmd_mkinvalid -> Clears PMD_SECT_VALID >> >> Once PMD_SECT_VALID gets cleared, it results in pmd_present() return false >> on the PMD entry. It will need another bit apart from PMD_SECT_VALID to re- >> affirm pmd_present() as true during the THP split process. To comply with >> above mentioned semantics, pmd_trans_huge() should also check pmd_present() >> first before testing presence of an actual transparent huge mapping. >> >> The solution: >> >> Ideally PMD_TYPE_SECT should have been used here instead. But it shares the >> bit position with PMD_SECT_VALID which is used for THP invalidation. Hence >> it will not be there for pmd_present() check after pmdp_invalidate(). >> >> A new software defined PMD_PRESENT_INVALID (bit 59) can be set on the PMD >> entry during invalidation which can help pmd_present() return true and in >> recognizing the fact that it still points to memory. >> >> This bit is transient. During the split process it will be overridden by a >> page table page representing normal pages in place of erstwhile huge page. >> Other pmdp_invalidate() callers always write a fresh PMD value on the entry >> overriding this transient PMD_PRESENT_INVALID bit, which makes it safe. >> >> [1]: https://lkml.org/lkml/2018/10/17/231 >> [2]: https://www.kernel.org/doc/Documentation/vm/transhuge.txt > > Hi Anshuman, > > One query on this. From my reading of the ARM ARM, bit 59 is not > an ignored bit. The exact requirements for hardware to be using > it are a bit complex though. > > It 'might' be safe to use it for this, but if so can we have a comment > explaining why. Also more than possible I'm misunderstanding things! We are using this bit 59 only when the entry is not active from MMU perspective i.e PMD_SECT_VALID is clear.