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=-8.2 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,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 037B3C433E1 for ; Tue, 18 Aug 2020 09:44:52 +0000 (UTC) Received: from merlin.infradead.org (merlin.infradead.org [205.233.59.134]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id B1ABC20B1F for ; Tue, 18 Aug 2020 09:44:51 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="J0nm6X/X" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org B1ABC20B1F Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=merlin.20170209; h=Sender:Content-Transfer-Encoding: Content-Type:Cc:List-Subscribe:List-Help:List-Post:List-Archive: List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:Date:Message-ID:From: References:To:Subject:Reply-To:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=3clsg15t0+ZAs+3T1oXpTRtVN9r2yy8Kd2S4hC4b/Cc=; b=J0nm6X/XqlXFziz79sIgzwWM/ wMBqXwT/OWNUoD7zwPj7AsENGkr51R2VPqQAZiCnA14rk2Rdf8UxcboDWNDO4JzIsZv7KeiSPIZxi m6AxLm+arhFYsA+iht/vrJ4Foy3FDWPH7q7oxtNjiF3kISaIVS/+rjRfCwkQSt6980n2tLECTzUxf Hvf41nzMCN5mL/4yQWryVU3vnMlm9/B/Ftw1xWbBF/2lSoEDNvPzB/s71LEywFf4K6hTKPqDG2Fz1 ESmAMVo6iCJstZ8w3CPchi+0hpKi0Z/Z+/wxSIkVmMYUY6QySuHYAvcL/FMjv0WJ6BebPHW5Gp3M1 /OEJXIc/A==; Received: from localhost ([::1] helo=merlin.infradead.org) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1k7y8l-0006xa-1n; Tue, 18 Aug 2020 09:42:47 +0000 Received: from foss.arm.com ([217.140.110.172]) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1k7y8i-0006x5-49 for linux-arm-kernel@lists.infradead.org; Tue, 18 Aug 2020 09:42:45 +0000 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 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-Language: en-US X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20200818_054244_271710_87009D87 X-CRM114-Status: GOOD ( 22.05 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Mark Rutland , Suzuki Poulose , catalin.marinas@arm.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Marc Zyngier , akpm@linux-foundation.org, will@kernel.org, linux-arm-kernel@lists.infradead.org Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org 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. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel