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[217.140.110.172]) by mx.google.com with ESMTP id g34si962003edb.182.2019.06.27.20.46.11 for ; Thu, 27 Jun 2019 20:46:11 -0700 (PDT) Received-SPF: pass (google.com: domain of anshuman.khandual@arm.com designates 217.140.110.172 as permitted sender) client-ip=217.140.110.172; Authentication-Results: mx.google.com; spf=pass (google.com: domain of anshuman.khandual@arm.com designates 217.140.110.172 as permitted sender) smtp.mailfrom=anshuman.khandual@arm.com 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 EEEA72B; Thu, 27 Jun 2019 20:46:10 -0700 (PDT) Received: from [10.162.40.144] (p8cg001049571a15.blr.arm.com [10.162.40.144]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 965093F706; Thu, 27 Jun 2019 20:46:08 -0700 (PDT) Subject: Re: [RFC 1/2] arm64/mm: Change THP helpers to comply with generic MM semantics To: Zi Yan Cc: linux-mm@kvack.org, Catalin Marinas , Will Deacon , Mark Rutland , Marc Zyngier , Suzuki Poulose , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org References: <1561639696-16361-1-git-send-email-anshuman.khandual@arm.com> <1561639696-16361-2-git-send-email-anshuman.khandual@arm.com> <7F685152-7C6C-4E99-99DF-03DDD03D6094@nvidia.com> From: Anshuman Khandual Message-ID: <5c490be8-5ac1-0a3a-32cf-d4e692fc59b5@arm.com> Date: Fri, 28 Jun 2019 09:16:35 +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: <7F685152-7C6C-4E99-99DF-03DDD03D6094@nvidia.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit 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 06/27/2019 09:01 PM, Zi Yan wrote: > On 27 Jun 2019, at 8:48, 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_mknotpresent -> 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(). >> >> PTE_SPECIAL never gets used for PMD mapping i.e there is no pmd_special(). >> Hence this bit can be set on the PMD entry during invalidation which can >> help in making pmd_present() return true and in recognizing the fact that >> it still points to memory. >> >> This bit is transient. During the split is process it will be overridden >> by a page table page representing the normal pages in place of erstwhile >> huge page. Other pmdp_invalidate() callers always write a fresh PMD value >> on the entry overriding this transient PTE_SPECIAL making it safe. In the >> past former pmd_[mk]splitting() functions used PTE_SPECIAL. >> >> [1]: https://lkml.org/lkml/2018/10/17/231 > > Just want to point out that lkml.org link might not be stable. > This one would be better: https://lore.kernel.org/linux-mm/20181017020930.GN30832@redhat.com/ Sure will update the link in the commit. Thanks !