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Tue, 24 Aug 2021 14:17:08 +0000 Subject: Re: can we finally kill off CONFIG_FS_DAX_LIMITED To: Dan Williams Cc: Heiko Carstens , Vasily Gorbik , Christian Borntraeger , Linux NVDIMM , linux-s390 , Gerald Schaefer , Christoph Hellwig References: <20210820054340.GA28560@lst.de> From: Joao Martins Message-ID: Date: Tue, 24 Aug 2021 15:17:02 +0100 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-ClientProxiedBy: LO2P265CA0314.GBRP265.PROD.OUTLOOK.COM (2603:10a6:600:a4::14) To BLAPR10MB4835.namprd10.prod.outlook.com (2603:10b6:208:331::11) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [10.175.176.190] (138.3.204.62) by LO2P265CA0314.GBRP265.PROD.OUTLOOK.COM (2603:10a6:600:a4::14) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4457.17 via Frontend Transport; Tue, 24 Aug 2021 14:17:06 +0000 X-MS-PublicTrafficType: Email X-MS-Office365-Filtering-Correlation-Id: 76466976-70cf-4aba-a769-08d96709dafc X-MS-TrafficTypeDiagnostic: MN2PR10MB3774: X-Microsoft-Antispam-PRVS: X-MS-Oob-TLC-OOBClassifiers: OLM:9508; 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I already suggested to >>> depend on ARCH_HAS_PTE_SPECIAL for ZONE_DEVICE there, which all modern >>> architectures have anyway. But the other odd special case is >>> CONFIG_FS_DAX_LIMITED which is just used for the xpram driver. Does >>> this driver still see use? If so can we make it behave like the >>> other DAX drivers and require a pgmap? I think the biggest missing >>> part would be to implement ARCH_HAS_PTE_DEVMAP for s390. >>> >> >> Gerald, >> >> Might you still be looking to help dcssblk get out of FS_DAX_LIMITED >> jail [1]? I recall Martin saying that 'struct page' overhead was >> prohibitive. I don't know if Joao's 'struct page' diet patches could >> help alleviate that at all (would require the filesystem to only >> allocate blocks in large page sizes). /me nods Either that or dynamically remapping the deduplicated tail page vmemmap areas when we punch holes in what was represented as a compound page (or when we collapse pages back together). Not sure how crazy the latter is. >> >> [1]: https://lore.kernel.org/r/20180523205017.0f2bc83e@thinkpad >