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Fri, 31 Mar 2023 11:38:45 +0000 (GMT) Received: from epsmgms1p1new.samsung.com (unknown [182.195.42.41]) by epsmtrp2.samsung.com (KnoxPortal) with ESMTP id 20230331113845epsmtrp208e66127ff1f61b4c3978fc8ca0e68b4~RfgvpHiOw0503605036epsmtrp2M; Fri, 31 Mar 2023 11:38:45 +0000 (GMT) X-AuditID: b6c32a48-9e7f970000008a8d-71-6426c6455a7a Received: from epsmtip1.samsung.com ( [182.195.34.30]) by epsmgms1p1new.samsung.com (Symantec Messaging Gateway) with SMTP id E2.A5.18071.546C6246; Fri, 31 Mar 2023 20:38:45 +0900 (KST) Received: from dell-Precision-7920-Tower.dsn.sec.samsung.com (unknown [10.229.83.99]) by epsmtip1.samsung.com (KnoxPortal) with ESMTPA id 20230331113845epsmtip1a9e322fd7df39cada90431bae9cae492~Rfgvb23PU0805808058epsmtip1S; Fri, 31 Mar 2023 11:38:45 +0000 (GMT) From: Kyungsan Kim To: willy@infradead.org Cc: lsf-pc@lists.linux-foundation.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-cxl@vger.kernel.org, a.manzanares@samsung.com, viacheslav.dubeyko@bytedance.com, dan.j.williams@intel.com, seungjun.ha@samsung.com, wj28.lee@samsung.com Subject: RE: Re: RE(2): FW: [LSF/MM/BPF TOPIC] SMDK inspired MM changes for CXL Date: Fri, 31 Mar 2023 20:38:45 +0900 Message-Id: <20230331113845.400220-1-ks0204.kim@samsung.com> X-Mailer: git-send-email 2.25.1 In-Reply-To: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Brightmail-Tracker: H4sIAAAAAAAAA+NgFlrAJsWRmVeSWpSXmKPExsWy7bCmma7rMbUUg2XHrC2mH1a0mD71AqPF +VmnWCz27D3JYnFvzX9Wi32v9zJbvOg8zmTRseENo8XvH3PYLDbef8fmwOXx78QaNo/NK7Q8 Fu95yeSx6dMkdo/JN5YzevRtWcXo8XmTXAB7VLZNRmpiSmqRQmpecn5KZl66rZJ3cLxzvKmZ gaGuoaWFuZJCXmJuqq2Si0+ArltmDtB1SgpliTmlQKGAxOJiJX07m6L80pJUhYz84hJbpdSC lJwC8wK94sTc4tK8dL281BIrQwMDI1OgwoTsjEWTPApOC1TMXvqauYFxJ28XIyeHhICJxM11 X9m7GLk4hAR2MEps+d3FBuF8YpS4PHE+I4TzjVHi8eclbDAt177sgWrZyyhx9eBPVpCEkEAX k8TJf9ogNpuAtsSfK+fBGkQExCWOTT3JCGIzC/xjlNhzWRLEFhYIlGhespUZxGYRUJV4vX0G UxcjBwevgI3EgT/GELvkJWZe+s4OYnMC7b0y/ScTiM0rIChxcuYTFoiR8hLNW2czg9wjIdDJ IdG3ZTXUoS4SB5feZ4WwhSVeHd/CDmFLSXx+txeqplji8et/UPESicNLfrNA2MYS724+ZwW5 h1lAU2L9Ln0QU0JAWeLILai1fBIdh/+yQ4R5JTrahCAaVSS2/1vODLPo9P5NUMM9JHY+W8UK CbV2RonVD2czTmBUmIXkm1lIvpmFsHgBI/MqRrHUguLc9NRiowITePQm5+duYgQnVS2PHYyz 337QO8TIxMF4iFGCg1lJhLfQWDVFiDclsbIqtSg/vqg0J7X4EKMpMKgnMkuJJucD03peSbyh iaWBiZmZobmRqYG5kjjvxw7lFCGB9MSS1OzU1ILUIpg+Jg5OqQams1ILe7gYPU0OWJXc/BOz dOlvq+miBntv139ldH29/pFTusOHk4/Wvvt58/ZxzXtyrIt1dqZv/HzimlyY5ovSHLfTKQaT f2pbfPty6ERPbePCA4IuNVvn71d2fHPo9/WPy5vsp8kJ33hzaU9iuCGDz7aJXdmO/66LXXTu CpVT0nKUTvhzPOK18WPmoP8lLrWf1xx/sf27xUcDWxuFRwu42GzZfP/LbFi23329o/qrAPdT EY4P7+/mqml6nni24Wi20ibnz1PnH1zXpqPxPmrS1wePpaX+VUse/lSR9aaDOyN417Vsjs95 N++GP+kX2fNVU+nWjokHFaN3Cn3+4Oe95L/GxO3V5cIarOW2dxxjFZVYijMSDbWYi4oTASaq mt8zBAAA X-Brightmail-Tracker: H4sIAAAAAAAAA+NgFrrLLMWRmVeSWpSXmKPExsWy7bCSnK7rMbUUg5ZnshbTDytaTJ96gdHi /KxTLBZ79p5ksbi35j+rxb7Xe5ktXnQeZ7Lo2PCG0eL3jzlsFhvvv2Nz4PL4d2INm8fmFVoe i/e8ZPLY9GkSu8fkG8sZPfq2rGL0+LxJLoA9issmJTUnsyy1SN8ugStj0SSPgtMCFbOXvmZu YNzJ28XIySEhYCJx7cse9i5GLg4hgd2MEm2fljNDJKQk3p9uY4ewhSXutxxhhSjqYJJYtaeH CSTBJqAt8efKeTYQW0RAXOLY1JOMIEXMIEVbL08HSwgL+EscfXWGFcRmEVCVeL19BlAzBwev gI3EgT/GEAvkJWZe+g62jBPooivTf4LNFxIwlrjcsRjsIF4BQYmTM5+wgNjMQPXNW2czT2AU mIUkNQtJagEj0ypGydSC4tz03GLDAsO81HK94sTc4tK8dL3k/NxNjOAY0NLcwbh91Qe9Q4xM HIyHGCU4mJVEeAuNVVOEeFMSK6tSi/Lji0pzUosPMUpzsCiJ817oOhkvJJCeWJKanZpakFoE k2Xi4JRqYGK+JR/40v/GH8NVsSuPfJ/pWrj8DTvb08fntPlERdQ5Jlcovvf+1GRVd/dBvMIp VkWDxXnSVc4fF+Q+UFQ9MkfQr/RG3tWwTq+OQ2aLfZf2MQS0+zdnedmbr9jwpeJZS79K8fml TLVaUXdr9pwSWfUqav1ShgcBj2/mPVUIk17Dt/Wn420XnztuinPW3Mwv9FVpqDy2eerte8Zm 3u4xCaZN63WN/i05eLYx0tbcXWiK0vWHf5790y3akTvl+aGkF/w7KsRWyUSvi9s2dW1PTegu j+a10zid99p0++9LeRG/YEF2zY6Ia2uvaTt4xe+ObVivo8N+Y7KyC8vtrlCBS293dXR9FCgz 195SyTbjthJLcUaioRZzUXEiABDyLN3wAgAA X-CMS-MailID: 20230331113845epcas2p313118617918ae2bf634c3c475fc5dbd8 X-Msg-Generator: CA Content-Type: text/plain; charset="utf-8" X-Sendblock-Type: AUTO_CONFIDENTIAL CMS-TYPE: 102P DLP-Filter: Pass X-CFilter-Loop: Reflected X-CMS-RootMailID: 20230331113845epcas2p313118617918ae2bf634c3c475fc5dbd8 References: X-Stat-Signature: eq9kuuwi7hmbsnyru35ywxisy4hq9mj7 X-Rspam-User: X-Rspamd-Queue-Id: 154AE40002 X-Rspamd-Server: rspam06 X-HE-Tag: 1680262730-364546 X-HE-Meta: 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 3oadBC/y XMr1IBx8xLx+lDSwuPRk0jl8DGkq6mmJ3tgZ8TSIePyM44n4vJwqP90Rtrj07JH/RkGOpdoGWoUsCj61+D/adJSkxFyBbuXTwbPocF4gx826H8oY= 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 Fri, Mar 24, 2023 at 02:55:02PM +0000, Matthew Wilcox wrote: >> No, that's not true. You can allocate kernel memory from ZONE_MOVABLE. >> You have to be careful when you do that, but eg filesystems put symlinks >> and directories in ZONE_MOVABLE, and zswap allocates memory from >> ZONE_MOVABLE. Of course, then you have to be careful that the kernel >> doesn't try to move it while you're accessing it. That's the tradeoff. > >I want to talk a little bit about what it would take to use MOVABLE >allocations for slab. > >Initially, one might presume that it is impossible to have slab use a >movable allocation. Usually, we need a relatively complex mechanism of >reference counting where one takes a reference on the page, uses it, >then puts the reference. Then migration can check the page reference >and if it's unused, it knows it's safe to migrate (much handwaving here, >of course it's more complex). > >The general case of kmalloc slabs cannot use MOVABLE allocations. >The API has no concept of "this pointer is temporarily not in use", >so we can never migrate any slab which has allocated objects. > >But for slab caches, individual objects may have access rules which allow >them to be moved. For example, we might be able to migrate every dentry >in a slab, then RCU-free the slab. Similarly for radix_tree_nodes. > >There was some work along these lines a few years ago: >https://lore.kernel.org/all/20190603042637.2018-16-tobin@kernel.org/ > >There are various practical problems with that patchset, but they can >be overcome with sufficient work. The question is: Why do we need to do >this work? What is the high-level motivation to make slab caches movable? Thank you for sharing us the case. We studied your sentences and the patchset. Let me summarize our understanding. A kernel context is migratable at certain in case the reference count of a page is traceable and it is 0. As I answered previously, our intention is the attribute of CXL DRAM page can be movable as well as unmovable. A memory allocator context should be able to determine it.