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b=NW7ahbfGKNgneVtM+N5j1WhBNzJBjJYeEZ+35XxVOzGS00SwxOIdkF1Z173QC815lTt22K5muC92PeBmVt6Rt2JMrpVIweYb0UUmYb+T3lvDOx+6FovSma3uHSBEHAuaAB6tZXpEb+tGlMT4nlZFgF9346Qabz+LOkufpgFW2Z8= Authentication-Results-Original: dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=arm.com; Received: from AM9PR08MB7120.eurprd08.prod.outlook.com (2603:10a6:20b:3dc::22) by AS2PR08MB8288.eurprd08.prod.outlook.com (2603:10a6:20b:557::8) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.8769.29; Wed, 28 May 2025 06:15:45 +0000 Received: from AM9PR08MB7120.eurprd08.prod.outlook.com ([fe80::2933:29aa:2693:d12e]) by AM9PR08MB7120.eurprd08.prod.outlook.com ([fe80::2933:29aa:2693:d12e%2]) with mapi id 15.20.8769.025; Wed, 28 May 2025 06:15:45 +0000 Message-ID: <68604797-f3a9-4555-aaf2-228f6e7f820a@arm.com> Date: Wed, 28 May 2025 11:45:36 +0530 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 2/2] mm: Optimize mremap() by PTE batching To: Lorenzo Stoakes Cc: akpm@linux-foundation.org, Liam.Howlett@oracle.com, vbabka@suse.cz, jannh@google.com, pfalcato@suse.de, linux-mm@kvack.org, linux-kernel@vger.kernel.org, david@redhat.com, peterx@redhat.com, ryan.roberts@arm.com, mingo@kernel.org, libang.li@antgroup.com, maobibo@loongson.cn, zhengqi.arch@bytedance.com, baohua@kernel.org, anshuman.khandual@arm.com, willy@infradead.org, ioworker0@gmail.com, yang@os.amperecomputing.com, baolin.wang@linux.alibaba.com, ziy@nvidia.com, hughd@google.com References: <20250527075049.60215-1-dev.jain@arm.com> <20250527075049.60215-3-dev.jain@arm.com> <3d5d5813-7764-4667-937f-cec52f8b03c9@lucifer.local> <422a63ea-9211-4e4b-a37c-8d4c5e964b53@arm.com> <888a8314-b200-40b7-ab52-f30f0af93374@lucifer.local> <8ef715e7-76a2-4004-ac10-3cc10912c67d@arm.com> <67cf2131-da6b-4a1d-828e-52f0ff7fd0fb@lucifer.local> <170036fa-727c-4d2b-8ed4-0a1b6e4cf8d3@arm.com> <53a39d5f-6ea2-403f-8837-fb00d7f8d6b8@lucifer.local> Content-Language: en-US From: Dev Jain In-Reply-To: <53a39d5f-6ea2-403f-8837-fb00d7f8d6b8@lucifer.local> Content-Type: text/plain; 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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 28 May 2025 06:16:20.9907 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 5c27cf4c-17ad-4a8d-fbd0-08dd9daf2a49 X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d;Ip=[4.158.2.129];Helo=[outbound-uk1.az.dlp.m.darktrace.com] X-MS-Exchange-CrossTenant-AuthSource: AM3PEPF0000A78E.eurprd04.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: DB4PR08MB7933 X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: 77507180008 X-Stat-Signature: pwopghi4bhiq9ghefdx1183mqu3uxmjh X-Rspam-User: X-HE-Tag: 1748412987-720131 X-HE-Meta: U2FsdGVkX1/929z8Z9TZcalU181M4yxL4qfloqeWgQUSxAlekMxRd5r6NF5Whv/rLxrB1/75J3j0cc2yC+49D0GjuiG3qonBo8zGdjj+r/z30WmJ5G9hiIdh5+zaOJ+wRTNR/KVlCBVy8nmTarFUBA2sGHxb3qtf2qsU3G5JdNAfO5g1nJp91nVsFWMPxCB9/oDMRRSpfI3IrnyozYMUpDxslLvTaeeobTvIR9GEofq3MwgSKRmvISvc1mL88uzeG4UT51PMBo0dk9qqtmTMu44TjYtYt1EGoSmK5QZxZiGUsqDxY4WUAiya3wrpmotIdgQhJA51wzR7JnUUeauIZeeUR3nno0nEmmq9AMoYIEOmE9OmezfQybk9TCtgKo3KKcfkyFlMYHAvbReHSogub2bCv5DtdXpd2E79cBIevg7zdmvoP2NcKIuoeYSTJzYP0uBvINzK70T5Iy5dzislSD8/fB8UxP+1nEYaYT9Vvi0h9FTY59j2Dznhm2qZbpdLdTP5p/6FjhfVG698SeyN4c5Gs2WDWoWOyFFuRkEYNjFwg/v7X0AQBULHRoseXLrJq6BqLVntM5vl4hLZLjPw+Qvwc6ReP1A1ULpPoNDvJp2cjs87qTtS69j3rjHXQhQKy6jGC4qco/BFJJuUTGy/QQnibyou+aG5uGanNo3Y0xfMyV04N5o9ircBCD7t8nyNztmh7WB36abe3ipYg8eZJvmPE+dhxgm7xPpdw68HPB4vRhkwKBR9wEmjQ2m/ZCqyluin9n0o5jad9mUGaIKSJoqGsyYw/kn1lHbQAHFzvMgo4ydQPMksvfsk0Wip9NDepDWzi4Cgljg2JTnXnfDgrZpJSoZbHLbi1SE61eQjU9dp9NMycS33Qa0SjDCF+3d5g6sX/rYSPq+q4HIg7kq/QzxrOktMAyOQ+9xN2lHADlrFQd26MbNiOkprfTmbtlQjLwQhDQUfhOGp9oRkYdT gIOeaiA1 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 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: List-Subscribe: List-Unsubscribe: On 28/05/25 10:19 am, Lorenzo Stoakes wrote: > On Wed, May 28, 2025 at 09:02:26AM +0530, Dev Jain wrote: >> On 27/05/25 10:16 pm, Lorenzo Stoakes wrote: >>> On Tue, May 27, 2025 at 10:08:59PM +0530, Dev Jain wrote: >>>> On 27/05/25 9:59 pm, Lorenzo Stoakes wrote: >>> [snip] >>>>> If I invoke split_huge_pmd(), I end up with a bunch of PTEs mapping the same >>>>> large folio. The folio itself is not split, so nr_ptes surely will be equal to >>>>> something >1 here right? >>>> Thanks for elaborating. >>>> >>>> So, >>>> >>>> Case 1: folio splitting => nr_ptes = 1 => the question of a/d bit smearing >>>> disappears. >>>> >>>> Case 2: page table splitting => consec PTEs point to the same large folio >>>> => nr_ptes > 1 => get_and_clear_full_ptes() will smear a/d bits on the >>>> new ptes, which is correct because we are still pointing to the same large >>>> folio. >>>> >>> OK awesome, I thought as much, just wanted to make sure :) we are good then. >>> >>> The accessed/dirty bits really matter at a folio granularity (and especially >>> with respect to reclaim/writeback which both operate at folio level) so the >>> smearing as you say is fine. >>> >>> This patch therefore looks fine, only the trivial comment fixup. >>> >>> I ran the series on my x86-64 setup (fwiw) with no build/mm selftest errors. >> Thanks! >> >> >>> Sorry to be a pain but could you respin with the commit message for this patch >>> updated to explicitly mention that the logic applies for the non-contPTE split >>> PTE case (and therefore also helps performance there)? That and the trivial >>> thing of dropping that comment. >> What do you mean by the non-contpte case? In that case the PTEs do not point >> to the same folio or are misaligned, and there will be no optimization. This > Split page table large folio. > >> patch is optimizing two things: 1) ptep_get() READ_ONCE accesses 2) reduction >> in number of TLBIs for contig blocks, both of which happen in the contpte case. >> > But it impacts split huge pages. Your code changes this behaviour. We need to > make this clear :) > >> In general, the patch should have a minor improvement on other arches because >> we are detecting a batch and processing it together, thus saving on a few >> function calls, but the main benefit is for arm64. > Ack, but you are changing this behaviour. The commit message doesn't make this > clear and seems to imply this only impacts contPTE cases. Or at least isn't > clear enough > > A simple additional paragraph like: > > 'Transparent huge pages which have been split into PTEs will also be impacted, > however the performance gain in this case is expected to be modest' Ah okay, sounds good. > > Will sort this out. > > Thanks! > >>> Then we should be good for a tag unless somebody else spots something >>> egregious :) >>> >>> Thanks for this! Good improvement. >>> >>> [snip] >>> >>> Cheers, Lorenzo