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 Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 31E48CFC29B for ; Fri, 21 Nov 2025 17:50:23 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 842126B0023; Fri, 21 Nov 2025 12:50:22 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 7F33E6B002E; Fri, 21 Nov 2025 12:50:22 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 6E1C96B008A; Fri, 21 Nov 2025 12:50:22 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0013.hostedemail.com [216.40.44.13]) by kanga.kvack.org (Postfix) with ESMTP id 5D0E06B0023 for ; Fri, 21 Nov 2025 12:50:22 -0500 (EST) Received: from smtpin23.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay04.hostedemail.com (Postfix) with ESMTP id 2FC581A0650 for ; Fri, 21 Nov 2025 17:50:22 +0000 (UTC) X-FDA: 84135353484.23.364FA23 Received: from mgamail.intel.com (mgamail.intel.com [198.175.65.15]) by imf16.hostedemail.com (Postfix) with ESMTP id 743D6180011 for ; Fri, 21 Nov 2025 17:50:19 +0000 (UTC) Authentication-Results: imf16.hostedemail.com; dkim=pass header.d=intel.com header.s=Intel header.b=FVyheUlJ; dmarc=pass (policy=none) header.from=intel.com; spf=pass (imf16.hostedemail.com: domain of dave.hansen@intel.com designates 198.175.65.15 as permitted sender) smtp.mailfrom=dave.hansen@intel.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1763747420; a=rsa-sha256; cv=none; b=o3o+Jp2qdbsBEAHSO2Mjx4qkkTD6igGDLNttGwfHi0X4m/NqbDtn20r2Atk/Av+LnOg8un aSL8Ov4xzMh6JZphUsJscteJ/BwTibTgEyrOs6Uy7GcNEo3LDndjLuwW0M+OPSa6D4Hl+N hXFkpi153ax59fMwb6vqi59mccdGYZg= ARC-Authentication-Results: i=1; imf16.hostedemail.com; dkim=pass header.d=intel.com header.s=Intel header.b=FVyheUlJ; dmarc=pass (policy=none) header.from=intel.com; spf=pass (imf16.hostedemail.com: domain of dave.hansen@intel.com designates 198.175.65.15 as permitted sender) smtp.mailfrom=dave.hansen@intel.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1763747420; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=lbpdozOYhy0BMLI8lFThkkg/d76zv4kCPJ/fwh1/PTM=; b=yyLctM8ZlrNKRmBp0yq0ELlK46NlEM5g7nK03q4Ba31SJZN+EBxT9/DM9OEKYITAWsTbtj /lGh/0sl9VbGvHHFhI69YuTtb3KL5XemOHQf/6rPjUeW1rmSILCVTgDI7tIxrAGCdLkdcN pPeB7MdfD7rYtmqRCBR78zToBbbeIzs= DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1763747420; x=1795283420; h=message-id:date:mime-version:subject:to:cc:references: from:in-reply-to:content-transfer-encoding; bh=YckklQG/5FwUbczH+x3i0icUWdCO2DJMQNFuo3RKWck=; b=FVyheUlJzVhJL3JGisYBBEkTlXOmx8yCJ5XvE8PCn2ptH6noFN/Mtv40 /I3vByKjLnHoqkLiww3VU/NOY/MlPRkLeUyAKG1V4CFhS1gXOxnCqP1Ej Wre4+R8kYUUFUarf0df/QEtQhtajubJQld0Gxz4+YYe8pnr3Jb/sPHMrn s6HPF2ZNSlaauiSS8ygN34RxGGXnX4jbQ/dbwNMewFZWC50AKwGA2Cg5i qUELmN3lWL+O4TXAQZ//bP/ZfZ02G/u90AcxR6rq7zz1pKMxyh9vDlC7b wh9W/eyiUnNhqxVSMypVcI+uLfGbc7BzeAwQHZ0KwYMzfZNgLYoZPo7Ba g==; X-CSE-ConnectionGUID: oyHf2g22RCOcYpyEPIpLZA== X-CSE-MsgGUID: cM2h7DEEReatPXff/kJc5A== X-IronPort-AV: E=McAfee;i="6800,10657,11620"; a="69465921" X-IronPort-AV: E=Sophos;i="6.20,216,1758610800"; d="scan'208";a="69465921" Received: from orviesa002.jf.intel.com ([10.64.159.142]) by orvoesa107.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 21 Nov 2025 09:50:17 -0800 X-CSE-ConnectionGUID: TALVmW4IQoSpoxqsJrQTWg== X-CSE-MsgGUID: pw0Ufr6DT1ewmiN2aWhOIA== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.20,216,1758610800"; d="scan'208";a="222378181" Received: from schen9-mobl4.amr.corp.intel.com (HELO [10.125.110.60]) ([10.125.110.60]) by orviesa002-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 21 Nov 2025 09:50:15 -0800 Message-ID: <2837ea3e-c0b8-46b0-b8da-bf06906d124d@intel.com> Date: Fri, 21 Nov 2025 09:50:14 -0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v7 30/31] x86/mm, mm/vmalloc: Defer kernel TLB flush IPIs under CONFIG_COALESCE_TLBI=y To: Valentin Schneider , linux-kernel@vger.kernel.org, linux-mm@kvack.org, rcu@vger.kernel.org, x86@kernel.org, linux-arm-kernel@lists.infradead.org, loongarch@lists.linux.dev, linux-riscv@lists.infradead.org, linux-arch@vger.kernel.org, linux-trace-kernel@vger.kernel.org Cc: Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , "H. Peter Anvin" , Andy Lutomirski , Peter Zijlstra , Arnaldo Carvalho de Melo , Josh Poimboeuf , Paolo Bonzini , Arnd Bergmann , Frederic Weisbecker , "Paul E. McKenney" , Jason Baron , Steven Rostedt , Ard Biesheuvel , Sami Tolvanen , "David S. Miller" , Neeraj Upadhyay , Joel Fernandes , Josh Triplett , Boqun Feng , Uladzislau Rezki , Mathieu Desnoyers , Mel Gorman , Andrew Morton , Masahiro Yamada , Han Shen , Rik van Riel , Jann Horn , Dan Carpenter , Oleg Nesterov , Juri Lelli , Clark Williams , Yair Podemsky , Marcelo Tosatti , Daniel Wagner , Petr Tesarik , Shrikanth Hegde References: <20251114150133.1056710-1-vschneid@redhat.com> <20251114151428.1064524-10-vschneid@redhat.com> Content-Language: en-US From: Dave Hansen Autocrypt: addr=dave.hansen@intel.com; keydata= xsFNBE6HMP0BEADIMA3XYkQfF3dwHlj58Yjsc4E5y5G67cfbt8dvaUq2fx1lR0K9h1bOI6fC oAiUXvGAOxPDsB/P6UEOISPpLl5IuYsSwAeZGkdQ5g6m1xq7AlDJQZddhr/1DC/nMVa/2BoY 2UnKuZuSBu7lgOE193+7Uks3416N2hTkyKUSNkduyoZ9F5twiBhxPJwPtn/wnch6n5RsoXsb ygOEDxLEsSk/7eyFycjE+btUtAWZtx+HseyaGfqkZK0Z9bT1lsaHecmB203xShwCPT49Blxz VOab8668QpaEOdLGhtvrVYVK7x4skyT3nGWcgDCl5/Vp3TWA4K+IofwvXzX2ON/Mj7aQwf5W iC+3nWC7q0uxKwwsddJ0Nu+dpA/UORQWa1NiAftEoSpk5+nUUi0WE+5DRm0H+TXKBWMGNCFn c6+EKg5zQaa8KqymHcOrSXNPmzJuXvDQ8uj2J8XuzCZfK4uy1+YdIr0yyEMI7mdh4KX50LO1 pmowEqDh7dLShTOif/7UtQYrzYq9cPnjU2ZW4qd5Qz2joSGTG9eCXLz5PRe5SqHxv6ljk8mb ApNuY7bOXO/A7T2j5RwXIlcmssqIjBcxsRRoIbpCwWWGjkYjzYCjgsNFL6rt4OL11OUF37wL QcTl7fbCGv53KfKPdYD5hcbguLKi/aCccJK18ZwNjFhqr4MliQARAQABzUVEYXZpZCBDaHJp c3RvcGhlciBIYW5zZW4gKEludGVsIFdvcmsgQWRkcmVzcykgPGRhdmUuaGFuc2VuQGludGVs LmNvbT7CwXgEEwECACIFAlQ+9J0CGwMGCwkIBwMCBhUIAgkKCwQWAgMBAh4BAheAAAoJEGg1 lTBwyZKwLZUP/0dnbhDc229u2u6WtK1s1cSd9WsflGXGagkR6liJ4um3XCfYWDHvIdkHYC1t MNcVHFBwmQkawxsYvgO8kXT3SaFZe4ISfB4K4CL2qp4JO+nJdlFUbZI7cz/Td9z8nHjMcWYF IQuTsWOLs/LBMTs+ANumibtw6UkiGVD3dfHJAOPNApjVr+M0P/lVmTeP8w0uVcd2syiaU5jB aht9CYATn+ytFGWZnBEEQFnqcibIaOrmoBLu2b3fKJEd8Jp7NHDSIdrvrMjYynmc6sZKUqH2 I1qOevaa8jUg7wlLJAWGfIqnu85kkqrVOkbNbk4TPub7VOqA6qG5GCNEIv6ZY7HLYd/vAkVY E8Plzq/NwLAuOWxvGrOl7OPuwVeR4hBDfcrNb990MFPpjGgACzAZyjdmYoMu8j3/MAEW4P0z F5+EYJAOZ+z212y1pchNNauehORXgjrNKsZwxwKpPY9qb84E3O9KYpwfATsqOoQ6tTgr+1BR CCwP712H+E9U5HJ0iibN/CDZFVPL1bRerHziuwuQuvE0qWg0+0SChFe9oq0KAwEkVs6ZDMB2 P16MieEEQ6StQRlvy2YBv80L1TMl3T90Bo1UUn6ARXEpcbFE0/aORH/jEXcRteb+vuik5UGY 5TsyLYdPur3TXm7XDBdmmyQVJjnJKYK9AQxj95KlXLVO38lczsFNBFRjzmoBEACyAxbvUEhd GDGNg0JhDdezyTdN8C9BFsdxyTLnSH31NRiyp1QtuxvcqGZjb2trDVuCbIzRrgMZLVgo3upr MIOx1CXEgmn23Zhh0EpdVHM8IKx9Z7V0r+rrpRWFE8/wQZngKYVi49PGoZj50ZEifEJ5qn/H Nsp2+Y+bTUjDdgWMATg9DiFMyv8fvoqgNsNyrrZTnSgoLzdxr89FGHZCoSoAK8gfgFHuO54B lI8QOfPDG9WDPJ66HCodjTlBEr/Cwq6GruxS5i2Y33YVqxvFvDa1tUtl+iJ2SWKS9kCai2DR 3BwVONJEYSDQaven/EHMlY1q8Vln3lGPsS11vSUK3QcNJjmrgYxH5KsVsf6PNRj9mp8Z1kIG qjRx08+nnyStWC0gZH6NrYyS9rpqH3j+hA2WcI7De51L4Rv9pFwzp161mvtc6eC/GxaiUGuH BNAVP0PY0fqvIC68p3rLIAW3f97uv4ce2RSQ7LbsPsimOeCo/5vgS6YQsj83E+AipPr09Caj 0hloj+hFoqiticNpmsxdWKoOsV0PftcQvBCCYuhKbZV9s5hjt9qn8CE86A5g5KqDf83Fxqm/ vXKgHNFHE5zgXGZnrmaf6resQzbvJHO0Fb0CcIohzrpPaL3YepcLDoCCgElGMGQjdCcSQ+Ci FCRl0Bvyj1YZUql+ZkptgGjikQARAQABwsFfBBgBAgAJBQJUY85qAhsMAAoJEGg1lTBwyZKw l4IQAIKHs/9po4spZDFyfDjunimEhVHqlUt7ggR1Hsl/tkvTSze8pI1P6dGp2XW6AnH1iayn yRcoyT0ZJ+Zmm4xAH1zqKjWplzqdb/dO28qk0bPso8+1oPO8oDhLm1+tY+cOvufXkBTm+whm +AyNTjaCRt6aSMnA/QHVGSJ8grrTJCoACVNhnXg/R0g90g8iV8Q+IBZyDkG0tBThaDdw1B2l asInUTeb9EiVfL/Zjdg5VWiF9LL7iS+9hTeVdR09vThQ/DhVbCNxVk+DtyBHsjOKifrVsYep WpRGBIAu3bK8eXtyvrw1igWTNs2wazJ71+0z2jMzbclKAyRHKU9JdN6Hkkgr2nPb561yjcB8 sIq1pFXKyO+nKy6SZYxOvHxCcjk2fkw6UmPU6/j/nQlj2lfOAgNVKuDLothIxzi8pndB8Jju KktE5HJqUUMXePkAYIxEQ0mMc8Po7tuXdejgPMwgP7x65xtfEqI0RuzbUioFltsp1jUaRwQZ MTsCeQDdjpgHsj+P2ZDeEKCbma4m6Ez/YWs4+zDm1X8uZDkZcfQlD9NldbKDJEXLIjYWo1PH hYepSffIWPyvBMBTW2W5FRjJ4vLRrJSUoEfJuPQ3vW9Y73foyo/qFoURHO48AinGPZ7PC7TF vUaNOTjKedrqHkaOcqB185ahG2had0xnFsDPlx5y In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 743D6180011 X-Rspamd-Server: rspam07 X-Stat-Signature: qjuz48erzhphsyk4fhuhfg33u41ojrex X-Rspam-User: X-HE-Tag: 1763747419-92599 X-HE-Meta: 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 D+qiSXQq K2OWkccmLco5XZXA7ie0VTI8nx6i2Doh0SDdW7QMnbsg/mgYryI/cNY9CEkNfTqSyIntxfTPoCndKUwJCyn80kndmflpS8sEwKz54aBIRGNazeYH5Qo5/1GoguFKd67zDAEYVCOo1RHORIIbQaEfMCHTVAIl0Yb+5Ke7nphIYgQgeTXFoT70+ozUXkk1zWLqsSwo1XRNlsXYYDPPqfLSkeckM8wm5Hm+YHE1I/Z5gVUlx2SSbGhgXNkg0NHyOju1QJ43JOgj1Hj4qz5ETkTBSEMVb9bX0WYZBGcEr 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 11/21/25 09:37, Valentin Schneider wrote: > On 19/11/25 10:31, Dave Hansen wrote: >> On 11/14/25 07:14, Valentin Schneider wrote: >>> +static bool flush_tlb_kernel_cond(int cpu, void *info) >>> +{ >>> + return housekeeping_cpu(cpu, HK_TYPE_KERNEL_NOISE) || >>> + per_cpu(kernel_cr3_loaded, cpu); >>> +} >> >> Is it OK that 'kernel_cr3_loaded' can be be stale? Since it's not part >> of the instruction that actually sets CR3, there's a window between when >> 'kernel_cr3_loaded' is set (or cleared) and CR3 is actually written. >> >> Is that OK? >> >> It seems like it could lead to both unnecessary IPIs being sent and for >> IPIs to be missed. >> > > So the pattern is > > SWITCH_TO_KERNEL_CR3 > FLUSH > KERNEL_CR3_LOADED := 1 > > KERNEL_CR3_LOADED := 0 > SWITCH_TO_USER_CR3 > > > The 0 -> 1 transition has a window between the unconditional flush and the > write to 1 where a remote flush IPI may be omitted. Given that the write is > immediately following the unconditional flush, that would really be just > two flushes racing with each other, Let me fix that for you. When you wrote "a remote flush IPI may be omitted" you meant to write: "there's a bug." ;) In the end, KERNEL_CR3_LOADED==0 means, "you don't need to send this CPU flushing IPIs because it will flush the TLB itself before touching memory that needs a flush". SWITCH_TO_KERNEL_CR3 FLUSH // On kernel CR3, *AND* not getting IPIs KERNEL_CR3_LOADED := 1 > but I could punt the kernel_cr3_loaded > write above the unconditional flush. Yes, that would eliminate the window, as long as the memory ordering is right. You not only need to have the KERNEL_CR3_LOADED:=1 CPU set that variable, you need to ensure that it has seen the page table update. > The 1 -> 0 transition is less problematic, worst case a remote flush races > with the CPU returning to userspace and it'll get interrupted back to > kernelspace. It's also not just "returning to userspace". It could well be *in* userspace by the point the IPI shows up. It's not the end of the world, and the window isn't infinitely long. But there certainly is still a possibility of getting spurious interrupts for the precious NOHZ_FULL task while it's in userspace.