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]) by smtp.lore.kernel.org (Postfix) with ESMTP id 73270C54798 for ; Thu, 7 Mar 2024 17:37:04 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id E25A46B0246; Thu, 7 Mar 2024 12:37:03 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id DD3DB6B0249; Thu, 7 Mar 2024 12:37:03 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id C745F6B024A; Thu, 7 Mar 2024 12:37:03 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0014.hostedemail.com [216.40.44.14]) by kanga.kvack.org (Postfix) with ESMTP id B469B6B0246 for ; Thu, 7 Mar 2024 12:37:03 -0500 (EST) Received: from smtpin16.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay07.hostedemail.com (Postfix) with ESMTP id 683F8160AF3 for ; Thu, 7 Mar 2024 17:37:03 +0000 (UTC) X-FDA: 81870948726.16.405DC08 Received: from mgamail.intel.com (mgamail.intel.com [192.198.163.7]) by imf12.hostedemail.com (Postfix) with ESMTP id 8FC9D40005 for ; Thu, 7 Mar 2024 17:37:00 +0000 (UTC) Authentication-Results: imf12.hostedemail.com; dkim=pass header.d=intel.com header.s=Intel header.b=Scs6Jg7G; spf=pass (imf12.hostedemail.com: domain of dave.hansen@intel.com designates 192.198.163.7 as permitted sender) smtp.mailfrom=dave.hansen@intel.com; dmarc=pass (policy=none) header.from=intel.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1709833021; a=rsa-sha256; cv=none; b=NRlLT4sYqrhp7LmTloVE9lXUoREDaX1k0GQgjlCHcen8tm4vr8NfDI9MLyTX9Q6XPPQDyN q8p9BPRPjqMRgQa5Ff4ebCNa+7MCBM5yc5db1ZLS6cjqTaLmHyeasOiN5Z2aZxA47qtjz4 r0Qe0rLBss959TG0dYlP70E/9w8tWyI= ARC-Authentication-Results: i=1; imf12.hostedemail.com; dkim=pass header.d=intel.com header.s=Intel header.b=Scs6Jg7G; spf=pass (imf12.hostedemail.com: domain of dave.hansen@intel.com designates 192.198.163.7 as permitted sender) smtp.mailfrom=dave.hansen@intel.com; dmarc=pass (policy=none) header.from=intel.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1709833021; 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=j5APa47CD/VeHMN8ErQx0miuRjH52iQuo8eg/KgDSwE=; b=4ZyU51GpJnATrYGsP9G+ZOCQVPn6badLAliQ1CIw6R+BuK9FNyPzl/Y2tdei78qEfg9l6R I9AI50qs9vPwI5Zb7uUIIyudpyZxB0dk4ofyOIOcae/2wx+nsrA0x4KxXPnsdK8xVc7T53 oSOwiiOQFnHy+ZFiP2wCJI9m/acPioU= DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1709833021; x=1741369021; h=message-id:date:mime-version:subject:to:cc:references: from:in-reply-to:content-transfer-encoding; bh=d+uJbvrJni6Hk3MuASRhRdMts5GdhPUHOSsQge7P9kU=; b=Scs6Jg7G78UWGw6J9hTuq6a51RNA/fPwfQpHibVats+U5aqHnstMu2lr AeKp6U/X+9tIVXLByFZqSeHAMiOmIo7olNngtsj0m002GN1h2kkyEaZr8 dbdY26eY4J8LNZVN0Cvq+JUshc2ywGo8iSX4PGHgBhkvX+mTbTYTZzRx2 4TwvGeUBaFEjou2XnpkRQlTtARY8tPY9R7JXhwGWFo7W5GSgNzKwLxYqq RI5b+K7eb+iYUJNsvB5zGDd8axpSU7+bQrw2DU/+DsJeRnx4Sq2QUkvL1 7z+xBbnj6dhTPwEQzszMwwA8sQiwX3KBKcfB1v1yKIGpC90VWo5nA0CPP w==; X-IronPort-AV: E=McAfee;i="6600,9927,11006"; a="29963053" X-IronPort-AV: E=Sophos;i="6.07,107,1708416000"; d="scan'208";a="29963053" Received: from orviesa004.jf.intel.com ([10.64.159.144]) by fmvoesa101.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 07 Mar 2024 09:36:59 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.07,107,1708416000"; d="scan'208";a="14844346" Received: from yuqiangu-mobl.amr.corp.intel.com (HELO [10.209.57.176]) ([10.209.57.176]) by orviesa004-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 07 Mar 2024 09:36:59 -0800 Message-ID: Date: Thu, 7 Mar 2024 09:36:58 -0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 1/3] x86/mm: fix LAM cr3 mask inconsistency during context switch Content-Language: en-US To: Yosry Ahmed , Andrew Morton Cc: Thomas Gleixner , Ingo Molnar , Borislav Petkov , Peter Zijlstra , Andy Lutomirski , "Kirill A. Shutemov" , x86@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20240307133916.3782068-1-yosryahmed@google.com> <20240307133916.3782068-2-yosryahmed@google.com> 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: <20240307133916.3782068-2-yosryahmed@google.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 8FC9D40005 X-Stat-Signature: wijdy9hn3qmftjfw5obk3w5tyqh1cap1 X-Rspam-User: X-HE-Tag: 1709833020-502257 X-HE-Meta: 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 2aJj6TEr ApTY637UVpTRzU+1b5zR4W5QP8a2GhC10v8FBapeJeuCVw67BX2qxVX5Ur3O5bHKYahabRZa8vLJm+s+wGLyHTVnzxYH9IgVqDdUIdW8ovKQLmxz1u4jCrw362q8+GvyMnSpk/EkUnAhJ/Vam6k6fqG6EqTvxpYWaiXZiU/ABnC4pNg4t404YGLvA3PyBK3yFXdwcOXpoDdh7eMY/BVRYjorF2F9G4Qvoo29Cc2Cf/26Z2spCANqw96ZkqxPPgkEXNLexwxL1G8dCX/bku3da1bTszyBTTW/B8cBqWJTTk0Mf4RV3gVqrF55vkGrzmkZVOoyyBQuTpQ4FH+8= 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: I know we all have different rules, but any time you could spend absorbing: https://www.kernel.org/doc/html/next/process/maintainer-tip.html would be appreciated, especially: > The condensed patch description in the subject line should start with > a uppercase letter and should be written in imperative tone. On 3/7/24 05:39, Yosry Ahmed wrote: > In switch_mm_irqs_off(), we read the 'mm->context.lam_cr3_mask' into > 'new_lam', which is later passed to load_new_mm_cr3(). However, there is > a call to set_tlbstate_lam_mode() in between which will read > 'mm->context.lam_cr3_mask' again and set 'cpu_tlbstate.lam' accordingly. > If we race with another thread updating 'mm->context.lam_cr3_mask', the > value in 'cpu_tlbstate.lam' could end up being different from CR3. Your description is fine (modulo the we's). But I slightly reworded it to make it more plainly readable: LAM can only be enabled when a process is single-threaded. But _kernel_ threads can temporarily use a single-threaded process's mm. That means that a context-switching kernel thread can race and observe the mm's LAM metadata (mm->context.lam_cr3_mask) change. The context switch code does two logical things with that metadata: populate CR3 and populate 'cpu_tlbstate.lam'. If it hits this race, 'cpu_tlbstate.lam' and CR3 can end up out of sync. This de-synchronization is currently harmless. But it is confusing and might lead to warnings or real bugs. -- > Fix the problem by updating set_tlbstate_lam_mode() to return the LAM > mask that was set to 'cpu_tlbstate.lam', and use that mask in > switch_mm_irqs_off() when writing CR3. Use READ_ONCE to make sure we > read the mask once and use it consistenly. Spell checking is also appreciated. ... > -static inline void set_tlbstate_lam_mode(struct mm_struct *mm) > +static inline unsigned long set_tlbstate_lam_mode(struct mm_struct *mm) > { > - this_cpu_write(cpu_tlbstate.lam, > - mm->context.lam_cr3_mask >> X86_CR3_LAM_U57_BIT); > + unsigned long lam = READ_ONCE(mm->context.lam_cr3_mask); > + > + this_cpu_write(cpu_tlbstate.lam, lam >> X86_CR3_LAM_U57_BIT); > this_cpu_write(tlbstate_untag_mask, mm->context.untag_mask); > + return lam; > } The comments about races need to be _here_ so that the purpose of the READ_ONCE() is clear. It would also be nice to call out the rule that this can only meaningfully be called once per context switch. > @@ -633,7 +628,12 @@ void switch_mm_irqs_off(struct mm_struct *unused, struct mm_struct *next, > barrier(); > } > > - set_tlbstate_lam_mode(next); > + /* > + * Even if we are not actually switching mm's, another thread could have > + * updated mm->context.lam_cr3_mask. Make sure tlbstate_lam_cr3_mask() > + * and the loaded CR3 use the up-to-date mask. > + */ I kinda dislike how the comment talks about the details of what set_tlbstate_lam_mode() does. It would be much better to put the meat of this comment at the set_tlbstate_lam_mode() definition. > + new_lam = set_tlbstate_lam_mode(next); > if (need_flush) { > this_cpu_write(cpu_tlbstate.ctxs[new_asid].ctx_id, next->context.ctx_id); > this_cpu_write(cpu_tlbstate.ctxs[new_asid].tlb_gen, next_tlb_gen); This is less a complaint about your change and more of the existing code, but I wish it was more obvious that set_tlbstate_lam_mode() is logically shuffling data (once) from 'next' into the tlbstate. The naming makes it sound like it is modifying the tlbstate of 'next'. But I don't have any particularly brilliant ideas to fix it either. Maybe just: /* new_lam is effectively cpu_tlbstate.lam */ > @@ -705,7 +705,6 @@ void initialize_tlbstate_and_flush(void) > > /* LAM expected to be disabled */ > WARN_ON(cr3 & (X86_CR3_LAM_U48 | X86_CR3_LAM_U57)); > - WARN_ON(mm_lam_cr3_mask(mm)); > > /* > * Assert that CR4.PCIDE is set if needed. (CR4.PCIDE initialization > @@ -724,7 +723,7 @@ void initialize_tlbstate_and_flush(void) > this_cpu_write(cpu_tlbstate.next_asid, 1); > this_cpu_write(cpu_tlbstate.ctxs[0].ctx_id, mm->context.ctx_id); > this_cpu_write(cpu_tlbstate.ctxs[0].tlb_gen, tlb_gen); > - set_tlbstate_lam_mode(mm); > + WARN_ON(set_tlbstate_lam_mode(mm)); The "set_" naming bugs me in both of the sites that get modified here. I'd be with a new name that fits better, if we can think of one.