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NULL pointer dereference when booting ppc64_guest_defconfig in QEMU on -next To: Harry Yoo , Nathan Chancellor Cc: =?UTF-8?Q?Thomas_Wei=C3=9Fschuh?= , Michal Clapinski , Andrew Morton , Thomas Gleixner , Steven Rostedt , Masami Hiramatsu , linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org, linux-kernel@vger.kernel.org References: <20260319233745.GA769346@ax162> From: Mathieu Desnoyers Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: YT4PR01CA0253.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:b01:10f::6) To YT2PR01MB9175.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:b01:be::5) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: YT2PR01MB9175:EE_|YT6PR01MB10892:EE_ X-MS-Office365-Filtering-Correlation-Id: 19126c4a-e384-4b8d-74a3-08de867d3656 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: 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before the creation of >> the first mm at boot. > > But how does this work when someone calls mm_cpumask() on init_mm early? > Looks like it will behave incorrectly because get_rss_stat_items_size() > returns zero? It doesn't work as expected at all. I missed that all users of mm_cpumask() end up relying on get_rss_stat_items_size(), which now calls percpu_counter_tree_items_size(), which depends on initialization from percpu_counter_tree_subsystem_init(). If you add a call to percpu_counter_tree_subsystem_init in arch/powerpc/kernel/setup_arch() just before: VM_WARN_ON(cpumask_test_cpu(smp_processor_id(), mm_cpumask(&init_mm))); cpumask_set_cpu(smp_processor_id(), mm_cpumask(&init_mm)); Does the warning go away ? Alternatively, would could use a lazy initialization invoking percpu_counter_tree_subsystem_init from percpu_counter_tree_items_size when the initialization is not already done. Any preference ? Mathieu -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com