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NULL pointer dereference when booting ppc64_guest_defconfig in QEMU on -next To: "Harry Yoo (Oracle)" Cc: Harry Yoo , Nathan Chancellor , =?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> <7780a471-9d99-40a7-ade7-0c4594ac36c7@efficios.com> 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: YQZPR01CA0039.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:c01:86::24) To YT2PR01MB9175.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:b01:be::5) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: YT2PR01MB9175:EE_|YT2PR01MB8438:EE_ X-MS-Office365-Filtering-Correlation-Id: 82068799-6116-498c-7f3f-08de86850fc1 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2026-03-20 09:21, Harry Yoo (Oracle) wrote: > On Fri, Mar 20, 2026 at 08:35:46AM -0400, Mathieu Desnoyers wrote: >> On 2026-03-20 00:17, Harry Yoo wrote: >> [...] >>>> [1]: https://lore.kernel.org/20260227153730.1556542-4-mathieu.desnoyers@efficios.com/ >>> >>> @Mathieu: In patch 1/3 description, >>>> Changes since v7: >>>> - Explicitly initialize the subsystem from start_kernel() right >>>> after mm_core_init() so it is up and running 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 ? > > Hmm it goes away, but I'm not sure if it is it okay to use nr_cpu_ids > before setup_nr_cpu_ids() is called? AFAIU on powerpc setup_nr_cpu_ids() is called near the end of smp_setup_cpu_maps(), which is called early in setup_arch, at least before the two lines which use mm_cpumask. >> 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. > > So this probably isn't a way to go? I'd favor explicit initialization, so the inter-dependencies are clear. > Hmm perhaps we should treat init_mm as a special case in > mm_cpus_allowed() and mm_cpumask(). I'd prefer not to go there if boot sequence permits and keep things simple. I think we're in a situation very similar to tree RCU, here is what is done in rcu_init_geometry: static bool initialized; if (initialized) { /* * Warn if setup_nr_cpu_ids() had not yet been invoked, * unless nr_cpus_ids == NR_CPUS, in which case who cares? */ WARN_ON_ONCE(old_nr_cpu_ids != nr_cpu_ids); return; } old_nr_cpu_ids = nr_cpu_ids; initialized = true; Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com