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b=sH1bvS66tYWyssScB5U5W+6yI64kW9PEgpfUF/sWtKjITnZDYvB9r9QWkxb43jfGM zenGTMnJrUvbUYr7BSjNH3xYrX4QeMHcdJAG7n0vfv6vC/IDPMlmvZxw/BlGXhfnew 1vxTLMpqKhkujwTmBBc249LCJkqMOZnddRHtuPmR+4mEei4v9vGfiCX0WOveVD5mqE sBmQz/YP2sxDQxTC+6T+ielbgQmhmOE6GuOPD+Pu/cot6sy7RANs08qGzCmHWcwXja /XVffeAKWummLz4xiHeuy1RW2l6/RTQxM2PKsI7m5/9UNTGBCav96JgY0HLDtUTu/u H8NCahNA8DG4Q== Date: Fri, 20 Mar 2026 22:21:56 +0900 From: "Harry Yoo (Oracle)" To: Mathieu Desnoyers Cc: Harry Yoo , Nathan Chancellor , Thomas =?iso-8859-1?Q?Wei=DFschuh?= , 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 Subject: Re: NULL pointer dereference when booting ppc64_guest_defconfig in QEMU on -next Message-ID: References: <20260319233745.GA769346@ax162> <7780a471-9d99-40a7-ade7-0c4594ac36c7@efficios.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <7780a471-9d99-40a7-ade7-0c4594ac36c7@efficios.com> X-Rspam-User: X-Rspamd-Queue-Id: C6684100010 X-Rspamd-Server: rspam08 X-Stat-Signature: cmcm49389kraopn4gccyicqmhaej1nce X-HE-Tag: 1774012918-834812 X-HE-Meta: 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 CDLaHnHU /MM4G Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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? > 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? Hmm perhaps we should treat init_mm as a special case in mm_cpus_allowed() and mm_cpumask(). -- Cheers, Harry / Hyeonggon