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b=DjOwOV8G1UiUJj0rYo3i3CjWTQsqdK4Y3ob3KXpGLF/fLE7rSmAwIbvrJqA/D9yrv jJQYi6st2sdrgA9J31dkO5SIddorWAN15xloNrijFdByxAbJcv+4YFcMc3NK2GCMxW tWrEA2wx6C1zknjNJo9sjNXJeTrl+Sj7ZY96gHqMD82AHbVlbt2BYKOynF4iRnnzJ2 5cCy3JyMbOoFb1iZHQ5PTGeuT8Oks/dnxp+VMab/esljySMyBH0s/SwmG9j2XPd7LI eyDTNY1/G1JwNS1C+VI/Wl9m9bB7/WZVqlS95izhV8pqVx6DORMo2FTFLwzNDVp/kN o75KfJ1c5h2rg== Date: Mon, 23 Mar 2026 10:53:57 +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> <7458d8fd-5922-4e0b-9cd5-91880282aaa3@efficios.com> <7a8faee8-0eb5-4e58-a6d5-ef711791e3f4@efficios.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <7a8faee8-0eb5-4e58-a6d5-ef711791e3f4@efficios.com> X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 26B3314000E X-Stat-Signature: dusscyuyoa5w7qshju1qrmu7g5wcapby X-Rspam-User: X-HE-Tag: 1774230839-850572 X-HE-Meta: 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 7LIvatsa q8MWCSJcWhZ1xXGirJ368CEpGpz5rn5AtkjK6mQlfrHdTf3nR5Fp+zpYYAKINbocBTk59awmHi/Iyy/NCtZjq4tCQr8fgJdzOcBAlZqcxnzK2PquyGCpYiVvGcaFTcOP1q3/uQoWDSU6SiwARxv/iPvMykxthraLZwTH0GiG5Ffex02f1F00R48VV9zO6OGCfR4d7KTeJCKovr3696rpLWi/5ujN6nHqdCy5NRUmUKWSEj1SeFmMmdo9oRGO1G1/Tuma1e7DiYkpUNnapZ1FkejrOOgPR+ygshP5y7lpOB1O7c/UmBMo9tWyuUpjUP6TbG/pkCqqTvakCddTyUN5KZozwaqVmgsWe5y5rnADCcLzwkR3pXCxwiWa2JRUhXW5ONIEIzolXzGuFvVUxTli9SGtK6w== 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 10:20:37AM -0400, Mathieu Desnoyers wrote: > On 2026-03-20 09:31, Mathieu Desnoyers wrote: > > 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: > > [...] > > One thing we could do to catch this kind of init sequence issue > is to add a WARN_ON_ONCE in percpu_counter_tree_items_size: > > size_t percpu_counter_tree_items_size(void) > { > if (WARN_ON_ONCE(!nr_cpus_order)) > return 0; > return counter_config->nr_items * sizeof(struct percpu_counter_tree_level_item); Looks good! -- Cheers, Harry / Hyeonggon