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Fri, 20 Mar 2026 18:33:28 -0700 (PDT) From: Ritesh Harjani (IBM) To: Mathieu Desnoyers , "Harry Yoo (Oracle)" , linuxppc-dev@lists.ozlabs.org Cc: Harry Yoo , Nathan Chancellor , Thomas =?utf-8?Q?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, Srikar Dronamraju , Madhavan Srinivasan Subject: Re: NULL pointer dereference when booting ppc64_guest_defconfig in QEMU on -next In-Reply-To: <7a8faee8-0eb5-4e58-a6d5-ef711791e3f4@efficios.com> Date: Sat, 21 Mar 2026 06:42:41 +0530 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> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-version: 1.0 Content-type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit ++ linuxppc-dev Mathieu Desnoyers writes: > 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: Even though powerpc is showing the warning because of VM_WARN_ON_ONCE(), but this looks more of a generic problem, where use of mm_cpumask() before and after percpu_counter_tree_items_size() could lead to different results (as you also pointed above). Looks like this is causing regressions in linux-next with warnings similar to what Harry also pointed out. Do we have any solution for this, or are we planning to hold on to this patch[1] and maybe even remove it temporarily from linux-next, until this is fixed? [1]: https://lore.kernel.org/all/20260227153730.1556542-1-mathieu.desnoyers@efficios.com/ [ 0.000000] WARNING: arch/powerpc/mm/mmu_context.c:106 at switch_mm_irqs_off+0x1a0/0x1d0, CPU#2: swapper/0 [ 0.000000] Modules linked in: [ 0.000000] CPU: 2 UID: 0 PID: 0 Comm: swapper Not tainted 7.0.0-rc4-next-20260317-00008-g5585e414f073 #4 PREEMPTLAZY [ 0.000000] Hardware name: IBM PowerNV (emulated by qemu) POWER10 0x801200 opal:v7.1 PowerNV [ 0.000000] NIP: c00000000008f3b0 LR: c00000000008f330 CTR: c000000000090e20 [ 0.000000] REGS: c000000003cb79b0 TRAP: 0700 Not tainted (7.0.0-rc4-next-20260317-00008-g5585e414f073) [ 0.000000] MSR: 9000000002021033 CR:24022224 XER: 00000000 <...> [ 0.000000] NIP [c00000000008f3b0] switch_mm_irqs_off+0x1a0/0x1d0 [ 0.000000] LR [c00000000008f330] switch_mm_irqs_off+0x120/0x1d0 [ 0.000000] Call Trace: [ 0.000000] [c000000003cb7c50] [0500210400000080] 0x500210400000080 (unreliable) [ 0.000000] [c000000003cb7cb0] [c0000000000ad850] start_using_temp_mm+0x34/0xb0 [ 0.000000] [c000000003cb7cf0] [c0000000000ae8b8] patch_mem+0x110/0x530 [ 0.000000] [c000000003cb7d70] [c000000000077f30] ftrace_modify_code+0x114/0x154 [ 0.000000] [c000000003cb7dd0] [c00000000036a690] ftrace_process_locs+0x408/0x810 [ 0.000000] [c000000003cb7ec0] [c0000000030584ec] ftrace_init+0x68/0x1c4 [ 0.000000] [c000000003cb7f30] [c00000000300d3b8] start_kernel+0x680/0xc44 [ 0.000000] [c000000003cb7fe0] [c00000000000e99c] start_here_common+0x1c/0x20 -ritesh