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Tue, 1 Jul 2025 14:59:19 +0000 Message-ID: Date: Tue, 1 Jul 2025 22:59:12 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [BUG] Kernel panic in __migrate_swap_task() on 6.16-rc2 (NULL pointer dereference) To: Libo Chen , Jirka Hladky , Abhigyan ghosh CC: , Chen Yu References: <07D9F8BC-47A7-4E87-8655-C978C056E308@gmail.com> <31bd3395-cfe3-4af5-bc1c-fa8d26629b93@intel.com> Content-Language: en-US From: "Chen, Yu C" In-Reply-To: Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-ClientProxiedBy: SG2PR02CA0126.apcprd02.prod.outlook.com (2603:1096:4:188::11) To DM4PR11MB6020.namprd11.prod.outlook.com (2603:10b6:8:61::19) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DM4PR11MB6020:EE_|SA3PR11MB7525:EE_ X-MS-Office365-Filtering-Correlation-Id: 703d1c77-a3f3-4fb7-448d-08ddb8afdb44 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|376014|1800799024; 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>>>                                            p->flags |= PF_EXITING >>>                                         exit_mm >>>                                            p->mm = NULL; >>> >>>     migrate_swap_stop >>>       __migrate_swap_task((arg->src_task, arg->dst_cpu) >>>        count_memcg_event_mm(p->mm, NUMA_TASK_SWAP)# p->mm is NULL >>> >>> Could you please help check if the following debug patch works, >> >> Attached the patch: >> >> diff --git a/kernel/sched/core.c b/kernel/sched/core.c >> index 8988d38d46a3..82fc966b390c 100644 >> --- a/kernel/sched/core.c >> +++ b/kernel/sched/core.c >> @@ -3364,7 +3364,12 @@ static void __migrate_swap_task(struct task_struct *p, int cpu) >>  { >>      __schedstat_inc(p->stats.numa_task_swapped); >>      count_vm_numa_event(NUMA_TASK_SWAP); >> -    count_memcg_event_mm(p->mm, NUMA_TASK_SWAP); >> +    if (unlikely(!p->mm)) { > > I am starting to wonder if we should keep this check and add a big fat warning > like "there is a bug here, please report it!" rather than brick the kernel. > A kernel panic, for sure, helps catch bugs like this more than a line of dmesg, > so it's a tradeoff I guess. What do you think? > I agree with you that adding something like WARN_ON_ONCE() could help, and it seems that the kernel panic is not expected in this scenario because this feature is a statistic calculation rather than the critical logic. Maybe waiting for Jirka's feedback to decide the next step. thanks, Chenyu > > Thanks, > Libo > >> +        trace_printk("!! (%d %s) flags=%lx\n", p->pid, p->comm, >> +                p->flags); >> +    } else { >> +        count_memcg_event_mm(p->mm, NUMA_TASK_SWAP); >> +    } >> >>      if (task_on_rq_queued(p)) { >>          struct rq *src_rq, *dst_rq; >