linux-kernel - Re: [PATCH v3] mm: Make vmalloc_dump

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Date:   Mon, 28 Nov 2022 17:13:43 +0800
From:   "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
To:     "Zhang, Qiang1" <qiang1.zhang@...el.com>,
        "paulmck@...nel.org" <paulmck@...nel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "frederic@...nel.org" <frederic@...nel.org>,
        "joel@...lfernandes.org" <joel@...lfernandes.org>
CC:     "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] mm: Make vmalloc_dump_obj() call in clean context



On 2022/11/28 16:33, Zhang, Qiang1 wrote:
> On 2022/11/23 7:05, Zhang, Qiang1 wrote:
>>
>> Gently ping  😊
>>
>> Thanks
>> Zqiang
>>
>>> Currently, the mem_dump_obj() is invoked in call_rcu(), the
>>> call_rcu() is maybe invoked in non-preemptive code segment, for 
>>> object allocated from vmalloc(), the following scenarios may occur:
>>>
>>>        CPU 0
>>> tasks context
>>>   spin_lock(&vmap_area_lock)
>>>       Interrupt context
>>>           call_rcu()
>>>             mem_dump_obj
>>>               vmalloc_dump_obj
>>>                 spin_lock(&vmap_area_lock) <--deadlock
>>>
>>> and for PREEMPT-RT kernel, the spinlock will convert to sleepable 
>>> lock, so the vmap_area_lock spinlock not allowed to get in 
>>> non-preemptive code segment. therefore, this commit make the 
>>> vmalloc_dump_obj() call in a clean context.
>>>
>>> Signed-off-by: Zqiang <qiang1.zhang@...el.com>
>>> ---
>>> v1->v2:
>>> add IS_ENABLED(CONFIG_PREEMPT_RT) check.
>>> v2->v3:
>>> change commit message and add some comment.
>>>
>>> mm/util.c    |  4 +++-
>>> mm/vmalloc.c | 25 +++++++++++++++++++++++++
>>> 2 files changed, 28 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/mm/util.c b/mm/util.c
>>> index 12984e76767e..2b0222a728cc 100644
>>> --- a/mm/util.c
>>> +++ b/mm/util.c
>>> @@ -1128,7 +1128,9 @@ void mem_dump_obj(void *object)
>>> 		return;
>>>
>>> 	if (virt_addr_valid(object))
>>> -		type = "non-slab/vmalloc memory";
>>> +		type = "non-slab memory";
>>> +	else if (is_vmalloc_addr(object))
>>> +		type = "vmalloc memory";
>>> 	else if (object == NULL)
>>> 		type = "NULL pointer";
>>> 	else if (object == ZERO_SIZE_PTR)
>>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c index 
>>> ccaa461998f3..4351eafbe7ab 100644
>>> --- a/mm/vmalloc.c
>>> +++ b/mm/vmalloc.c
>>> @@ -4034,6 +4034,31 @@ bool vmalloc_dump_obj(void *object)
>>> 	struct vm_struct *vm;
>>> 	void *objp = (void *)PAGE_ALIGN((unsigned long)object);
>>>
>>> +	/* for non-vmalloc addr, return directly */
>>> +	if (!is_vmalloc_addr(objp))
>>> +		return false;
>>> +
>>> +	/**
>>> +	 * for non-Preempt-RT kernel, return directly. otherwise not
>>> +	 * only needs to determine whether it is in the interrupt context
>>> +	 * (in_interrupt())to avoid deadlock, but also to avoid acquire
>>> +	 * vmap_area_lock spinlock in disables interrupts or preempts
>>> +	 * critical sections, because the vmap_area_lock spinlock convert
>>> +	 * to sleepable lock
>>> +	 */
>>> +	if (IS_ENABLED(CONFIG_PREEMPT_RT) && !preemptible())
>>> +		return false;
>>> +
>>> +	/**
>>> +	 * get here, for Preempt-RT kernel, it means that we are in
>>> +	 * preemptible context(preemptible() is true), it also means
>>> +	 * that the in_interrupt() will return false.
>>> +	 * for non-Preempt-RT kernel, only needs to determine whether
>>> +	 * it is in the interrupt context(in_interrupt()) to avoid deadlock
>>> +	 */
>>> +	if (in_interrupt())
>>> +		return false;
>>
>>
>> We want mem_dump_obj() to work properly in the interrupt context. But with this if statement, it's impossible to work properly.
> 
> This is to avoid the following scenarios, because, call_rcu() can be invoked in hard irq or
> softirq context, so mem_dump_obj() not dump some details info.

OK. Sorry, I'm confusing your issue with what I'm doing right now.

https://lkml.org/lkml/2022/11/16/913

I need "if (in_interrupt() && spin_is_locked(&vmap_area_lock))". So
mem_dump_obj() can work well in interrupt, except the task was
interrupted in the critical section of vmap_area_lock.


> 
> CPU 0
> tasks context
>    spin_lock(&vmap_area_lock)
>        Interrupt  or softirq context
>            call_rcu()
>              mem_dump_obj
>               vmalloc_dump_obj
>                  spin_lock(&vmap_area_lock) <--deadlock
> 
> because mem_dump_obj() only used by RCU,   I'm not sure if this modification is appropriate, 
> need to hear from Paul.
> 
> Thanks
> Zqiang
> 
> 
>>
>> Here's my test case:
>> void *tst_p;
>>
>> void my_irqwork_handler(struct irq_work *work) {
>>        void *p = tst_p;
>>
>>        printk("enter my_irqwork_handler: CPU=%d, locked=%d\n", smp_processor_id(), tst_is_locked());
>>        mem_dump_obj(p);
>>        vfree(p);
>> }
>>
>> static void test_mem_dump(void)
>> {
>>        struct irq_work work = IRQ_WORK_INIT_HARD(my_irqwork_handler);
>>
>>        tst_p = vmalloc(PAGE_SIZE);
>>        if (!tst_p) {
>>                printk("vmalloc failed\n");
>>                return;
>>        }
>>        printk("enter test_mem_dump: CPU=%d\n", smp_processor_id());
>>
>>        //tst_lock();
>>        irq_work_queue(&work);
>>        //tst_unlock();
>>
>>        printk("leave test_mem_dump: CPU=%d\n", smp_processor_id()); }
>>
>> Test result:
>> [   45.212941] enter test_mem_dump: CPU=0
>> [   45.213280] enter my_irqwork_handler: CPU=0, locked=0
>> [   45.213546]  vmalloc memory
>> [   45.213996] leave test_mem_dump: CPU=0
>>
>>> +
>>> 	vm = find_vm_area(objp);
>>> 	if (!vm)
>>> 		return false;
>>> --
>>> 2.25.1
>>
>>
>> -- 
>> Regards,
>>  Zhen Lei

-- 
Regards,
  Zhen Lei