RE: [PATCH v3] mm: Make vmalloc_dump_obj() call in clean context

[Zhang, Qiang1]

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@xxxxxxxxx>
>> ---
>> 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.

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