Re: [PATCH v11 2/4] uacce: add uacce driver

[zhangfei <zhangfei.gao@linaro.org>]

Hi, Dave

On 2020/1/16 上午12:43, Dave Jiang wrote:
>
>
> On 1/15/20 4:18 AM, zhangfei wrote:
>> Hi, Greg
>>
>> On 2020/1/14 下午10:59, Greg Kroah-Hartman wrote:
>>> On Mon, Jan 13, 2020 at 11:34:55AM +0800, zhangfei wrote:
>>>> Hi, Greg
>>>>
>>>> Thanks for the review.
>>>>
>>>> On 2020/1/12 上午3:40, Greg Kroah-Hartman wrote:
>>>>> On Sat, Jan 11, 2020 at 10:48:37AM +0800, Zhangfei Gao wrote:
>>>>>> +static int uacce_fops_open(struct inode *inode, struct file *filep)
>>>>>> +{
>>>>>> +    struct uacce_mm *uacce_mm = NULL;
>>>>>> +    struct uacce_device *uacce;
>>>>>> +    struct uacce_queue *q;
>>>>>> +    int ret = 0;
>>>>>> +
>>>>>> +    uacce = xa_load(&uacce_xa, iminor(inode));
>>>>>> +    if (!uacce)
>>>>>> +        return -ENODEV;
>>>>>> +
>>>>>> +    if (!try_module_get(uacce->parent->driver->owner))
>>>>>> +        return -ENODEV;
>>>>> Why are you trying to grab the module reference of the parent device?
>>>>> Why is that needed and what is that going to help with here?
>>>>>
>>>>> This shouldn't be needed as the module reference of the owner of the
>>>>> fileops for this module is incremented, and the "parent" module 
>>>>> depends
>>>>> on this module, so how could it be unloaded without this code being
>>>>> unloaded?
>>>>>
>>>>> Yes, if you build this code into the kernel and the "parent" 
>>>>> driver is a
>>>>> module, then you will not have a reference, but when you remove that
>>>>> parent driver the device will be removed as it has to be unregistered
>>>>> before that parent driver can be removed from the system, right?
>>>>>
>>>>> Or what am I missing here?
>>>> The refcount here is preventing rmmod "parent" module after fd is 
>>>> opened,
>>>> since user driver has mmap kernel memory to user space, like mmio, 
>>>> which may
>>>> still in-use.
>>>>
>>>> With the refcount protection, rmmod "parent" module will fail until
>>>> application free the fd.
>>>> log like: rmmod: ERROR: Module hisi_zip is in use
>>> But if the "parent" module is to be unloaded, it has to unregister the
>>> "child" device and that will call the destructor in here and then you
>>> will tear everything down and all should be good.
>>>
>>> There's no need to "forbid" a module from being unloaded, even if it is
>>> being used.  Look at all networking drivers, they work that way, right?
>> Thanks Greg for the kind suggestion.
>>
>> I still have one uncertainty.
>> Does uacce has to block process continue accessing the mmapped area 
>> when remove "parent" module?
>> Uacce can block device access the physical memory when parent module 
>> call uacce_remove.
>> But application is still running, and suppose it is not the kernel 
>> driver's responsibility to call unmap.
>>
>> I am looking for some examples in kernel,
>> looks vfio does not block process continue accessing when 
>> vfio_unregister_iommu_driver either.
>>
>> In my test, application will keep waiting after rmmod parent, until 
>> ctrl+c, when unmap is called.
>> During the process, kernel does not report any error.
>>
>> Do you have any advice?
>
> Would it work to call unmap_mapping_range() on the char dev 
> inode->i_mappings? I think you need to set the vma->fault function ptr 
> for the vm_operations_struct in the original mmap(). After the 
> mappings are unmapped, you can set a state variable to trigger the 
> return of VM_FAULT_SIGBUS in the ->fault function when the user app 
> accesses the mmap region again and triggers a page fault. The user app 
> needs to be programmed to catch exceptions to deal with that.

Thanks Dave for the advice.
Will look into it, may need some time to investigate.
I would like to make an additional patch for this issue, since it does 
not impact the main function.

Thanks