Re: Does get_user_pages_fast lock the user pages in memory in my case?

[Huang Shijie <shijie8@gmail.com>]

Minchan Kim a??e??:
> On Mon, 20 Apr 2009 13:05:33 +0800
> Huang Shijie <shijie8@gmail.com> wrote:
>
>   
>> Minchan Kim a??e??:
>>     
>>> On Mon, 20 Apr 2009 11:28:00 +0800
>>> Huang Shijie <shijie8@gmail.com> wrote:
>>>
>>> I will summarize your method. 
>>> Is right ?
>>>
>>>
>>> kernel(driver)					application 
>>>
>>> 						posix_memalign(buffer)
>>> 						ioctl(buffer)
>>>
>>> ioctl handler
>>> get_user_pages(pages);
>>> /* This pages are mapped at user's vma' 
>>> address space */
>>> vaddr = vmap(pages);
>>> /* This pages are mapped at vmalloc space */
>>> .
>>> .
>>> <after sometime, 
>>> It may change to other process context>
>>> .
>>> .
>>> interrupt handler in your driver 
>>> memcpy(vaddr, src, len); 
>>> notify_user();
>>>
>>> 						processing(buffer);
>>>
>>> It's rather awkward use case of get_user_pages. 
>>>
>>> If you want to share one big buffer between kernel and user, 
>>> You can vmalloc and remap_pfn_range.
>>>   
>>>       
>> The v4l2 method IO_METHOD_MMAP does use the vmaloc() method you told above ,
>> our driver also support this method,we user vmalloc /remap_vmalloc_range().
>>
>> But the v4l2 method IO_METHOD_USERPTR must use the method I told above.
>>     
>
> I can't understand IO_METHOD_USERPTR's benefit compared with IO_METHOD_MMAP. 
> I think both solution can support that application programmer can handle buffer as like pointer and kernel can reduce copy overhead from kernel to user. 
>
>   
yes ,I agree with you .
But the application programmers do not know which method is more efficient.

> Why do you have to support IO_METHOD_USERPTR?
>   
just for fun. For the v4l2 spec has the method ,why I don't realize it?

> If you can justify your goal, we can add locked GUP. 
>
>   
I can't .

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