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 Fri, 17 Apr 2009 15:01:01 +0800
> Huang Shijie <shijie8@gmail.com> wrote:
>
>   
>>    I'm writting a driver for a video card with the V4L2 interface .
>>    V4L2 interface supports the USER-POINTER method for the video frame 
>> handling.
>>
>>    VLC player supports the USER-POINTER method,while MPALYER does not.
>>
>>    In the USER-POINTER method, VLC will call the posix_memalign() to 
>> allocate
>> 203 pages in certain PAL mode (that is 720*576*2) for a single frame.
>>    In my driver , I call the get_user_pages_fast() to obtain the pages 
>> array,and then call
>> the vmap() to map the pages to VMALLOC space for the memcpy().The code 
>> shows below:
>>    ....................
>>    get_user_pages_fast();
>>    ...
>>    f->data = vmap();
>>    .......................
>>     
>
>
> What I understand is that you get the pages of posix_memalign by get_user_pages_fast 
> and then that pages are mapped at kernel vmalloc space by vmap. 
>
> Is it for removing copy overhead from kernel to user ?
>
>   
I need a large range of virtual contigous memory to store my video 
frame(about 203 pages). When I received a full frame ,I will queue the 
buffer in
a VIDIOC queue,which will be remove by the VIDIOC_DQBUF.
>>    In comments, it said :
>> "
>> +/**
>> + * get_user_pages_fast() - pin user pages in memory
>> + * @start:     starting user address
>> + * @nr_pages:  number of pages from start to pin
>> + * @write:     whether pages will be written to
>> + * @pages:     array that receives pointers to the pages pinned.
>> + *             Should be at least nr_pages long.
>> "
>>
>>    But after I digged the code of kswap and the get_user_pages(called by 
>> get_user_pages_fast),
>> I did not find how the pages pinned in memory.I really need the pages 
>> pinned in memory.
>>
>>    Assume page A is one of the pages obtained by get_user_pages_fast() 
>> during page-fault.
>>
>> [1] page A will on the LRU_ACTIVE_ANON list;
>>    the _count of page A increment by one;
>>    PTE for page A will be set ACCESSED.
>>
>> [2] kswapd will scan the lru list,and move page A from LRU_ACTIVE_ANON  
>> to LRU_INACTIVE_ANON.
>>    In the shrink_page_list(), there is nothing can stop page A been 
>> swapped out.
>>    I don't think the page_reference() can move page A back to 
>> LRU_ACTIVE_ANON.In my driver,
>>    I am not sure if the VLC can access the page A.
>>
>>    Is this a bug? or I miss something?
>>    Thanks .
>>     
>
> If above my assumption is right, It's not a BUG. 
> You get the application's pages by get_user_pages_fast. 
> 'Page pinning' means it shouldn't be freed. 
> Application's pages always can be swapped out. 
> If you don't want to swap out the page, you should use mlock. 
> If you use mlock, kernel won't insert the page to lru [in]active list.
> So the page never can be swapped out. 
>
>   
Yes, it not a bug .

I read the kernel code again. In my case ,the kernel will pin the pages 
in memory.
I missed function is_page_cache_freeable() in the pageout().

In my case, is_page_cache_freeable()will return false ,for 
page_count(page) is 3 now:
<1> one is from alloc_page_* in page fault.
<2> one is from get_usr_pages()
<3> one is from add_to_swap() in shrink_page_list()

So ,there is no need to use the mlock, it will mess my driver.
is_page_cache_freeable()will return PAGE_KEEP, and page is locked in 
swap cache.

Unfortunately, the page is unmaped, and the PTE of the page has been 
replaced by a swp_entry_t .
When the process read the page ,it will raise a page fault again, the 
kernel will find the page in the
swap cache, and requeue the page in LRU_ACTIVE_ANON, ---I think it is a 
vicious circle for the kernel.

I think there two places to put back the gup() pages.
<1> isolate_page_glable()
<2> in the shrink_page_list(), before called the try_to_unmap().
KOSAKI Motohiro 's patch takes effect in the second place.
I think the first place is better.








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