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

[Minchan Kim <minchan.kim@gmail.com>]

On Mon, 20 Apr 2009 10:15:55 +0800
Huang Shijie <shijie8@gmail.com> wrote:

> 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.'

I can't understand your point. 
Sorry for that. 

Could you explain more detail relation (user buffer which is allocated by posix_memalign) and (kernel buffer which is mapped by vmap) ?

> >>    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()

One more, try_to_unmap will call page_cache_release. 
So, count is 2. 

> 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.
>

I can't understand your point exactly yet.
But what I mean is following as in user mode

posix_memalignq(&buffer);
mlock(buffer,  buffer_len); 

I will not dirty your driver. 
Do I miss something ?

> 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.
> 
> 
> 
> 
> 
> 
> 
> 


-- 
Kinds Regards
Minchan Kim

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