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

[Huang Shijie <shijie8@gmail.com>]

   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();
   .......................

   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 .


 






 

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