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

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

Hi

> "
> +/**
> + * 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 .

BUG.

We are talking about it just now.

see the following thread in lkml
	"[RFC][PATCH 0/6] IO pinning(get_user_pages()) vs fork race fix"


but unfortunately, we don't have no painful fix. perhaps you need change
your code...





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