User space memory

[Christoph Seitz]

Am 12.03.2013 14:08, schrieb Prabhu nath:
> 
> 
> On Sun, Mar 10, 2013 at 11:30 PM, Christoph Seitz <c.seitz at tu-bs.de
> <mailto:c.seitz at tu-bs.de>> wrote:
> 
>     Hi all,
> 
>     I have some problems allocation Memory the right way and use it in my
>     kernel module.
> 
>     I use a char device for reading and writing from/to a pcie dma card.
>     Especially the read function makes me some headache. The user allocates
>     some memory with posix_memalign and call the read function on the
>     device, so that the devices knows where to write to. My driver now uses
>     get_user_pages() to pin the user pages. The memory has never been
>     written or read by the user, so it's not yet in the RAM, right? And
>     get_user_pages returns a valid number of pages, but for every page the
>     same struct. (respectively the same pointer). Is there any way to ensure
>     that the user pages are in the ram and get_user_pages returns a valid
>     page array? 
> 
> 
> If you know the RAM physical address range you can figure out by doing
> the following
>     *page_to_pfn(page_ptr) << 12*;
>     where page_ptr is a struct page * returned by get_user_pages().
>    *page_to_pfn()* will return the pfn of the corresponding page frame
> and left shifting by 12 bits will give you page frame base address.

Maybe my description was a little bit misleading. I have the problem,
that if I don't write to that allocated memory in the user application,
I won't get any valid pages from get_user_pages. The reason seems to be,
that thees pages never got faulted and so the user memory never gets a
page frame. Is there any chance for a kernel module to force a page
fault or to assign the user memory a page frame?
I found out, if I use the force flag with get_user_pages, the pages get
faulted, but there has to be a nicer way than using the force flag.

Regards,
Chris