Re: User space memory

[Prabhu nath <gprabhunath@xxxxxxxxx>]

On Tue, Mar 12, 2013 at 7:59 PM, <Valdis.Kletnieks@xxxxxx> wrote:

On Tue, 12 Mar 2013 18:38:05 +0530, Prabhu nath said:

> On Sun, Mar 10, 2013 at 11:30 PM, Christoph Seitz <c.seitz@xxxxxxxx> wrote:

> > 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.

Unfortunately, that doesn't actually tell you what Christoph was
worried about - is the page *currently* in RAM?  For that, you need
to check some bits in the pfn once you find it.

Also, note the following:

It's not always 12, because not everything uses a 4K page - consider hugepage
support, or Power and Itanium where the pages are bigger and often several
different sizes are supported.  There's an API for the current page size. Use
it. :)

 Oops.. it should be  page_to_pfn(page_ptr) << PAGE_SHIFT

Also, there's an API for pinning pages so they *stay* in RAM so you can target
them for I/O.  Use that. ;)

-- 
Regards,
Prabhunath G
Linux Trainer
Bangalore
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