User space memory

[Prabhu nath]

On Tue, Mar 12, 2013 at 7:59 PM, <Valdis.Kletnieks at vt.edu> 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 at tu-bs.de>
> 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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