Re: [RFC 5/7] LTTng instrumentation mm

Dave Hansen
2007-11-19T18:07:30Z

On Fri, 2007-11-16 at 09:47 -0500, Mathieu Desnoyers wrote:
> * Dave Hansen (haveblue@us.ibm.com) wrote:
> > For most (all?) architectures, the PFN and the virtual address in the
> > kernel's linear are interchangeable with pretty trivial arithmetic. All
> > pages have a pfn, but not all have a virtual address. Thus, I suggested
> > using the pfn. What kind of virtual addresses are you talking about?
> >
>
> Hrm, in asm-generic/memory_model.h, we have various versions of
> __page_to_pfn. Normally they all cast the result to (unsigned long),
> except for :
>
>
> #elif defined(CONFIG_SPARSEMEM_VMEMMAP)
>
> /* memmap is virtually contigious. */
> #define __pfn_to_page(pfn) (vmemmap + (pfn))
> #define __page_to_pfn(page) ((page) - vmemmap)
>
> So I guess the result is a pointer ? Should this be expected ?

Nope. 'pointer - pointer' is an integer. Just solve this equation for
integer:

'pointer + integer = pointer'

-- Dave