Re: [RFC 5/7] LTTng instrumentation mm

Mathieu Desnoyers
2007-11-16T14:47:55Z

* Dave Hansen (haveblue@us.ibm.com) wrote:
> On Thu, 2007-11-15 at 16:51 -0500, Mathieu Desnoyers wrote:
> > * Dave Hansen (haveblue@us.ibm.com) wrote:
> > > > On Tue, 2007-11-13 at 14:33 -0500, Mathieu Desnoyers wrote:
> > > > linux-2.6-lttng/mm/page_io.c 2007-11-13 09:49:35.000000000 -0500
> > > > @@ -114,6 +114,7 @@ int swap_writepage(struct page *page, st
> > > > rw |= (1 << BIO_RW_SYNC);
> > > > count_vm_event(PSWPOUT);
> > > > set_page_writeback(page);
> > > > + trace_mark(mm_swap_out, "address %p", page_address(page));
> > > > unlock_page(page);
> > > > submit_bio(rw, bio);
> > > > out:
> > >
> > > I'm not sure all this page_address() stuff makes any sense on highmem
> > > systems. How about page_to_pfn()?
> >
> > Knowing which page frame number has been swapped out is not always as
> > relevant as knowing the page's virtual address (when it has one). Saving
> > both the PFN and the page's virtual address could give us useful
> > information when the page is not mapped.
>
> 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 ?

Mathieu