Re: [RFC 5/7] LTTng instrumentation mm

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* 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

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68

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