On Wed, 08 Jun 2011, Dave Hansen wrote:
> On Wed, 2011-06-08 at 15:18 -0400, Eric B Munson wrote:
> > -#define __pfn_to_page(pfn) \
> > -({ unsigned long __pfn = (pfn); \
> > - struct mem_section *__sec = __pfn_to_section(__pfn); \
> > - __section_mem_map_addr(__sec) + __pfn; \
> > +#ifdef CONFIG_DEBUG_MEMORY_MODEL
> > +#define __pfn_to_page(pfn) \
> > +({ unsigned long __pfn = (pfn); \
> > + struct mem_section *__sec = __pfn_to_section(__pfn); \
> > + struct page *__page = __section_mem_map_addr(__sec) + __pfn; \
> > + WARN_ON(__page->flags == 0); \
> > + __page; \
>
> What was the scenario you're trying to catch here? If you give a really
> crummy __pfn, you'll probably go off the end of one of the mem_section[]
> arrays, and get garbage back for __sec. You might also get a NULL back
> from __section_mem_map_addr() if the section is possibly valid, but just
> not present on this particular system.
>
> I _think_ the only kind of bug this will catch is if you have a valid
> section, with a valid section_mem_map[] but still manage to find
> yourself with an 'struct page' unclaimed by any zone and thus
> uninitialized.
This is the case I was going after. I will rework for a V2 based on the
feedback here.
>
> You could catch a lot more cases by being a bit more paranoid:
>
> void check_pfn(unsigned long pfn)
> {
> int nid;
>
> // hacked in from pfn_to_nid:
> // Don't actually do this, add a new helper near pfn_to_nid()
> // Can this even fit in the physnode_map?
> if (pfn / PAGES_PER_ELEMENT > ARRAY_SIZE(physnode_map))
> WARN();
>
> // Is there a valid nid there?
> nid = pfn_to_nid(pfn);
> if (nid == -1)
> WARN();
>
> // check against NODE_DATA(nid)->node_start_pfn;
> // check against NODE_DATA(nid)->node_spanned_pages;
> }
> > })
> > +#else
> > +#define __pfn_to_page(pfn) \
> > +({ unsigned long __pfn = (pfn); \
> > + struct mem_section *__sec = __pfn_to_section(__pfn); \
> > + __section_mem_map_addr(__sec) + __pfn; \
> > +})
> > +#endif /* CONFIG_DEBUG_MEMORY_MODEL */
>
> Instead of making a completely new __pfn_to_page() in the debugging
> case, I'd probably do something like this:
>
> #ifdef CONFIG_DEBUG_MEMORY_MODEL
> #define check_foo(foo) {\
> some_check_here(foo);\
> WARN_ON(foo->flags);\
> }
> #else
> #define check_foo(foo) do{}while(0)
> #endif;
>
> #define __pfn_to_page(pfn) \
> ({ unsigned long __pfn = (pfn); \
> struct mem_section *__sec = __pfn_to_section(__pfn); \
> struct page *__page = __section_mem_map_addr(__sec) + __pfn; \
> check_foo(page) \
> __page; \
> })
>
> That'll make sure that the two copies of __pfn_to_page() don't
> accidentally diverge. It also makes it a lot easier to read, I think.
>
> -- Dave
>
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