asm-generic: add an optional pfn_valid check to page_to_phys

page_to_pfn is usually implemented by pointer arithmetics on the memory
map, which means that bogus input can lead to even more bogus output.

Powerpc had a pfn_valid check on the intermediate pfn in the page_to_phys
implementation when CONFIG_DEBUG_VIRTUAL is defined, which seems
generally useful, so add that to the generic version.

Signed-off-by: Christoph Hellwig <[e-mail]>
Reviewed-by: Thomas Huth <[e-mail]>
Signed-off-by: Arnd Bergmann <[e-mail]>

include/asm-generic/memory_model.h

···
 #define page_to_pfn __page_to_pfn
 #define pfn_to_page __pfn_to_page
 
+#ifdef CONFIG_DEBUG_VIRTUAL
+#define page_to_phys(page)						\
+({									\
+	unsigned long __pfn = page_to_pfn(page);			\
+									\
+	WARN_ON_ONCE(!pfn_valid(__pfn));				\
+	PFN_PHYS(__pfn);						\
+})
+#else
 #define page_to_phys(page)	PFN_PHYS(page_to_pfn(page))
+#endif /* CONFIG_DEBUG_VIRTUAL */
 #define phys_to_page(phys)	pfn_to_page(PHYS_PFN(phys))
 
 #endif /* __ASSEMBLY__ */