mm: introduce offset_in_thp

Mirroring offset_in_page(), this gives you the offset within this
particular page, no matter what size page it is.  It optimises down to
offset_in_page() if CONFIG_TRANSPARENT_HUGEPAGE is not set.

Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Reviewed-by: David Hildenbrand <[email protected]>
Reviewed-by: William Kucharski <[email protected]>
Reviewed-by: Zi Yan <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: "Kirill A. Shutemov" <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

Author: Matthew Wilcox (Oracle)

include/linux/mm.h

@@ -1594,6 +1594,7 @@ static inline void clear_page_pfmemalloc(struct page *page)
 extern void pagefault_out_of_memory(void);
 
 #define offset_in_page(p)	((unsigned long)(p) & ~PAGE_MASK)
+#define offset_in_thp(page, p)	((unsigned long)(p) & (thp_size(page) - 1))
 
 /*
  * Flags passed to show_mem() and show_free_areas() to suppress output in