mm: Introduce pXX_leaf_size()

A number of architectures have non-pagetable aligned huge/large pages.
For such architectures a leaf can actually be part of a larger entry.

Provide generic helpers to determine the size of a page-table leaf.

Signed-off-by: Peter Zijlstra (Intel) <[e-mail]>
Reviewed-by: Matthew Wilcox (Oracle) <[e-mail]>
Link: https://lkml.kernel.org/r/[e-mail]

include/linux/pgtable.h

···
 #define pmd_leaf(x)	0
 #endif
 
+#ifndef pgd_leaf_size
+#define pgd_leaf_size(x) (1ULL << PGDIR_SHIFT)
+#endif
+#ifndef p4d_leaf_size
+#define p4d_leaf_size(x) P4D_SIZE
+#endif
+#ifndef pud_leaf_size
+#define pud_leaf_size(x) PUD_SIZE
+#endif
+#ifndef pmd_leaf_size
+#define pmd_leaf_size(x) PMD_SIZE
+#endif
+#ifndef pte_leaf_size
+#define pte_leaf_size(x) PAGE_SIZE
+#endif
+
 #endif /* _LINUX_PGTABLE_H */