mm: define default MAX_PTRS_PER_* in include/pgtable.h

Commit c65e774fb3 ("x86/mm: Make PGDIR_SHIFT and PTRS_PER_P4D variable")
made PTRS_PER_P4D variable on x86 and introduced MAX_PTRS_PER_P4D as a
constant for cases which need a compile-time constant (e.g.  fixed-size
arrays).

powerpc likewise has boot-time selectable MMU features which can cause
other mm "constants" to vary.  For KASAN, we have some static
PTE/PMD/PUD/P4D arrays so we need compile-time maximums for all these
constants.  Extend the MAX_PTRS_PER_ idiom, and place default definitions
in include/pgtable.h.  These define MAX_PTRS_PER_x to be PTRS_PER_x unless
an architecture has defined MAX_PTRS_PER_x in its arch headers.

Clean up pgtable-nop4d.h and s390's MAX_PTRS_PER_P4D definitions while
we're at it: both can just pick up the default now.

Link: https://lkml.kernel.org/r/20210624034050.511391-4-dja@axtens.net
Signed-off-by: Daniel Axtens <dja@axtens.net>
Acked-by: Andrey Konovalov <andreyknvl@gmail.com>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Marco Elver <elver@google.com>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/s390/include/asm/pgtable.h

@@ -344,8 +344,6 @@ static inline int is_module_addr(void *addr)
 #define PTRS_PER_P4D	_CRST_ENTRIES
 #define PTRS_PER_PGD	_CRST_ENTRIES
 
-#define MAX_PTRS_PER_P4D	PTRS_PER_P4D
-
 /*
  * Segment table and region3 table entry encoding
  * (R = read-only, I = invalid, y = young bit):

include/asm-generic/pgtable-nop4d.h

@@ -9,7 +9,6 @@
 typedef struct { pgd_t pgd; } p4d_t;
 
 #define P4D_SHIFT		PGDIR_SHIFT
-#define MAX_PTRS_PER_P4D	1
 #define PTRS_PER_P4D		1
 #define P4D_SIZE		(1UL << P4D_SHIFT)
 #define P4D_MASK		(~(P4D_SIZE-1))

include/linux/pgtable.h

@@ -1592,4 +1592,26 @@ typedef unsigned int pgtbl_mod_mask;
 #define pte_leaf_size(x) PAGE_SIZE
 #endif
 
+/*
+ * Some architectures have MMUs that are configurable or selectable at boot
+ * time. These lead to variable PTRS_PER_x. For statically allocated arrays it
+ * helps to have a static maximum value.
+ */
+
+#ifndef MAX_PTRS_PER_PTE
+#define MAX_PTRS_PER_PTE PTRS_PER_PTE
+#endif
+
+#ifndef MAX_PTRS_PER_PMD
+#define MAX_PTRS_PER_PMD PTRS_PER_PMD
+#endif
+
+#ifndef MAX_PTRS_PER_PUD
+#define MAX_PTRS_PER_PUD PTRS_PER_PUD
+#endif
+
+#ifndef MAX_PTRS_PER_P4D
+#define MAX_PTRS_PER_P4D PTRS_PER_P4D
+#endif
+
 #endif /* _LINUX_PGTABLE_H */