ftrace: __start_mcount_loc should be .init.rodata

[John Reiser <jreiser@bitwagon.com>]

__start_mcount_loc[] is unused after init, yet occupies RAM forever
as part of .rodata.  152kiB is typical on a 64-bit architecture.  Instead,
__start_mcount_loc should be in the interval [__init_begin, __init_end)
so that the space is reclaimed after init.

__start_mcount_loc[] is generated during the load portion
of kernel build, and is used only by ftrace_init().  ftrace_init is declared
'__init' and is in .init.text, which is freed after init.
__start_mcount_loc is placed into .rodata by a call to MCOUNT_REC inside
the RO_DATA macro of include/asm-generic/vmlinux.lds.h.  The array *is*
read-only, but more importantly it is not used after init.  So the call to
MCOUNT_REC should be moved from RO_DATA to INIT_DATA.

This patch has been tested on x86_64 with CONFIG_DEBUG_PAGEALLOC=y
which verifies that the address range never is accessed after init.

[Please cc: me if replying only to the linux-kernel list.]

Signed off by: John Reiser <jreiser@BitWagon.com>

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index 6ad76bf..98b37cf 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -91,7 +91,8 @@
 #endif

 #ifdef CONFIG_FTRACE_MCOUNT_RECORD
-#define MCOUNT_REC()	VMLINUX_SYMBOL(__start_mcount_loc) = .; \
+#define MCOUNT_REC()	. = ALIGN(8);				\
+			VMLINUX_SYMBOL(__start_mcount_loc) = .; \
 			*(__mcount_loc)				\
 			VMLINUX_SYMBOL(__stop_mcount_loc) = .;
 #else
@@ -331,7 +332,6 @@
 	/* __*init sections */						\
 	__init_rodata : AT(ADDR(__init_rodata) - LOAD_OFFSET) {		\
 		*(.ref.rodata)						\
-		MCOUNT_REC()						\
 		DEV_KEEP(init.rodata)					\
 		DEV_KEEP(exit.rodata)					\
 		CPU_KEEP(init.rodata)					\
@@ -455,6 +455,7 @@
 	MEM_DISCARD(init.data)						\
 	KERNEL_CTORS()							\
 	*(.init.rodata)							\
+	MCOUNT_REC()							\
 	DEV_DISCARD(init.rodata)					\
 	CPU_DISCARD(init.rodata)					\
 	MEM_DISCARD(init.rodata)

--