ftrace: __start_mcount_loc should be .init.rodata

__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.

Signed-off-by: John Reiser <jreiser@BitWagon.com>
LKML-Reference: <4A6DF0B6.7080402@bitwagon.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 3 insertions, 2 deletions

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index 6ad76bf5fb40..98b37cf3ac6d 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)