ftrace: __start_mcount_loc should be .init.rodata

ftrace: __start_mcount_loc should be .init.rodata

[John Reiser]

__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)

-- 

Re: ftrace: __start_mcount_loc should be .init.rodata

[Steven Rostedt]

On Mon, 27 Jul 2009, John Reiser wrote:

> __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.

Are you sure about that?

> 
> __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) {		\

Isn't this in __init_rodata? Doesn't this get removed too?

I may be confused by all the uses of "init" in this file.

-- Steve


>  		*(.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)
> 
> -- 
> 
> 

Re: ftrace: __start_mcount_loc should be .init.rodata

[John Reiser]

Steven Rostedt wrote:
> 
> 
> On Mon, 27 Jul 2009, John Reiser wrote:
> 
>> __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.
> 
> Are you sure about that?

Data follows from a direct A<->B comparison.

My .config is same as Fedora Project, except for CONFIG_DEBUG_PAGEALLOC=y.
----- /var/log/messages
ftrace: allocating 18889 entries ...
	    # 18889 * 8 = 151112 bytes (147kiB) for __start_mcount_loc[]
		on 64-bit architecture
-----

With MCOUNT_REC() in original RO_DATA in include/asm-generic/vmlinux.lds.h:
===================================
----- /var/log/messages
kernel: Freeing initrd memory: 3134k freed
kernel: debug: unmapping init memory ffffffff8163d000..ffffffff81783000
	## The only message about unmapping;  length is 0x146000 or 1335296 bytes
kernel: Write protecting the kernel read-only data: 1844k   ## 148kiB more than later
-----

----- System.map
ffffffff8155c850 R __start_mcount_loc  # below unmapped range
ffffffff81581698 R __stop_mcount_loc
-----

----- "readelf --sections vmlinux" has these "\.init" strings:
  [12] __init_rodata     PROGBITS         ffffffff8155c850  0075c850  # _below_ unmapped range
  [15] .init.rodata      PROGBITS         ffffffff815f01c0  007f01c0  # _below_ unmapped range
  [25] .data.init_task   PROGBITS         ffffffff81612000  00a12000
  [28] .init.text        PROGBITS         ffffffff8163d000  00a3d000  # low end of unmapped range
  [29] .init.data        PROGBITS         ffffffff81675620  00a75620
  [30] .init.setup       PROGBITS         ffffffff8175d980  00b5d980
  [31] .initcall.init    PROGBITS         ffffffff8175eca0  00b5eca0
  [33] .x86_cpu_dev.init PROGBITS         ffffffff8175fa00  00b5fa00
  [39] .init.ramfs       PROGBITS         ffffffff81776000  00b76000  # still in unmapped range
   01     .data .init.rodata .data.cacheline_aligned .data.read_mostly
## Note that .init.rodata is in segment 1, with .data
   03     .data.init_task .data.page_aligned .smp_locks .init.text .init.data .init.setup .initcall.init .con_initcall.init .x86_cpu_dev.init .secu
rity_initcall.init .parainstructions .altinstructions .altinstr_replacement .exit.text .init.ramfs .data.percpu .data_nosave .bss
## Note that .init.text and .init.data are in segment 3
-----


With MCOUNT_REC() moved to INIT_DATA as patched to include/asm-generic/vmlinux.lds.h:
==================================
----- /var/log/messages:
kernel: Freeing initrd memory: 3134k freed
kernel: debug: unmapping init memory ffffffff81619000..ffffffff81784000
	## The only message about unmapping;  length is 0x16b000 or 1486848 bytes
kernel: Write protecting the kernel read-only data: 1696k   ## 148kiB less than before
-----

----- System.map
ffffffff816a28d0 T __start_mcount_loc  # within unmapped range
ffffffff816c7718 T __stop_mcount_loc
-----

----- "readelf --sections vmlinux" has these "\.init" strings:
  [14] .init.rodata      PROGBITS         ffffffff815cb1c0  007cb1c0  # _below_ unmapped range
  [24] .data.init_task   PROGBITS         ffffffff815ee000  009ee000
  [27] .init.text        PROGBITS         ffffffff81619000  00a19000  # low end of unmapped range
  [28] .init.data        PROGBITS         ffffffff81651620  00a51620
  [29] .init.setup       PROGBITS         ffffffff8175e7c0  00b5e7c0
  [30] .initcall.init    PROGBITS         ffffffff8175fae0  00b5fae0
  [32] .x86_cpu_dev.init PROGBITS         ffffffff81760840  00b60840
  [38] .init.ramfs       PROGBITS         ffffffff81777000  00b77000  # still in unmapped range
   01     .data .init.rodata .data.cacheline_aligned .data.read_mostly
   03     .data.init_task .data.page_aligned .smp_locks .init.text .init.data .init.setup .initcall.init .con_initcall.init .x86_cpu_dev.init .secu
rity_initcall.init .parainstructions .altinstructions .altinstr_replacement .exit.text .init.ramfs .data.percpu .data_nosave .bss
-----

Perusing the source:
arch/x86/mm/init_64.c: free_init_pages() called from
   ./arch/x86/kernel/alternative.c:		free_init_pages("SMP alternatives",
   ./arch/x86/mm/init_64.c:	free_init_pages("unused kernel memory",
   ./arch/x86/mm/init_64.c:	free_init_pages("initrd memory", start, end);
All other calls also are architecture-specific and in other architectures,
and also are similar to these.


>> @@ -331,7 +332,6 @@
>>  	/* __*init sections */						\
>>  	__init_rodata : AT(ADDR(__init_rodata) - LOAD_OFFSET) {		\
> 
> Isn't this in __init_rodata? Doesn't this get removed too?

In my vmlinux it is _not_ removed; it is with .rodata, not .init.rodata
(and not .init.data), and is outside (below) the removed range.

-- 

Re: ftrace: __start_mcount_loc should be .init.rodata

[Steven Rostedt]

On Mon, 27 Jul 2009, John Reiser wrote:

> Steven Rostedt wrote:
> > 
> > 
> > On Mon, 27 Jul 2009, John Reiser wrote:
> > 
> >> __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.
> > 
> > Are you sure about that?
> 
> Data follows from a direct A<->B comparison.
> 
> My .config is same as Fedora Project, except for CONFIG_DEBUG_PAGEALLOC=y.
> ----- /var/log/messages
> ftrace: allocating 18889 entries ...
> 	    # 18889 * 8 = 151112 bytes (147kiB) for __start_mcount_loc[]
> 		on 64-bit architecture
> -----
> 
> With MCOUNT_REC() in original RO_DATA in include/asm-generic/vmlinux.lds.h:
> ===================================
> ----- /var/log/messages
> kernel: Freeing initrd memory: 3134k freed
> kernel: debug: unmapping init memory ffffffff8163d000..ffffffff81783000
> 	## The only message about unmapping;  length is 0x146000 or 1335296 bytes
> kernel: Write protecting the kernel read-only data: 1844k   ## 148kiB more than later
> -----
> 
> ----- System.map
> ffffffff8155c850 R __start_mcount_loc  # below unmapped range
> ffffffff81581698 R __stop_mcount_loc
> -----
> 
> ----- "readelf --sections vmlinux" has these "\.init" strings:
>   [12] __init_rodata     PROGBITS         ffffffff8155c850  0075c850  # _below_ unmapped range
>   [15] .init.rodata      PROGBITS         ffffffff815f01c0  007f01c0  # _below_ unmapped range
>   [25] .data.init_task   PROGBITS         ffffffff81612000  00a12000
>   [28] .init.text        PROGBITS         ffffffff8163d000  00a3d000  # low end of unmapped range
>   [29] .init.data        PROGBITS         ffffffff81675620  00a75620
>   [30] .init.setup       PROGBITS         ffffffff8175d980  00b5d980
>   [31] .initcall.init    PROGBITS         ffffffff8175eca0  00b5eca0
>   [33] .x86_cpu_dev.init PROGBITS         ffffffff8175fa00  00b5fa00
>   [39] .init.ramfs       PROGBITS         ffffffff81776000  00b76000  # still in unmapped range
>    01     .data .init.rodata .data.cacheline_aligned .data.read_mostly
> ## Note that .init.rodata is in segment 1, with .data
>    03     .data.init_task .data.page_aligned .smp_locks .init.text .init.data .init.setup .initcall.init .con_initcall.init .x86_cpu_dev.init .secu
> rity_initcall.init .parainstructions .altinstructions .altinstr_replacement .exit.text .init.ramfs .data.percpu .data_nosave .bss
> ## Note that .init.text and .init.data are in segment 3
> -----
> 
> 
> With MCOUNT_REC() moved to INIT_DATA as patched to include/asm-generic/vmlinux.lds.h:
> ==================================
> ----- /var/log/messages:
> kernel: Freeing initrd memory: 3134k freed
> kernel: debug: unmapping init memory ffffffff81619000..ffffffff81784000
> 	## The only message about unmapping;  length is 0x16b000 or 1486848 bytes
> kernel: Write protecting the kernel read-only data: 1696k   ## 148kiB less than before
> -----
> 
> ----- System.map
> ffffffff816a28d0 T __start_mcount_loc  # within unmapped range
> ffffffff816c7718 T __stop_mcount_loc
> -----
> 
> ----- "readelf --sections vmlinux" has these "\.init" strings:
>   [14] .init.rodata      PROGBITS         ffffffff815cb1c0  007cb1c0  # _below_ unmapped range
>   [24] .data.init_task   PROGBITS         ffffffff815ee000  009ee000
>   [27] .init.text        PROGBITS         ffffffff81619000  00a19000  # low end of unmapped range
>   [28] .init.data        PROGBITS         ffffffff81651620  00a51620
>   [29] .init.setup       PROGBITS         ffffffff8175e7c0  00b5e7c0
>   [30] .initcall.init    PROGBITS         ffffffff8175fae0  00b5fae0
>   [32] .x86_cpu_dev.init PROGBITS         ffffffff81760840  00b60840
>   [38] .init.ramfs       PROGBITS         ffffffff81777000  00b77000  # still in unmapped range
>    01     .data .init.rodata .data.cacheline_aligned .data.read_mostly
>    03     .data.init_task .data.page_aligned .smp_locks .init.text .init.data .init.setup .initcall.init .con_initcall.init .x86_cpu_dev.init .secu
> rity_initcall.init .parainstructions .altinstructions .altinstr_replacement .exit.text .init.ramfs .data.percpu .data_nosave .bss
> -----
> 
> Perusing the source:
> arch/x86/mm/init_64.c: free_init_pages() called from
>    ./arch/x86/kernel/alternative.c:		free_init_pages("SMP alternatives",
>    ./arch/x86/mm/init_64.c:	free_init_pages("unused kernel memory",
>    ./arch/x86/mm/init_64.c:	free_init_pages("initrd memory", start, end);
> All other calls also are architecture-specific and in other architectures,
> and also are similar to these.
> 
> 
> >> @@ -331,7 +332,6 @@
> >>  	/* __*init sections */						\
> >>  	__init_rodata : AT(ADDR(__init_rodata) - LOAD_OFFSET) {		\
> > 
> > Isn't this in __init_rodata? Doesn't this get removed too?
> 
> In my vmlinux it is _not_ removed; it is with .rodata, not .init.rodata
> (and not .init.data), and is outside (below) the removed range.

Thanks for this detailed explanation, I'll pull your patch in an run it 
through my tests, and then push it off.

-- Steve

[tip:tracing/core] ftrace: __start_mcount_loc should be .init.rodata

[tip-bot for John Reiser]

Commit-ID:  4b3b4c5e64ce26612646867ee354373620063534
Gitweb:     http://git.kernel.org/tip/4b3b4c5e64ce26612646867ee354373620063534
Author:     John Reiser <jreiser@bitwagon.com>
AuthorDate: Mon, 27 Jul 2009 11:23:50 -0700
Committer:  Steven Rostedt <rostedt@goodmis.org>
CommitDate: Sat, 12 Sep 2009 21:57:29 -0400

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>


---
 include/asm-generic/vmlinux.lds.h |    5 +++--
 1 files changed, 3 insertions(+), 2 deletions(-)

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)