linux-kernel - Re: [PATCH] mm/memblock: discard .text/.data if CONFIG_ARCH_KEEP

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Message-ID: <5f9e5d19-8a38-4e98-8cbb-e5501c76f740@arm.com>
Date: Mon, 6 May 2024 15:30:54 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: Wei Yang <richard.weiyang@...il.com>, arnd@...db.de, rppt@...nel.org
Cc: linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Subject: Re: [PATCH] mm/memblock: discard .text/.data if
 CONFIG_ARCH_KEEP_MEMBLOCK not set


On 5/6/24 06:51, Wei Yang wrote:
> When CONFIG_ARCH_KEEP_MEMBLOCK not set, we expect to discard related
> code and data. But it doesn't until CONFIG_MEMORY_HOTPLUG not set
> neither.

When CONFIG_ARCH_KEEP_MEMBLOCK is not set memblock information both for
normal and reserved memory get freed up but should the memblock related
code and data also be freed up as well ? Then I would also believe such
memory saving will be very minimal given CONFIG_ARCH_KEEP_MEMBLOCK code
is too limited scoped in the tree.

Also could you please explain how it is related to CONFIG_MEMORY_HOTPLUG
config being set or not.

> 
> This patch puts memblock's .text/.data into its own section, so that it
> only depends on CONFIG_ARCH_KEEP_MEMBLOCK to discard related code and
> data. After this, init size increase from 2420K to 2432K.

Is not this memory size saving some what insignificant to warrant a code
change ? Also is this problem applicable only to CONFIG_ARCH_KEEP_MEMBLOCK
config. Could you also provide details on how did you measure these numbers ?

> 
> Signed-off-by: Wei Yang <richard.weiyang@...il.com>
> ---
>  include/asm-generic/vmlinux.lds.h | 14 +++++++++++++-
>  include/linux/memblock.h          |  8 ++++----
>  2 files changed, 17 insertions(+), 5 deletions(-)
> 
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index f7749d0f2562..775c5eedb9e6 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -147,6 +147,14 @@
>  #define MEM_DISCARD(sec) *(.mem##sec)
>  #endif
>  
> +#if defined(CONFIG_ARCH_KEEP_MEMBLOCK)
> +#define MEMBLOCK_KEEP(sec)    *(.mb##sec)
> +#define MEMBLOCK_DISCARD(sec)
> +#else
> +#define MEMBLOCK_KEEP(sec)
> +#define MEMBLOCK_DISCARD(sec) *(.mb##sec)
> +#endif
> +
>  #ifndef CONFIG_HAVE_DYNAMIC_FTRACE_NO_PATCHABLE
>  #define KEEP_PATCHABLE		KEEP(*(__patchable_function_entries))
>  #define PATCHABLE_DISCARDS
> @@ -356,6 +364,7 @@
>  	*(.ref.data)							\
>  	*(.data..shared_aligned) /* percpu related */			\
>  	MEM_KEEP(init.data*)						\
> +	MEMBLOCK_KEEP(init.data*)					\
>  	*(.data.unlikely)						\
>  	__start_once = .;						\
>  	*(.data.once)							\
> @@ -573,6 +582,7 @@
>  		*(.ref.text)						\
>  		*(.text.asan.* .text.tsan.*)				\
>  	MEM_KEEP(init.text*)						\
> +	MEMBLOCK_KEEP(init.text*)					\
>  
>  
>  /* sched.text is aling to function alignment to secure we have same
> @@ -680,6 +690,7 @@
>  	KEEP(*(SORT(___kentry+*)))					\
>  	*(.init.data .init.data.*)					\
>  	MEM_DISCARD(init.data*)						\
> +	MEMBLOCK_DISCARD(init.data*)					\
>  	KERNEL_CTORS()							\
>  	MCOUNT_REC()							\
>  	*(.init.rodata .init.rodata.*)					\
> @@ -706,7 +717,8 @@
>  #define INIT_TEXT							\
>  	*(.init.text .init.text.*)					\
>  	*(.text.startup)						\
> -	MEM_DISCARD(init.text*)
> +	MEM_DISCARD(init.text*)						\
> +	MEMBLOCK_DISCARD(init.text*)
>  
>  #define EXIT_DATA							\
>  	*(.exit.data .exit.data.*)					\
> diff --git a/include/linux/memblock.h b/include/linux/memblock.h
> index e2082240586d..3e1f1d42dde7 100644
> --- a/include/linux/memblock.h
> +++ b/include/linux/memblock.h
> @@ -100,13 +100,13 @@ struct memblock {
>  
>  extern struct memblock memblock;
>  
> +#define __init_memblock        __section(".mbinit.text") __cold notrace \
> +						  __latent_entropy
> +#define __initdata_memblock    __section(".mbinit.data")
> +
>  #ifndef CONFIG_ARCH_KEEP_MEMBLOCK
> -#define __init_memblock __meminit
> -#define __initdata_memblock __meminitdata
>  void memblock_discard(void);
>  #else
> -#define __init_memblock
> -#define __initdata_memblock
>  static inline void memblock_discard(void) {}
>  #endif
>