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Date: Tue, 7 May 2024 08:16:47 +0000
From: Wei Yang <richard.weiyang@gmail.com>
To: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Wei Yang <richard.weiyang@gmail.com>, arnd@arndb.de, rppt@kernel.org,
	linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-mm@kvack.org
Subject: Re: [PATCH] mm/memblock: discard .text/.data if
 CONFIG_ARCH_KEEP_MEMBLOCK not set
Message-ID: <20240507081647.2x2l7fwjnyiud6ee@master>
Reply-To: Wei Yang <richard.weiyang@gmail.com>
References: <20240506012104.10864-1-richard.weiyang@gmail.com>
 <5f9e5d19-8a38-4e98-8cbb-e5501c76f740@arm.com>
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On Mon, May 06, 2024 at 03:30:54PM +0530, Anshuman Khandual wrote:
>
>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

If not freed, those functions would access unpredictable area.

>memory saving will be very minimal given CONFIG_ARCH_KEEP_MEMBLOCK code
>is too limited scoped in the tree.

Not very much, it shows 12K more in it.

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

This is in file include/asm-generic/vmlinux.lds.h.

MEM_KEEP/MEM_DISCARD is conditionally defined by CONFIG_MEMORY_HOTPLUG.
So even __init_memblock is defined as __meminit when CONFIG_ARCH_KEEP_MEMBLOCK
not set, it is not discarded.
                    
>>                  
>> 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

Yes, this is not significant.

>config. Could you also provide details on how did you measure these numbers ?
>

Kernel print related info in mem_init_print_info(). One of is it
init size, which includes init_data and init_text.

>> 
>> Signed-off-by: Wei Yang <richard.weiyang@gmail.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
>>  

-- 
Wei Yang
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