Re: [PATCH] export.h: reduce __ksymtab_strings string duplication by using "MS" section flags

[Jessica Yu]

+++ Jessica Yu [20/11/19 15:51 +0100]:
> Commit c3a6cf19e695 ("export: avoid code duplication in
> include/linux/export.h") refactors export.h quite nicely, but introduces
> a slight increase in memory usage due to using the empty string ""
> instead of NULL to indicate that an exported symbol has no namespace. As
> mentioned in that commit, this meant an increase of 1 byte per exported
> symbol without a namespace. For example, if a kernel configuration has
> about 10k exported symbols, this would mean that the size of
> __ksymtab_strings would increase by roughly 10kB.
>
> We can alleviate this situation by utilizing the SHF_MERGE and
> SHF_STRING section flags. SHF_MERGE|SHF_STRING indicate to the linker
> that the data in the section are null-terminated strings that can be
> merged to eliminate duplication. More specifically, from the binutils
> documentation - "for sections with both M and S, a string which is a
> suffix of a larger string is considered a duplicate. Thus "def" will be
> merged with "abcdef"; A reference to the first "def" will be changed to
> a reference to "abcdef"+3". Thus, all the empty strings would be merged
> as well as any strings that can be merged according to the cited method
> above. For example, "memset" and "__memset" would be merged to just
> "__memset" in __ksymtab_strings.
>
> As of v5.4-rc5, the following statistics were gathered with x86
> defconfig with approximately 10.7k exported symbols.
>
> Size of __ksymtab_strings in vmlinux:
> -------------------------------------
> v5.4-rc5: 213834 bytes
> v5.4-rc5 with commit c3a6cf19e695: 224455 bytes
> v5.4-rc5 with this patch: 205759 bytes
>
> So, we already see memory savings of ~8kB compared to vanilla -rc5 and
> savings of nearly 18.7kB compared to -rc5 with commit c3a6cf19e695 on top.
>
> Unfortunately, as of this writing, strings will not get deduplicated for
> kernel modules, as ld does not do the deduplication for
> SHF_MERGE|SHF_STRINGS sections for relocatable files (ld -r), which
> kernel modules are. A patch for ld is currently being worked on to
> hopefully allow for string deduplication in relocatable files in the
> future.
>
> Suggested-by: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Jessica Yu <jeyu@xxxxxxxxxx>
> ---
> include/asm-generic/export.h | 13 ++++++++++---
> include/linux/export.h       | 28 ++++++++++++++++++++++------
> 2 files changed, 32 insertions(+), 9 deletions(-)
>
> diff --git a/include/asm-generic/export.h b/include/asm-generic/export.h
> index fa577978fbbd..d0704f2602f4 100644
> --- a/include/asm-generic/export.h
> +++ b/include/asm-generic/export.h
> @@ -26,9 +26,16 @@
> .endm
>
> /*
> - * note on .section use: @progbits vs %progbits nastiness doesn't matter,
> - * since we immediately emit into those sections anyway.
> + * note on .section use: we specify @progbits vs %progbits since usage of
> + * "M" (SHF_MERGE) section flag requires it.
>  */
> +
> +#ifdef CONFIG_ARM
> +#define ARCH_PROGBITS %progbits
> +#else
> +#define ARCH_PROGBITS @progbits
> +#endif
> +
> .macro ___EXPORT_SYMBOL name,val,sec
> #ifdef CONFIG_MODULES
> 	.globl __ksymtab_\name
> @@ -37,7 +44,7 @@
> __ksymtab_\name:
> 	__put \val, __kstrtab_\name
> 	.previous
> -	.section __ksymtab_strings,"a"
> +	.section __ksymtab_strings,"aMS",ARCH_PROGBITS,1
> __kstrtab_\name:
> 	.asciz "\name"
> 	.previous
> diff --git a/include/linux/export.h b/include/linux/export.h
> index 201262793369..ab325a8e6bee 100644
> --- a/include/linux/export.h
> +++ b/include/linux/export.h
> @@ -81,16 +81,32 @@ struct kernel_symbol {
>
> #else
>
> +#ifdef CONFIG_ARM
> +#define ARCH_PROGBITS "%progbits"
> +#else
> +#define ARCH_PROGBITS "@progbits"
> +#endif

Just a comment, I don't like the duplication of this block, but I
wasn't sure where was the best place to have this define since
asm-generic/export.h and linux/export.h don't share includes, and
includes are generally kept to a minimum. If anyone has a better
idea, please let me know.