Re: [PATCH v3] export.h: reduce __ksymtab_strings string duplication by using "MS" section flags

[Masahiro Yamada]

On Fri, Dec 6, 2019 at 9:41 PM Jessica Yu <jeyu@xxxxxxxxxx> wrote:
>
> 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>
> ---
> v3:
>   - remove __KSTRTAB_ENTRY macros in favor of just putting the asm directly
>     in ___EXPORT_SYMBOL
>   - Document more clearly what the ___EXPORT_SYMBOL macro does
>
>  include/asm-generic/export.h |  8 +++++---
>  include/linux/export.h       | 27 ++++++++++++++++++++-------
>  2 files changed, 25 insertions(+), 10 deletions(-)
>
> diff --git a/include/asm-generic/export.h b/include/asm-generic/export.h
> index fa577978fbbd..23bc98e97a66 100644
> --- a/include/asm-generic/export.h
> +++ b/include/asm-generic/export.h
> @@ -26,9 +26,11 @@
>  .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 since usage of the "M" (SHF_MERGE)
> + * section flag requires it. Use '%progbits' instead of '@progbits' since the
> + * former apparently works on all arches according to the binutils source.
>   */
> +
>  .macro ___EXPORT_SYMBOL name,val,sec
>  #ifdef CONFIG_MODULES
>         .globl __ksymtab_\name
> @@ -37,7 +39,7 @@
>  __ksymtab_\name:
>         __put \val, __kstrtab_\name
>         .previous
> -       .section __ksymtab_strings,"a"
> +       .section __ksymtab_strings,"aMS",%progbits,1
>  __kstrtab_\name:
>         .asciz "\name"
>         .previous
> diff --git a/include/linux/export.h b/include/linux/export.h
> index 201262793369..18dcdcd118e7 100644
> --- a/include/linux/export.h
> +++ b/include/linux/export.h
> @@ -81,16 +81,29 @@ struct kernel_symbol {
>
>  #else
>
> -/* For every exported symbol, place a struct in the __ksymtab section */
> +/*
> + * For every exported symbol, do the following:

Just a nit.

You mention "an entry" twice
to talk about different classes of structures.

It might be better to be explicit about "which entry".


> + *
> + * - If applicable, place an entry in the __kcrctab section.

  "place a CRC entry in the __kcrctab section"
might be clearer ?


> + * - Put the name of the symbol and namespace (empty string "" for none) in
> + *   __ksymtab_strings.
> + * - Place an entry in the __ksymtab section.

 "Place a struct kernel_symbol entry in __ksymtab section"   ?
might be clearer ?


Other than that:
Reviewed-by: Masahiro Yamada <masahiroy@xxxxxxxxxx>


> + *
> + * note on .section use: we specify progbits since usage of the "M" (SHF_MERGE)
> + * section flag requires it. Use '%progbits' instead of '@progbits' since the
> + * former apparently works on all arches according to the binutils source.
> + */
>  #define ___EXPORT_SYMBOL(sym, sec, ns)                                 \
>         extern typeof(sym) sym;                                         \
> +       extern const char __kstrtab_##sym[];                            \
> +       extern const char __kstrtabns_##sym[];                          \
>         __CRC_SYMBOL(sym, sec);                                         \
> -       static const char __kstrtab_##sym[]                             \
> -       __attribute__((section("__ksymtab_strings"), used, aligned(1))) \
> -       = #sym;                                                         \
> -       static const char __kstrtabns_##sym[]                           \
> -       __attribute__((section("__ksymtab_strings"), used, aligned(1))) \
> -       = ns;                                                           \
> +       asm("   .section \"__ksymtab_strings\",\"aMS\",%progbits,1\n"   \
> +           "__kstrtab_" #sym ":                                \n"     \
> +           "   .asciz  \"" #sym "\"                            \n"     \
> +           "__kstrtabns_" #sym ":                              \n"     \
> +           "   .asciz  \"" ns "\"                              \n"     \
> +           "   .previous                                       \n");   \
>         __KSYMTAB_ENTRY(sym, sec)
>
>  #endif
> --
> 2.16.4
>


-- 
Best Regards
Masahiro Yamada