Re: [PATCH v2] export.h: reduce __ksymtab_strings string duplication by using "MS" section flags

[Jessica Yu <jeyu@kernel.org>]

+++ Masahiro Yamada [26/11/19 17:32 +0900]:
>On Tue, Nov 26, 2019 at 12:42 AM Jessica Yu <jeyu@kernel.org> 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@rasmusvillemoes.dk>
>> Signed-off-by: Jessica Yu <jeyu@kernel.org>
>> ---
>>
>> v2: use %progbits throughout and document the oddity in a comment.
>>
>>  include/asm-generic/export.h |  8 +++++---
>>  include/linux/export.h       | 27 +++++++++++++++++++++------
>>  2 files changed, 26 insertions(+), 9 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..3d835ca34d33 100644
>> --- a/include/linux/export.h
>> +++ b/include/linux/export.h
>> @@ -81,16 +81,31 @@ struct kernel_symbol {
>>
>>  #else
>>
>> +/*
>> + * 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 __KSTRTAB_ENTRY(sym)                                                   \
>> +       asm("   .section \"__ksymtab_strings\",\"aMS\",%progbits,1      \n"     \
>> +           "__kstrtab_" #sym ":                                        \n"     \
>> +           "   .asciz  \"" #sym "\"                                    \n"     \
>> +           "   .previous                                               \n")
>> +
>> +#define __KSTRTAB_NS_ENTRY(sym, ns)                                            \
>> +       asm("   .section \"__ksymtab_strings\",\"aMS\",%progbits,1      \n"     \
>> +           "__kstrtabns_" #sym ":                                      \n"     \
>> +           "   .asciz  " #ns "                                         \n"     \
>
>
>Hmm, it took some time for me to how this code works.
>
>ns is already a C string, then you added # to it,
>then I was confused.
>
>Personally, I prefer this code:
>" .asciz \"" ns "\" \n"
>
>so it looks in the same way as __KSTRTAB_ENTRY().
>
>
>
>BTW, you duplicated \"aMS\",%progbits,1" and ".previous"
>
>
>I would write it shorter, like this:
>
>
>#define ___EXPORT_SYMBOL(sym, sec, ns) \
>        extern typeof(sym) sym; \
>        extern const char __kstrtab_##sym[]; \
>        extern const char __kstrtabns_##sym[]; \
>        __CRC_SYMBOL(sym, sec); \
>        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)
>

Sure, I can change that. Just thought it'd be easier to read with the
separate macros. Thanks for your comments!