Messages in this thread Patch in this message | | | From | Jessica Yu <> | Subject | [PATCH v3] export.h: reduce __ksymtab_strings string duplication by using "MS" section flags | Date | Fri, 6 Dec 2019 13:41:02 +0100 |
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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> --- 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: + * + * - If applicable, place an entry in the __kcrctab section. + * - Put the name of the symbol and namespace (empty string "" for none) in + * __ksymtab_strings. + * - Place an entry in the __ksymtab section. + * + * 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
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