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authorJessica Yu <jeyu@kernel.org>2019-11-12 12:35:59 +0100
committerJessica Yu <jeyu@kernel.org>2019-12-16 10:35:33 +0100
commitce2b617ce8cbb7ba7a956299061bbc784131333c (patch)
tree3aa242cf80c4873c72e6549dc801a6416b58c2a5 /include/asm-generic
parentb6d0531ec7e2ae923395a1d5a80edb7ee51ac204 (diff)
export.h: reduce __ksymtab_strings string duplication by using "MS" section flags
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> Reviewed-by: Masahiro Yamada <masahiroy@kernel.org> Reviewed-by: Matthias Maennich <maennich@google.com> Signed-off-by: Jessica Yu <jeyu@kernel.org>
Diffstat (limited to 'include/asm-generic')
-rw-r--r--include/asm-generic/export.h8
1 files changed, 5 insertions, 3 deletions
diff --git a/include/asm-generic/export.h b/include/asm-generic/export.h
index afddc5442e92..365345f9a9e3 100644
--- a/include/asm-generic/export.h
+++ b/include/asm-generic/export.h
@@ -27,9 +27,11 @@
.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
.section ___ksymtab\sec+\name,"a"
@@ -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