Re: [PATCH v4] export.h: reduce __ksymtab_strings string duplication by using "MS" section flags

[Matthias Maennich <maennich@google.com>]

On Thu, Dec 12, 2019 at 03:16:13PM +0100, Jessica Yu 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>
>Reviewed-by: Masahiro Yamada <masahiroy@kernel.org>
>Signed-off-by: Jessica Yu <jeyu@kernel.org>
>---
>v4:
>  - fix the comment above ___EXPORT_SYMBOL to be more specific about what
>    entries are being placed in their respective sections.
>
> 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 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
>diff --git a/include/linux/export.h b/include/linux/export.h
>index 627841448293..c166d35e3d76 100644
>--- a/include/linux/export.h
>+++ b/include/linux/export.h
>@@ -82,16 +82,29 @@ 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 a CRC entry in the __kcrctab section.
>+ * - Put the name of the symbol and namespace (empty string "" for none) in
>+ *   __ksymtab_strings.
>+ * - Place a struct kernel_symbol 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");	\

nit: You might want to align the newline characters up to the asm line.

Thanks for working on this!

Reviewed-by: Matthias Maennich <maennich@google.com>

Cheers,
Matthias


> 	__KSYMTAB_ENTRY(sym, sec)
>
> #endif
>-- 
>2.16.4
>