Messages in this thread | | | Date | Tue, 29 Oct 2019 20:19:25 +0100 | From | Jessica Yu <> | Subject | Re: [PATCH 4/7] module: avoid code duplication in include/linux/export.h |
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+++ Rasmus Villemoes [27/09/19 13:07 +0200]: >On 27/09/2019 11.36, Masahiro Yamada wrote: >> include/linux/export.h has lots of code duplication between >> EXPORT_SYMBOL and EXPORT_SYMBOL_NS. >> >> To improve the maintainability and readability, unify the >> implementation. >> >> When the symbol has no namespace, pass the empty string "" to >> the 'ns' parameter. >> >> The drawback of this change is, it grows the code size. >> When the symbol has no namespace, sym->namespace was previously >> NULL, but it is now am empty string "". So, it increases 1 byte >> for every no namespace EXPORT_SYMBOL. >> >> A typical kernel configuration has 10K exported symbols, so it >> increases 10KB in rough estimation. >> >> I did not come up with a good idea to refactor it without increasing >> the code size. > >Can't we put the "aMS" flags on the __ksymtab_strings section? That >would make the empty strings free, and would also deduplicate the >USB_STORAGE string. And while almost per definition we don't have exact >duplicates among the names of exported symbols, we might have both a foo >and __foo, so that could save even more. > >I don't know if we have it already, but we'd need each arch to tell us >what symbol to use for @ in @progbits (e.g. % for arm). It seems most >are fine with @, so maybe a generic version could be > >#ifndef ARCH_SECTION_TYPE_CHAR >#define ARCH_SECTION_TYPE_CHAR "@" >#endif > >and then it would be >section("__ksymtab_strings,\"aMS\","ARCH_SECTION_TYPE_CHAR"progbits,1")
FWIW, I've just tinkered with this, and unfortunately the strings don't get deduplicated for kernel modules :-(
Apparently ld does not do the deduplication for SHF_MERGE|SHF_STRINGS sections for relocatable files (ld -r), which kernel modules are. See:
https://sourceware.org/ml/binutils/2009-07/msg00291.html
But, the strings do get deduplicated for vmlinux. Not sure if we can find a workaround for modules or if the benefit is significant enough if it only for vmlinux.
Thanks,
Jessica
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