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    SubjectRe: [PATCH 4/7] module: avoid code duplication in include/linux/export.h
    +++ 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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