linux-kernel - Re: [PATCH 4/7] module: avoid code duplication in include/linux/export.h

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Message-ID: <a2e6bdc2-3d35-a3dc-13ef-1ce32f77ef17@rasmusvillemoes.dk>
Date:   Tue, 29 Oct 2019 22:11:45 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Jessica Yu <jeyu@...nel.org>
Cc:     Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Matthias Maennich <maennich@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Martijn Coenen <maco@...roid.com>,
        Will Deacon <will.deacon@....com>,
        Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/7] module: avoid code duplication in
 include/linux/export.h

On 29/10/2019 20.19, Jessica Yu wrote:
> +++ Rasmus Villemoes [27/09/19 13:07 +0200]:
>> On 27/09/2019 11.36, Masahiro Yamada wrote:
>>>
>>> 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

I know <https://patches-gcc.linaro.org/patch/5858/> :)

> 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.

I think it's definitely worth if, even if it "only" benefits vmlinux for
now. And I still hope to revisit the --force-section-merge some day, but
it's very far down my priority list.

Rasmus