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

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Date:   Fri, 27 Sep 2019 13:07:33 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Jessica Yu <jeyu@...nel.org>
Cc:     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 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")

But I don't know if any tooling relies on the strings not being
deduplicated.

Rasmus