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

[Jessica Yu]

+++ Rasmus Villemoes [29/10/19 22:11 +0100]:
> 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/> :)

That is exactly what we need! :)

> > 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.

Yeah, I think it's worth having too.

If you don't have any extra cycles at the moment, and it's far down
your priority list, do you mind if I take a look and maybe try to push
that patch of yours upstream again? I don't know how successful I'd
be, but now since it's especially relevant for namespaces, it's
definitely worth looking at again.

Thanks!

Jessica