linux-kernel - Re: [PATCH] modules: Take a shortcut for checking if an address is in a module

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Date:	Thu, 12 Jun 2008 10:20:58 +1000
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	mingo@...e.hu, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] modules: Take a shortcut for checking if an address is in a module

On Thursday 12 June 2008 01:18:47 Arjan van de Ven wrote:
> On Wed, 11 Jun 2008 21:12:08 +1000
>
> Rusty Russell <rusty@...tcorp.com.au> wrote:
> > On Wednesday 11 June 2008 06:05:19 Arjan van de Ven wrote:
> > > From: Arjan van de Ven <arjan@...ux.intel.com>
> > > Subject: [PATCH] modules: Take a shortcut for checking if an
> > > address is in a module
> > >
> > > Various pieces of the kernel (lockdep, latencytop, etc) tend to
> > > store backtraces, sometimes at a relatively high frequency. In
> > > itself this isn't a big performance deal (after all you're using
> > > diagnostics features), but there have been some complaints from
> > > people who have over 100 modules loaded that this is a tad too slow.
> >
> > Nothing wrong with the idea, but how about a nice
> > arch_maybe_module_addr() macro rather than ifdefs in module.c?
>
> Everytime someone says that, I go crawl under my desk and weep quietly
> for 10 minutes.
>
> Yes ifdefs aren't nice. However, arch_foo_bar_baz() is equally bad, it
> just moves the uglyness around a little. It's like multiplying the dirt
> by three and then sweeping it under 3 different carpets. Sure in your
> area the room looks clean, but the total amount of crap didn't decrease.
>
> The only half-way sane way to do arch_has_foo is using weak functions,
> but unfortunately that runs into gcc issues to the point of not being
> usable without a great amount of care... the alternatives are both
> unpleasant and overkill. We really should try to keep the arch stuff as
> small as possible; a gazillion arch_foo_bar()'s is the opposite
> direction there ;(

OK, then how about something like this in linux/module.h:

#ifndef MODULE_ADDR_MIN
#define MODULE_ADDR_MIN OUL
#endif
#ifndef MODULE_ADDR_MAX
#define MODULE_ADDR_MAX -1UL
#endif

Or, we could actually make those as variables and update them in module.c 
itself.  No arch changes required, and pretty easy to understand.

Cheers,
Rusty.
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