linux-kernel - RE: [PATCH] xen/tmem: cleanup

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Date:	Mon, 06 Feb 2012 16:49:42 +0000
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Dan Magenheimer" <dan.magenheimer@...cle.com>
Cc:	"Jeremy Fitzhardinge" <jeremy@...p.org>,
	<xen-devel@...ts.xensource.com>,
	"Konrad Wilk" <konrad.wilk@...cle.com>,
	<linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] xen/tmem: cleanup

>>> On 06.02.12 at 17:37, Dan Magenheimer <dan.magenheimer@...cle.com> wrote:
>>  From: Jan Beulich [mailto:JBeulich@...e.com]
>> Sent: Monday, February 06, 2012 1:13 AM
>> To: Dan Magenheimer
>> Cc: Jeremy Fitzhardinge; xen-devel@...ts.xensource.com; Konrad Wilk; 
> linux-kernel@...r.kernel.org 
>> Subject: RE: [PATCH] xen/tmem: cleanup
>> 
>> >>> On 04.02.12 at 17:42, Dan Magenheimer <dan.magenheimer@...cle.com> wrote:
>> >>  From: Jan Beulich [mailto:JBeulich@...e.com]
>> >> Subject: [PATCH] xen/tmem: cleanup
>> >>
>> >> Use 'bool' for boolean variables. Do proper section placement.
>> >> Eliminate an unnecessary export.
>> >>
>> >> Signed-off-by: Jan Beulich <jbeulich@...e.com>
>> >> Cc: Dan Magenheimer <dan.magenheimer@...cle.com>
>> >>
>> >> -int tmem_enabled __read_mostly;
>> >> -EXPORT_SYMBOL(tmem_enabled);
>> >> +bool __read_mostly tmem_enabled = false;
>> >
>> > tmem_enabled is used in xen/drivers/xen-selfballoon.c
>> 
>> Which can't be built as a module, and hence the symbol doesn't need
>> exporting. This patch (of course, I'm tempted to say) survived build
>> testing.
> 
> Yes, correct.  BUT... I think only the reason xen-selfballoon.c
> can't be built as a module is because the MM variable vm_committed_as
> (or an access function) is not exported.  Ideally xen-selfballoon.c
> probably should be a module but putting a core MM change in
> the critical path of a Xen-only-related enhancement seemed
> a recipe for sure failure.

No, this isn't the main reason (as you say further down, this could
easily be adjusted for) afaict. The main reason is that
register_xen_selfballooning() is being called from non-modular
code (xen-balloon.c), which could be made a module too, but in
turn has at least one reference that wouldn't be nice to become
an export (balloon_set_new_target()).

Jan

> Konrad, if you (1) disagree entirely, or (2) want to remove the
> tmem_enabled EXPORT_SYMBOL now and add it back later if/when
> the core MM change happens, I'll leave that up to you.
> 
> If (2), the MM change should be added to the minor-tmem-related-
> changes-that-need-to-be-eventually-upstreamed list ;-)
> 
> Dan



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