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

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Date:	Mon, 6 Feb 2012 09:22:26 -0800 (PST)
From:	Dan Magenheimer <dan.magenheimer@...cle.com>
To:	Konrad Wilk <konrad.wilk@...cle.com>,
	Jan Beulich <JBeulich@...e.com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	xen-devel@...ts.xensource.com, linux-kernel@...r.kernel.org
Subject: RE: [PATCH] xen/tmem: cleanup

> From: Konrad Rzeszutek Wilk
> Subject: Re: [PATCH] xen/tmem: cleanup
> 
> On Mon, Feb 06, 2012 at 04:49:42PM +0000, Jan Beulich wrote:
> > >>> 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, you're right, I had forgotten about that "chained" dependency.
Thanks and FWIW, on the entire original patch:

Acked-by: Dan Magenheimer <dan.magenheimer@...cle.com>
 
> It would be nice if all of it could become modules. That way HVM
> device driver domains could load the whole thing without having much
> built-in code in the kernel.
> 
> Is it possible to do that?

Konrad, my module expertise is very low, so I will leave that
to Jan or someone else to answer or look into.

Dan
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