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

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Message-ID: <20120206170224.GD23240@phenom.dumpdata.com>
Date:	Mon, 6 Feb 2012 12:02:24 -0500
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Jan Beulich <JBeulich@...e.com>
Cc:	Dan Magenheimer <dan.magenheimer@...cle.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	xen-devel@...ts.xensource.com, linux-kernel@...r.kernel.org
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()).

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?
> 
> 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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