linux-kernel - Re: [PATCH] fix pgd

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Message-ID: <20110222171341.GD31195@random.random>
Date:	Tue, 22 Feb 2011 18:13:41 +0100
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Jan Beulich <JBeulich@...ell.com>,
	Ian Campbell <Ian.Campbell@...rix.com>,
	Andi Kleen <andi@...stfloor.org>,
	Hugh Dickins <hughd@...gle.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Xen-devel@...ts.xensource.com" <Xen-devel@...ts.xensource.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Johannes Weiner <jweiner@...hat.com>,
	Larry Woodman <lwoodman@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] fix pgd_lock deadlock

On Tue, Feb 22, 2011 at 09:08:54AM -0800, Jeremy Fitzhardinge wrote:
> On 02/22/2011 06:34 AM, Andrea Arcangeli wrote:
> > On Tue, Feb 22, 2011 at 02:22:53PM +0000, Jan Beulich wrote:
> >> If everyone's okay with putting a couple of "if (xen_pv_domain())"
> >> into mm/fault.c - sure. I would have thought that this wouldn't be
> >> liked, hence the suggestion to make this depend on seeing the
> >> backlink be non-NULL.
> > I prefer the if (xen_pv_domain) so it also gets optimized away
> > at build time when CONFIG_XEN=n. I think it's also more self
> > explanatory to read.
> 
> Eh, very not keen about that.  I'd only consider it as a last resort.
> 
> In practice CONFIG_XEN is always enabled in distros, so the conditional
> would always be done at runtime.

That's ok, it's still better than setting a page filed IMHO. As the
page flag would be set conditional to xen_pv_domain() anyway. I
wouldn't like to make things more complicated than they already are,
this will make it more explicit why that spinlock is needed too.
Unless we foresee other hypervisors using a generic functionality we
don't need one.
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