linux-kernel - Re: [PATCH v7 1/4] mm: introduce compaction and migration for virtio ballooned pages

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Message-ID: <20120815085218.GG4052@csn.ul.ie>
Date:	Wed, 15 Aug 2012 09:52:18 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Rafael Aquini <aquini@...hat.com>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	Rik van Riel <riel@...hat.com>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Minchan Kim <minchan@...nel.org>
Subject: Re: [PATCH v7 1/4] mm: introduce compaction and migration for virtio
 ballooned pages

On Tue, Aug 14, 2012 at 05:00:49PM -0300, Rafael Aquini wrote:
> On Tue, Aug 14, 2012 at 10:35:25PM +0300, Michael S. Tsirkin wrote:
> > > > > +/* __isolate_lru_page() counterpart for a ballooned page */
> > > > > +bool isolate_balloon_page(struct page *page)
> > > > > +{
> > > > > +	if (WARN_ON(!movable_balloon_page(page)))
> > > > 
> > > > Looks like this actually can happen if the page is leaked
> > > > between previous movable_balloon_page and here.
> > > > 
> > > > > +		return false;
> > > 
> > > Yes, it surely can happen, and it does not harm to catch it here, print a warn and
> > > return.
> > 
> > If it is legal, why warn? For that matter why test here at all?
> >
> 
> As this is a public symbol, and despite the usage we introduce is sane, the warn
> was placed as an insurance policy to let us know about any insane attempt to use
> the procedure in the future. That was due to a nice review nitpick, actually.
> 
> Even though the code already had a test to properly avoid this race you
> mention, I thought that sustaining the warn was a good thing. As I told you,
> despite real, I've never got (un)lucky enough to stumble across that race window
> while testing the patch.
> 
> If your concern is about being too much verbose on logging, under certain
> conditions, perhaps we can change that test to a WARN_ON_ONCE() ?
> 
> Mel, what are your thoughts here?
>  

I viewed it as being defensive programming. VM_BUG_ON would be less
useful as it can be compiled out. If the race can be routinely hit then
multiple warnings is instructive in itself. I have no strong feelings
about this though. I see little harm in making the check but in light of
this conversation add a short comment explaining that the check should
be redundant.

-- 
Mel Gorman
SUSE Labs
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