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

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Message-ID: <20120626165258.GY11413@one.firstfloor.org>
Date:	Tue, 26 Jun 2012 18:52:58 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Mel Gorman <mel@....ul.ie>
Cc:	Rafael Aquini <aquini@...hat.com>, linux-mm@...ck.org,
	Rik van Riel <riel@...hat.com>,
	Andi Kleen <andi@...stfloor.org>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH 1/4] mm: introduce compaction and migration for virtio ballooned pages

> 
> What shocked me actually is that VM_BUG_ON code is executed on
> !CONFIG_DEBUG_VM builds and has been since 2.6.36 due to commit [4e60c86bd:
> gcc-4.6: mm: fix unused but set warnings]. I thought the whole point of
> VM_BUG_ON was to avoid expensive and usually unnecessary checks. Andi,
> was this deliberate?

The idea was that the compiler optimizes it away anyways.

I'm not fully sure what putback_balloon_page does, but if it just tests
a bit (without non variable test_bit) it should be ok.

-Andi
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