linux-kernel - Re: [bug] SLUB + mm/slab.c boot crash in -rc9

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Message-ID: <20080415210818.GA1339@elte.hu>
Date:	Tue, 15 Apr 2008 23:08:18 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Christoph Lameter <clameter@....com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	linux-kernel@...r.kernel.org, Mel Gorman <mel@....ul.ie>,
	Nick Piggin <npiggin@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>, Yinghai.Lu@....com,
	apw@...dowen.org,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [bug] SLUB + mm/slab.c boot crash in -rc9

* Ingo Molnar <mingo@...e.hu> wrote:

> 
> * Christoph Lameter <clameter@....com> wrote:
> 
> > On Tue, 15 Apr 2008, Ingo Molnar wrote:
> > 
> > > my current guess would have been some bootmem regression/interaction 
> > > that messes up the buddy bitmaps - but i just reverted to the v2.6.24 
> > > version of bootmem.c and that crashes too ...
> > 
> > The simplest solution for now may be to go with your workaround 
> > increasing SECTION_SIZE_BITS to 27. [...]
> 
> the bug's effects are so severe that this is the last thing i'd like 
> to do.

more verbosely: we sometimes do "blind" reverts, if it's reasonably 
established (or strongly suspected) that a revert makes a bug less 
severe. We do this even if we dont fully understand the bug and its 
effects and time runs out - on the assumption that we wont get worse 
than the old code was.

but what i'd not really like to do are blind _non-revert_ changes. With 
your suggested change we'd introduce a seemingly innocious but still 
wholly new (and untested) memory setup layout on the most popular Linux 
kernel memory config in existence. (!PAE 32-bit is still being run on 
more than 50% of the Linux desktops - around 80% runs 32-bit kernels.)

And as this bug demonstrates it, seemingly small differences appear to 
have large effects so we cannot know in what direction that would go - 
we might turn a rare regression into a common regression. I'd rather 
release with this bug being unfixed than with tweaking it just because 
the effect seems less severe on a totally unrepresentative set of 
systems.

	Ingo
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