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

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

>
> * Christoph Lameter <clameter@sgi.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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