From: Mel (mel at domain csn.ul.ie)
Date: Mon 15 Oct 2001 - 12:57:15 IST
On Mon, 15 Oct 2001, Mark Dennehy wrote:
> Ref:[Dave Airlie, Mon, 15/10/01 at domain 10:10 +0100]
> > get kgdb up and running and get another PC to act as a debug terminal..
> > of course the oops is useless without having been run through ksymoops..
> > you can manually debug it using System.map yourself...
> Ich, that'd be like pulling your toenails out with your teeth
> through the ball of your foot :(
not that bad, but it's a silly exercise. Run oops through ksymoops and
it'll be much easier to figure out what the hell is going wrong
> I've figured out what was happening - a simple buffer overrun
> which was hidden because the overrun was somehow getting
> whatever code ran after init_module. :(
It might be worth your while enabling SLAB_DEBUG_SUPPORT in mm/slab.c . In
2.4, this was changed to CONFIG_SLAB_DEBUG but I think it may be the same
thing, I don't have time to decipher 2.2.19's slab code to confirm but I
think it behaves kinda similar in this case
What CONFIG_SLAB_DEBUG does in 2.4 at least is add support for
SLAB_RED_ZONE and SLAB_POISON . SLAB_POISON poisons new objects in the
slab with a known test pattern for checking access to uninitialised
fields. SLAB_RED_ZONE pads one word either side of new objects with a
known test pattern to catch buffer overruns.
This is most useful when you are using the slab allocator for your own
caches but as kmalloc uses a cache of various sized memory chunks (see
/proc/slabinfo and you'll see them), the same ***may*** hold true. If you
overrun, you should see a message along the line of "Bad front redzone" in
2.2.19 and just BUG() is called on 2.4.9 during kmem_cache_free.
Unfortunatly, by then it may be too late but at least you'll be told that
memory was messed up due to overruns if you are luckly :-)
Disclaimer: I'm very new to looking at the slab allocator. I haven't
actually tested any of this, it based on simply reading the code and what
I suspect happens but don't actually know for a fact. It's quiet possible
I'm talking utter shit and way off the mark
-- Mel Gorman, President UL Computer Society http://www.csn.ul.ie/~mel
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