linux-kernel - Re: [Bug #11342] Linux 2.6.27-rc3: kernel BUG at mm/vmalloc.c

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Date:	Wed, 27 Aug 2008 12:24:08 -0400
From:	"Parag Warudkar" <parag.lkml@...il.com>
To:	"Alan Cox" <alan@...rguk.ukuu.org.uk>
Cc:	"Adrian Bunk" <bunk@...nel.org>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Rusty Russell" <rusty@...tcorp.com.au>,
	"Alan D. Brunelle" <Alan.Brunelle@...com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	"Kernel Testers List" <kernel-testers@...r.kernel.org>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Arjan van de Ven" <arjan@...ux.intel.com>,
	"Ingo Molnar" <mingo@...e.hu>, linux-embedded@...r.kernel.org
Subject: Re: [Bug #11342] Linux 2.6.27-rc3: kernel BUG at mm/vmalloc.c - bisected

On Wed, Aug 27, 2008 at 9:21 AM, Alan Cox <alan@...rguk.ukuu.org.uk> wrote:
>> By your logic though, XFS on x86 should work fine with 4K stacks -
>> many will attest that it does not and blows up due to stack issues.
>>
>> I have first hand experiences of things blowing up with deep call
>> chains when using 4K stacks where 8K worked just fine on same
>> workload.
>>
>> So there is definitely some other problem with 4K stacks.
>
> Nothing of the sort. If it blows up with a 4K stack it will almost
> certainly blow up with an 8K stack *eventually* - when a heavy stack usage
> coincides with a heavy stack using IRQ handler.
>
> You won't catch it in simple testing, you won't catch it in trivial
> simulation and it'll be incredibly hard to reproduce. Not the kind of bug
> you want in a production system really. IRQ stacks make things much more
> predictable.


I see - so if I end up having a workload on 8k where heavy stack using
IRQs and deep kernel call chains come at the same time - even 8K will
blow up.
So 4K will blow too except that it doesn't require IRQs also to use
heavy stack, just XFS is good enough :)

It then seems like the IRQs using lot of stack is not so much of a
problem in the current kernel as much as deeper call chains and stack
usage of normal non-irq path code is.
So 8k makes it possible for the deeper call chains of non-irq path to
survive since they get better part of the 8K to themselves and IRQs
can do with less almost always.

At least that's what I can derive from the fact that we do not have
lots of reports of 8K stack blowing up.

Thanks

Parag
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