linux-kernel - Re: 4KSTACKS + DEBUG

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Date:	Thu, 30 Aug 2007 00:53:56 +0200
From:	"Jesper Juhl" <jesper.juhl@...il.com>
To:	"Eric Sandeen" <sandeen@...hat.com>
Cc:	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: 4KSTACKS + DEBUG_STACKOVERFLOW harmful

On 30/08/2007, Eric Sandeen <sandeen@...hat.com> wrote:
> Noticed today that the combination of 4KSTACKS and DEBUG_STACKOVERFLOW
> config options is a bit deadly.
>
> DEBUG_STACKOVERFLOW warns in do_IRQ if we're within THREAD_SIZE/8 of the
> end of useable stack space, or 512 bytes on a 4k stack.
>
> If we are, then it goes down the dump_stack path, which uses most, if
> not all, of the remaining stack, thereby turning a well-intentioned
> warning into a full-blown catastrophe.
>
...
>
> 448 bytes to tell us that we're within 512 bytes (or less) of certain
> doom... and I think there's call overhead on top of that?
>
> The large stack usage in those 2 functions is due to big char arrays, of
> size KSYM_NAME_LEN (128 bytes) and KSYM_SYMBOL_LEN (223 bytes).
>
> IOW, the stack warning effectively reduces useful stack left in our itty
> bitty 4k stacks by over 10%.
>
> Any suggestions for ways around this?  The warning is somewhat helpful,
> and I guess the obvious option is to lighten up the dump_stack path, but
> it's still effectively reducing precious available stack space by some
> amount.
>
A first step could be to allocate those two char arrays with kmalloc()
instead of on the stack, but then I guess that dump_stack() gets
called from places where we may not really want to be calling
kmalloc(). I guess we could allocate the buffers earlier (like at boot
time) and store pointers somewhere where dump stack can get to them
later when it needs them.


> With CONFIG_DEBUG_STACK_USAGE, we could print at oops time: "oh, and by
> the way, you blew your stack" if there is no zeroed stack space left, as
> a post-mortem.  Even without that option, I think we could still check
> whether the *current* %esp at oops time has gone too far?  But if we
> blew the stack, returned, and *then* oops, I think it'd be hard to know
> without the DEBUG_STACK_USAGE option that we ran out of room.
>

We could also simply have it warn at a higher limit, like 1024 bytes
instead of 512. But I guess then we would get too many false positives
and make it less useful.

-- 
Jesper Juhl <jesper.juhl@...il.com>
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