From owner-freebsd-arch  Thu Dec 20  7:47:27 2001
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Date: Thu, 20 Dec 2001 09:47:23 -0600
From: Alfred Perlstein <bright@mu.org>
To: Poul-Henning Kamp <phk@freebsd.org>
Cc: arch@freebsd.org
Subject: Re: Kernel stack size and stacking: do we have a problem ?
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* Poul-Henning Kamp <phk@freebsd.org> [011220 04:46] wrote:
> 
> As most of you have probably heard, I'm working on a stacking
> disk I/O layer (http://freefall.freebsd.org/~phk/Geom).
> 
> This is as far as I know, only the third freely stackable subsystem
> in the kernel, the first two being VFS/filesystems and netgraph.
> 
> The problem with stacking layered systems is that the naïve and
> simple implementation, just calling into the layer below, has
> basically unbounded kernel stack usage.

I think you're thinking way too hard about this, what would make
sense is a near surefire way to catch stack overflow along with a
panic message that was clear like "increase upages in kernel".

Btw, Windows catches this and somehow assigns additional kernel
stack pages to processes (or at least it seems).  Do a search for
"MinSp" or "MinSps".

Lastly I would really avoid redesigning the VOPs however one
suggestion would be to define an entry/exit function so that
instead of having traditional stacking code like so:

my_layer(void *foo)
{

   if (mytest()) {
      NEXT_LAYER(foo);
      something;
   }
   return ERROR;
}

You could have:

my_layer_entry(void *foo)
{

  if (mytest())
     return (foo, next_layer_ptr);
  else
     return ERROR;
}

my_layer_exit(void *foo)
{

   something;

}

This would keep the stack at a near constant level at the expense
of programming complexity.  If you think about it though, it's quite
like how network interrupts are handled, hardware queues the packets
and then software runs after hardware returns.

-Alfred

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