linux-kernel - Re: [PATCH][RFC] 4K stacks default, not a debug thing any more...?

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Date:	Wed, 1 Aug 2007 20:04:03 +1000
From:	Neil Brown <neilb@...e.de>
To:	"Dan Merillat" <dan.merillat@...il.com>
Cc:	"Eric Sandeen" <sandeen@...deen.net>,
	"Satyam Sharma" <satyam.sharma@...il.com>,
	"Alan Cox" <alan@...rguk.ukuu.org.uk>,
	"Andrea Arcangeli" <andrea@...e.de>,
	"Matt Mackall" <mpm@...enic.com>,
	"Rene Herman" <rene.herman@...il.com>,
	"Ray Lee" <ray-lk@...rabbit.org>, "Bodo Eggert" <7eggert@....de>,
	"Jeremy Fitzhardinge" <jeremy@...p.org>,
	"Jesper Juhl" <jesper.juhl@...il.com>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	"William Lee Irwin III" <wli@...omorphy.com>,
	"David Chinner" <dgc@....com>,
	"Arjan van de Ven" <arjan@...radead.org>
Subject: Re: [PATCH][RFC] 4K stacks default, not a debug thing any more...?

On Wednesday August 1, dan.merillat@...il.com wrote:
> 
> The other issue is with the layered IO design - no matter what we
> configure the stack size to, it is still possible to create a set of
> translation layers that will cause it to crash regularly:  XFS on
> dm_crypt on loop on XFS on dm_crypt on loop on ad infinitum.

No, this does not use indefinite stack.

loop will schedule each request to be handled by a kernel thread, so
requests to 'loop' are serialised, never stacked.

In 2.6.22, generic_make_request detects and serialises recursive calls,
so unlimited recursion is not possible there either.

It is still possible to do
  dm on dm on dm on dm on md on md on md on md

and calls to ->issue_flush_fn or ->unplug_fn could use an arbitrarily
large amount of stack.  But the stack usage of each stage is very
small so it is unlikely to be a problem (though it should still be
fixed).

NeilBrown
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