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

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Date:	Thu, 12 Jul 2007 08:09:15 +1000
From:	Neil Brown <neilb@...e.de>
To:	Daniel Phillips <phillips@...nq.net>
Cc:	Zan Lynx <zlynx@....org>, Jesper Juhl <jesper.juhl@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH][RFC] 4K stacks default, not a debug thing any more...?

On Wednesday July 11, phillips@...nq.net wrote:
> On Wednesday 11 July 2007 10:54, Zan Lynx wrote:
> > Jesper Juhl wrote:
> > > Hi,
> > >
> > > I'm wondering if it's time to make 4K stacks the default and to start
> > > considering removing the 8K stack option alltogether soon?
> > >
> > > One of the big problem spots was XFS, but that got some stack usage
> > > fixes recently, and the 4K stack option has been around for quite a
> > > while now, so people really should have gotten around to fixing any
> > > code that can't handle it.   Are there still any big problem areas
> > > remaining?
> >
> > Has anyone fixed the infrequent crashes with 4K stacks and ext3 -> LVM
> > snapshot -> LVM -> DM mirror -> libata?
> 
> Ahem: ext3 -> LVM snapshot -> LVM -> DM mirror -> DM crypt -> md -> libata,
> or worse.
> 
> No, it's not fixed.  The model is wrong.  Virtual block drivers should not
> be callling submit_bio.  The recursive IO submissions should be handled
> on a dedicated stack, most probably allocated as part of the request queue.
> This could be done easily in device mapper and md, or better, in
> submit_bio.

Maybe you should read that latest kernel source code.  Particularly
generic_make_request in block/ll_rw_blk.c.
The comment preceding it read:

/*
 * We only want one ->make_request_fn to be active at a time,
 * else stack usage with stacked devices could be a problem.
 * So use current->bio_{list,tail} to keep a list of requests
 * submited by a make_request_fn function.
 * current->bio_tail is also used as a flag to say if
 * generic_make_request is currently active in this task or not.
 * If it is NULL, then no make_request is active.  If it is non-NULL,
 * then a make_request is active, and new requests should be added
 * at the tail
 */

NeilBrown
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