linux-kernel - Re: 2.6.19 file content corruption on ext3

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Date:	Tue, 19 Dec 2006 12:03:48 -0800 (PST)
From:	dean gaudet <dean@...tic.org>
To:	Linus Torvalds <torvalds@...l.org>
cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...l.org>, andrei.popa@...eo.ro,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Hugh Dickins <hugh@...itas.com>,
	Florian Weimer <fw@...eb.enyo.de>,
	Marc Haber <mh+linux-kernel@...schlus.de>,
	Martin Michlmayr <tbm@...ius.com>
Subject: Re: 2.6.19 file content corruption on ext3

On Mon, 18 Dec 2006, Linus Torvalds wrote:

> On Tue, 19 Dec 2006, Nick Piggin wrote:
> > 
> > We never want to drop dirty data! (ignoring the truncate case, which is
> > handled privately by truncate anyway)
> 
> Bzzt.
> 
> SURE we do.
> 
> We absolutely do want to drop dirty data in the writeout path.
> 
> How do you think dirty data ever _becomes_ clean data?
> 
> In other words, yes, we _do_ want to test-and-clear all the pgtable bits 
> _and_ the PG_dirty bit. We want to do it for:
>  - writeout
>  - truncate
>  - possibly a "drop" event (which could be a case for a journal entry that 
>    becomes stale due to being replaced or something - kind of "truncate" 
>    on metadata)
> 
> because both of those events _literally_ turn dirty state into clean 
> state.
> 
> In no other circumstance do we ever want to clear a dirty bit, as far as I 
> can tell. 

i admit this may not be entirely relevant, but it seems like a good place 
to bring up an old problem:  when a disk dies with lots of queued writes 
it can totally bring a system to its knees... even after the disk is 
removed.  i wrote up something about this a while ago:

http://lkml.org/lkml/2005/8/18/243

so there's another reason to "clear a dirty bit"... well, in fact -- drop 
the pages entirely.

-dean
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