linux-ext4 - Re: [patch] fix up lock order reversal in writeback

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Message-Id: <20101118121756.ddeed28f.akpm@linux-foundation.org>
Date:	Thu, 18 Nov 2010 12:17:56 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	Nick Piggin <npiggin@...nel.dk>, "Ted Ts'o" <tytso@....edu>,
	Jan Kara <jack@...e.cz>, linux-fsdevel@...r.kernel.org,
	linux-ext4@...r.kernel.org, linux-btrfs@...r.kernel.org
Subject: Re: [patch] fix up lock order reversal in writeback

On Thu, 18 Nov 2010 13:02:43 -0600
Eric Sandeen <sandeen@...hat.com> wrote:

> On 11/18/10 12:36 PM, Andrew Morton wrote:
> > On Thu, 18 Nov 2010 12:04:21 -0600 Eric Sandeen <sandeen@...hat.com> wrote:
> > 
> >> On 11/18/10 11:10 AM, Andrew Morton wrote:
> >>> On Thu, 18 Nov 2010 08:55:18 -0600 Eric Sandeen <sandeen@...hat.com> wrote:
> >>>
> >>>>> Can we just delete writeback_inodes_sb_nr_if_idle() and
> >>>>> writeback_inodes_sb_if_idle()?  The changelog for 17bd55d037a02 is
> >>>>> pretty handwavy - do we know that deleting these things would make a
> >>>>> jot of difference?
> >>>>
> >>>> Really?  I thought it was pretty decent ;)
> >>>>
> >>>> Anyway, xfstests 204, "Test out ENOSPC flushing on small filesystems."
> >>>> shows the problem clearly, IIRC.  I should have included that in the
> >>>> changelog, I suppose, sorry.
> >>>
> >>> Your email didn't really impart any information :(
> >>>
> >>> I suppose I could accidentally delete those nasty little functions in a
> >>> drivers/parport patch then wait and see if anyone notices.
> >>>
> >>
> >> Um, ok, then, to answer the question directly :
> >>
> >> No, please don't delete those functions, it will break ENOSPC handling
> >> in ext4 as shown by xfstests regression test #204 ...
> >>
> > 
> > If those functions "fix" a testcase then it was by sheer luck, and the
> > fs's ENOSPC handling is still busted.
> > 
> > For a start writeback_inodes_sb_if_idle() is a no-op if the device
> > isn't idle!  
> 
> so writeback is already in progress and it's already doing what we need,
> right?  Space is being freed up as we speak in that case.

With no guarantee that it's being freed at a sufficient rate.

> > Secondly, if the device _was_ idle,
> > writeback_inodes_sb_if_idle() uses a work handoff to another thread,
> > which means that the work might not get executed for another six weeks.
> 
> We start it quite early, before things are critical.
> 
> Yeah, it's not bulletproof but it is tons better.

Translation: "it papers over a bug".

Look, if this was a little best-effort poke-writeback-now performance
tweak then fine.  But as an attempt to prevent a synchronous and bogus
ENOSPC error it's just hopeless.

Guys, fix the thing for real, and take that hack out.
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