Re: [PATCH 2/3] xfs: delalloc and quota softlimit timers are incoherent

[Dave Chinner <david@fromorbit.com>]

On Tue, Nov 12, 2024 at 03:48:35PM -0800, Darrick J. Wong wrote:
> On Wed, Nov 13, 2024 at 09:05:15AM +1100, Dave Chinner wrote:
> > From: Supriya Wickrematillake <sup@sgi.com>
> 
> Wow, there are still people working at SGI??

huh, that's a git-send-email screwup. It must have pulled that from
the quote of the initial quota commit:

> > This debug was introduced in:
> > 
> > commit 0d5ad8383061fbc0a9804fbb98218750000fe032
> > Author: Supriya Wickrematillake <sup@sgi.com>
> > Date:   Wed May 15 22:44:44 1996 +0000
> > 
> >     initial checkin
> >     quotactl syscall functions.

Here, and decided to ignore the actual "From: dchinner@redhat" tag
in the local commit message.

Great work, git!


> > The very first quota support commit back in 1996. This is zero-day
> > debug for Irix and, as it turns out, a zero-day bug in the debug
> > code because the delalloc code on Irix didn't update the softlimit
> > timers, either.
> > 
> > IOWs, this issue has been in the code for 28 years.
> > 
> > We obviously don't care if soft limit timers are a bit rubbery when
> > we have delalloc reservations in memory. Production systems running
> > quota reports have been exposed to this situation for 28 years and
> > nobody has noticed it, so the debug code is essentially worthless at
> > this point in time.
> > 
> > We also have the on-disk dquot verifiers checking that the soft
> > limit timer is running whenever the dquot is over the soft limit
> > before we write it to disk and after we read it from disk. These
> > aren't firing, so it is clear the issue is purely a temporary
> > in-memory incoherency that I never would have noticed had the test
> > not silently failed to unmount the filesystem.
> > 
> > Hence I'm simply going to trash this runtime debug because it isn't
> > useful in the slightest for catching quota bugs.
> > 
> > Signed-off-by: Dave Chinner <dchinner@redhat.com>
> 
> Agreed!  I've hit this once in a blue moon and didn't think it was
> especially useful either.
> 
> Reviewed-by: Darrick J. Wong <djwong@kernel.org>

Thanks!

-Dave.

-- 
Dave Chinner
david@fromorbit.com