linux-ext4 - Re: ext4: fix reference counting bug on block allocation error

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Date:	Sun, 14 Aug 2016 21:46:29 +0200
From:	Greg KH <gregkh@...uxfoundation.org>
To:	Vegard Nossum <vegard.nossum@...cle.com>
Cc:	Theodore Ts'o <tytso@....edu>, adilger.kernel@...ger.ca,
	linux-ext4@...r.kernel.org,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Subject: Re: ext4: fix reference counting bug on block allocation error

On Sun, Aug 14, 2016 at 09:35:59PM +0200, Vegard Nossum wrote:
> On 08/14/2016 09:09 PM, Greg KH wrote:
> > On Sun, Aug 14, 2016 at 09:02:48PM +0200, Vegard Nossum wrote:
> > > The commit which is being fixed is ancient:
> > > 
> > > $ git describe 8556e8f3b6
> > > v2.6.28-5758-g8556e8f3
> > > 
> > > It's probably already in the base of every current stable tree, no?
> > 
> > Huh?  Ok, this odd:
> > 	$ git describe --contains 8556e8f3b6
> > 	fatal: cannot describe '8556e8f3b6c4c11601ce1e9ea8090a6d8bd5daae'
> > 
> > Yet just a plain 'git describe' does work...
> > 
> > That's what threw me off, I only use --contains as that shows the
> > release the commit is in.
> > 
> > Ok, that makes me feel a bit better (that my scripts didn't miss the
> > patch, it was just old), but I wonder what is going on with git...
> > 
> 
> How odd.
> 
> There is this:
> 
> http://www.spinics.net/lists/git/msg246837.html
> 
> """
> Yes, the "describe --contains" algorithm uses timestamps to cut off the
> traversal, so it can do the wrong thing if there's clock skew. It has a
> "slop" margin of one day, but skew larger than that can fool it.
> """
> 
> It looks like ancestors of 87d8fe1 don't work:
> 
> $ for commit in $(git rev-list 87d8fe1^^..87d8fe1); do git describe
> --contains $commit; done
> v2.6.29-rc1~40^2~12
> fatal: cannot describe '0087d9fb3f29f59e8d42c8b058376d80e5adde4c'
> 
> So maybe it's because the parent commit is 2 days in the future:
> 
> $ git log --format=fuller 87d8fe1^^..87d8fe1
> commit 87d8fe1ee6b8d2f95076142d58c440dba4e7bdc2
> Author:     Theodore Ts'o <tytso@....edu>
> AuthorDate: Sat Jan 3 09:47:09 2009 -0500
> Commit:     Theodore Ts'o <tytso@....edu>
> CommitDate: Sat Jan 3 09:47:09 2009 -0500
> [...]
> 
> commit 0087d9fb3f29f59e8d42c8b058376d80e5adde4c
> Author:     Aneesh Kumar K.V <aneesh.kumar@...ux.vnet.ibm.com>
> AuthorDate: Mon Jan 5 21:49:12 2009 -0500
> Commit:     Theodore Ts'o <tytso@....edu>
> CommitDate: Mon Jan 5 21:49:12 2009 -0500
> [...]

Ouch, that's some clock skew.

Thanks for working this out, I've now applied the patch to the stable
trees.

greg k-h
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