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Date: Fri, 17 Aug 2012 09:15:58 -0400
From: "Theodore Ts'o" <tytso@mit.edu>
To: Fengguang Wu <fengguang.wu@intel.com>
Cc: Marti Raudsepp <marti@juffo.org>,
        Kernel hackers <linux-kernel@vger.kernel.org>,
        ext4 hackers <linux-ext4@vger.kernel.org>, maze@google.com
Subject: Re: NULL pointer dereference in ext4_ext_remove_space on 3.5.1
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Thanks Fengguang:

For the record, I was able to find my own easy repro, last night using
only a 220 meg partition:

# mke2fs -t ext4 -b 1024 -J size=1 /dev/vdc
# mount -t ext2 /dev/vdc /vdc
# mkdir /vdc/a
# cd /vdc/a
# seq 1 210000  | xargs -n 1 fallocate -l 1m
# seq 1 2 210000  | xargs /bin/rm
# mkdir /vdc/b
# cd /vdc/b
# seq 1 103 | xargs -n 1 fallocate -l 1g
# cd /
# umount /vdc
# mount -t ext4 -o commit=10000 /dev/vdc /vdc
# rm -rf /vdc/b

For future reference, there are a couple of things that are of
interest to ext4 developers when trying to create repro's:

1)  The use of mounting with ext2 to speed up the setup.

2)  The first two "seq ... | xargs ..." commands to create a very
fragmented file system.

3) Using a 1k block size file system to stress the extent tree code
and htree directory (since its easier to make larger tree structure).

4)  The use of the mount option commit=10000 to test what happens when
the journal is full (without using a nice, fast device such as RAID array
or without burning write cycles on an expensive flash device.)

						- Ted