linux-kernel - Re: NULL pointer dereference in ext4_ext_remove

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Message-ID: <20120817131558.GA11439@thunk.org>
Date:	Fri, 17 Aug 2012 09:15:58 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Fengguang Wu <fengguang.wu@...el.com>
Cc:	Marti Raudsepp <marti@...fo.org>,
	Kernel hackers <linux-kernel@...r.kernel.org>,
	ext4 hackers <linux-ext4@...r.kernel.org>, maze@...gle.com
Subject: Re: NULL pointer dereference in ext4_ext_remove_space on 3.5.1

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
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