linux-ext4 - Re: double free of blocks occurred during online defrag

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Date:	Wed, 25 Feb 2009 16:14:46 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Kazuya Mio <k-mio@...jp.nec.com>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: double free of blocks occurred during online defrag

On Wed, Feb 25, 2009 at 03:39:52PM +0900, Kazuya Mio wrote:
> Hi Aneesh,
> 
> When I remove the file that is running online defrag, the following error occurs
> after closing the file descriptor:
> 
> Jan 22 17:06:52 G3-OPC-SVR2 kernel: EXT4-fs error (device hda8):
> ext4_mb_release_inode_pa: free 2048, pa_free 1562
> Jan 22 17:06:52 G3-OPC-SVR2 kernel: EXT4-fs error (device hda8): mb_free_blocks:
> double-free of inode 0's block 802817(bit 0 in group 98)
> Jan 22 17:06:52 G3-OPC-SVR2 kernel: EXT4-fs error (device hda8): mb_free_blocks:
> double-free of inode 0's block 802818(bit 1 in group 98)
> Jan 22 17:06:52 G3-OPC-SVR2 kernel: EXT4-fs error (device hda8): mb_free_blocks:
> double-free of inode 0's block 802819(bit 2 in group 98)
> Jan 22 17:06:52 G3-OPC-SVR2 kernel: EXT4-fs error (device hda8): mb_free_blocks:
> double-free of inode 0's block 802820(bit 3 in group 98)
> Jan 22 17:06:52 G3-OPC-SVR2 kernel: EXT4-fs error (device hda8): mb_free_blocks:
> double-free of inode 0's block 802821(bit 4 in group 98)
> Jan 22 17:06:52 G3-OPC-SVR2 kernel: EXT4-fs error (device hda8): mb_free_blocks:
> double-free of inode 0's block 802822(bit 5 in group 98)
> 
> So, online defrag calls ext4_discard_preallocations() at the end of
> ext4_defrag() to avoid double-free error.
> However, above error hasn't occurred since applying your patch posted on Nov
> 6th, 2008 because this error is caused by the same reason of your report.
> http://marc.info/?l=linux-ext4&m=122599787406193&w=4
> 
> What is the status of this patch?

We dropped the patch because I found that the double free in my case was
not exactly due the explanation given in the patch above.

I asked to drop the patch in

http://article.gmane.org/gmane.comp.file-systems.ext4/10199

I also found that the patch is not completely correct. The meta-data
blocks which are added to the free_list are not allocated from any
prealloc space.

So what you are seeing may be a different problem which the patch is
hiding from happening.  I guess you will have to look more closely at why the
double-free is happening in your case.

-aneesh
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