linux-ext4 - Re: [PATCH 3/4]ext4: Return exchanged blocks count to user space in failure

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Date:	Fri, 4 Sep 2009 11:35:56 +0800
From:	Peng Tao <bergwolf@...il.com>
To:	Greg Freemyer <greg.freemyer@...il.com>
Cc:	Akira Fujita <a-fujita@...jp.nec.com>,
	Theodore Tso <tytso@....edu>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH 3/4]ext4: Return exchanged blocks count to user space in 
	failure

On Thu, Sep 3, 2009 at 9:48 PM, Greg Freemyer<greg.freemyer@...il.com> wrote:
> On Thu, Sep 3, 2009 at 1:13 AM, Peng Tao<bergwolf@...il.com> wrote:
>> Hi, Greg,
>>
>> On Thu, Sep 3, 2009 at 4:59 AM, Greg Freemyer<greg.freemyer@...il.com> wrote:
>>> Peng,
>>>
>>> I have not looked at the code very closely, but can you tell me where
>>> a file corruption can take place?   Not completing the replacement of
>>> extents with donor extents is one thing.  Corrupting the original file
>>> contents is another.
>> The file corruption is mainly because of the half done replacement.
>>
>> My test case is here:
>> http://marc.info/?l=linux-ext4&m=124992522305319&w=2
>>
>> With Akira's previous patch
>> (http://marc.info/?l=linux-ext4&m=124937430627867&w=2),
>> EXT4_IOC_MOVE_EXT does not panic the kernel any more. But it leaves
>> the orig file's extent tree corrupted.
>
> Is this highly repeatable, e4defrag using EXT4_IOC_MOVE_EXT corrupts
> sparse files?
It is the EXT4_IOC_MOVE_EXT ioctl corrupts sparse files because it
doesn't handle well with file holes.
The e4defrag program  loops to call EXT4_IOC_MOVE_EXT for each
non-continuous parts of a file, so it doesn't expose file holes to
kernel.
But since the ioctl interface is publicly usable, I do suggest making
it behave better, at lease don't bug the kernel, because I did hit
null pointer reference in my last time testing the new ioctl. So IMHO,
Akira's previous patch (http://patchwork.ozlabs.org/patch/30707/) is
really necessary.

>
> If so, it seems like a pretty major bug that will be exposed to
> userspace when 2.6.31 goes final.
>
> It seems to me at a minimum a Kconfig option should be added to enable
> the ioctl to userspace and that it should have depends on EXPERIMENTAL
> and default to NO for now.
>
> We don't want people thinking that this feature is stable in 2.6.31.
I do agree that the feature is still unstable ATM.

>
> Greg
> --
> Greg Freemyer
> Head of EDD Tape Extraction and Processing team
> Litigation Triage Solutions Specialist
> http://www.linkedin.com/in/gregfreemyer
> Preservation and Forensic processing of Exchange Repositories White Paper -
> <http://www.norcrossgroup.com/forms/whitepapers/tng_whitepaper_fpe.html>
>
> The Norcross Group
> The Intersection of Evidence & Technology
> http://www.norcrossgroup.com
>



-- 
Cheers,
Peng Tao
State Key Laboratory of Networking and Switching Technology
Beijing Univ. of Posts and Telecoms.
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