linux-kernel - Re: [PATCH v2] ext4: fix hole punch failure when depth is greater than 0

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Date:	Mon, 2 Jul 2012 16:51:43 +0530
From:	Ashish Sangwan <ashishsangwan2@...il.com>
To:	Lukáš Czerner <lczerner@...hat.com>
Cc:	sandeen@...hat.com, Ted Tso <tytso@....edu>,
	linux-kernel@...r.kernel.org, linux-ext4@...r.kernel.org,
	Namjae Jeon <linkinjeon@...il.com>
Subject: Re: [PATCH v2] ext4: fix hole punch failure when depth is greater
 than 0

I will try to elaborate more about the problem and steps to reproduce:
Linux version 3.4.0 on X86 box.

Step1:
First create a small partition as it would be easy to fragment quickly.
The main intent for fragmenting the partition is to create a file with
at least 2 extent indexes.
You can also choose some other method to create such a file.
I created 200Mb partition and while formating used blocksize as 4KB
and used attached script to fragment.
This script fragments the partition such that each extent is exactly
6FS blocks OR 24KB in length.
Linux#> ./fragment.sh /mnt/
6+0 records in
6+0 records out
24576 bytes (24.0KB) copied, 0.087997 seconds, 272.7KB/s
cp: write error: No space left on device
Partition filled
rm: can't remove '/mnt//file1.7564': No such file or directory
fragmented partition /mnt/ with 24KB files

Step2:
Create a file with 342 extents i.e 2 extent indexes
[root@...tnik ashish]# dd if=linux-3.4.tar.bz2 of=check_1 bs=24576 count=342
342+0 records in
342+0 records out
8404992 bytes (8.4 MB) copied, 0.0913289 s, 92.0 MB/s
[root@...tnik ashish]# cp check_1 /mnt/check_2
[root@...tnik ashish]# debugfs  /dev/sdb1
debugfs 1.41.14 (22-Dec-2010)
debugfs:  dump_extents check_2
Level Entries       Logical      Physical Length Flags
 0/ 1   1/  2     0 -  2039 32792             2040
 1/ 1   1/340     0 -     5  5771 -  5776       6
 1/ 1   2/340     6 -    11  5783 -  5788      6
<------- continue like wise till 340 ------------->
 1/ 1 340/340  2034 -  2039  9835 -  9840   6
 0/ 1   2/  2  2040 -  2051  5764              12
 1/ 1   1/  2  2040 -  2045  9847 -  9852     6
 1/ 1   2/  2  2046 -  2051  9859 -  9864     6

Step3: Punch hole at offset 4KB and length of hole is also 4KB
There is attached fallocate.c
[root@...tnik ashish]# ./fallacote.x86 /mnt/check_2
ret = 0
[root@...tnik ashish]#

Check extent tree:
[root@...tnik ashish]# debugfs  /dev/sdb1
debugfs 1.41.14 (22-Dec-2010)
debugfs:  dump_extents check_2
Level Entries       Logical      Physical Length Flags
 0/ 1   1/  3     0 -     5 32792                 6
 1/ 1   1/  2     0 -     1  5771 -  5772      2
 1/ 1   2/  2     2 -     5  5773 -  5776      4
 0/ 1   2/  3     6 -  2039  9871           2034
 1/ 1   1/339     6 -    11  5783 -  5788    6
<------- continue like wise till 339 ------------->
 1/ 1 339/339  2034 -  2039  9835 -  9840   6
 0/ 1   3/  3  2040 -  2051  5764             12
 1/ 1   1/  2  2040 -  2045  9847 -  9852    6
 1/ 1   2/  2  2046 -  2051  9859 -  9864    6

It is clearly visible that punching hole just split the first extent into 2
but failed to remove the required blocks.

Step4: To cross check, take diff of 2 files.
[root@...tnik ashish]# diff check_1 /mnt/check_2
[root@...tnik ashish]#

The 2 files are exactly same.

After applying this patch, correct results will be observed.

On Mon, Jul 2, 2012 at 2:42 PM, Lukáš Czerner <lczerner@...hat.com> wrote:
> On Mon, 2 Jul 2012, Ashish Sangwan wrote:
>
>> Date: Mon, 2 Jul 2012 11:03:26 +0530
>> From: Ashish Sangwan <ashishsangwan2@...il.com>
>> To: sandeen@...hat.com, Lukáš Czerner <lczerner@...hat.com>,
>>     Ted Tso <tytso@....edu>
>> Cc: linux-kernel@...r.kernel.org, linux-ext4@...r.kernel.org,
>>     Namjae Jeon <linkinjeon@...il.com>
>> Subject: Re: [PATCH v2] ext4: fix hole punch failure when depth is greater
>>     than 0
>>
>> Hi Ted, Lukas, Eric,
>>
>> Did any of you get a chance to look at it?
>
> I am sorry for the delay. I have been trying to reproduce this
> problem without any success so far. But is was on ppc64 machine, so
> I'll try that on x86_64 and then review the patch.
>
> Thanks!
> -Lukas
>
>>
>> On Fri, Jun 22, 2012 at 6:19 AM, Namjae Jeon <linkinjeon@...il.com> wrote:
>> > Hi. Eric.
>> >
>> > This is the patch for punch hole issue.
>> >
>> > Thanks.
>> >
>>

View attachment "fallacote.c" of type "text/x-csrc" (650 bytes)

Download attachment "fragment.sh" of type "application/x-sh" (279 bytes)