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Date:	Mon, 18 Jun 2012 20:40:55 +0900
From:	Namjae Jeon <linkinjeon@...il.com>
To:	Lukáš Czerner <lczerner@...hat.com>
Cc:	Ashish Sangwan <ashishsangwan2@...il.com>,
	linux-ext4@...r.kernel.org
Subject: Re: Punching hole using fallocate is not removing the uninit extent
 from extent tree

2012/6/18, Lukáš Czerner <lczerner@...hat.com>:
> On Mon, 28 May 2012, Ashish Sangwan wrote:
>
>> Date: Mon, 28 May 2012 19:38:05 +0530
>> From: Ashish Sangwan <ashishsangwan2@...il.com>
>> To: linux-ext4@...r.kernel.org
>> Subject: Punching hole using fallocate is not removing the uninit extent
>> from
>>     extent tree
>
> Hi Ashish,
>
> I am looking at you patch, however I am not able to reproduce this.
> Can you please send more information (script preferably) on how to
> reproduce this problem ?
>
Hi. Lukas.
If you use the below script, you can easily reproduce this problem.
And I can not attach script file, so I write script code in this mail.
you can paste it to file.

fragmentation.sh
-----------------------------------------------------------------------
dd if=/dev/zero of=12kb bs=4096 count=6
index=0
while [ $? == 0 ]
do
index=$(($index + 1))
cp 12kb $1/file1.$index
done
echo "Partition filled"
sync
df -h
index=0
while [ $? == 0 ]
do
index=$(($index + 2))
sync
rm  $1/file1.$index
done
sync
echo "fragmented partition $1 with 4KB files"
-------------------------------------------------------------------------

1. create smaill partition(500MB) to reproduce rapidly.
2. plz run this script to make fragmentation partition after making
dummy directory like this.
mkdir test_dir
./fragmentation.sh test_dir

3. dd if=/dev/zero of=d_file bs=1024 count=10240
4. you can see file depth is not zero.(dump_extents d_file
5. and try to fallocate offset : 4096 lengh 4096.

> Also what kernel version did try this on ?
We are using 3.0.20 kernel patched your punch hole patches.

Please let us know in case of any queries.

Thanks.

>
> Thanks!
> -Lukas
>
>>
>> I have created a formatted EXT4 partition such that every single
>> extent is exactly 6blocks (24KB) of length.
>> I used hole punch on 2 different files.
>>
>> CASE 1: In first situation, file size is 72KB. There are total 3
>> extents each 24KB length. Using fallocate to punch hole starting at
>> offset 4096 and length 4096,
>> dump_extents gives the following expected output :
>>
>> Before punching hole :
>> Level Entries       Logical      Physical Length Flags
>> 0/ 0   1/  2     0 -     5  1856 -  1861      6
>> 0/ 0   2/  2     6 -    11  1868 -  1873      6
>>
>> After punching hole :
>> Level Entries       Logical      Physical Length Flags
>> 0/ 0   1/  3     0 -     0  1856 -  1856      1
>> 0/ 0   2/  3     2 -     5  1858 -  1861      4
>> 0/ 0   3/  3     6 -    11  1868 -  1873      6
>>
>> The 1st extent: 0-5, is splitted into 3 extents, "0-0", "1-1", "2-5"
>> Extent 1-1 is first marked as uninitialized in function
>> ext4_ext_map_blocks() and later removed from the extent tree by
>> ext4_ext_remove_space().
>>
>> CASE 2: File size is 9.4MB. There are total 400 extents each 24KB
>> length, depth of extent tree at root header is 1 and there are 2 index
>> entries.
>>
>> dump_extents output before punching hole:
>> Level Entries       Logical      Physical Length Flags
>> 0/ 1   1/  2     0 -  2039  1922           2040
>> 1/ 1   1/340     0 -     5  1856 -  1861      6
>> 1/ 1   2/340     6 -    11  1868 -  1873      6
>> < Continued likewise till 340/340 >
>> 1/ 1 340/340  2034 -  2039  5942 -  5947      6
>> 0/ 1   2/  2  2040 -  2399  1923            360
>> 1/ 1   1/ 60  2040 -  2045  5954 -  5959      6
>> 1/ 1   2/ 60  2046 -  2051  5966 -  5971      6
>> < Continued likewise till 60/60 >
>> 1/ 1  60/ 60  2394 -  2399  6662 -  6667      6
>>
>> dump_extents output after punching hole :
>> 0/ 1   1/  3     0 -     5  1922              6
>> 1/ 1   1/  3     0 -     0  1856 -  1856      1
>> 1/ 1   2/  3     1 -     1  1857 -  1857      1 Uninit
>> 1/ 1   3/  3     2 -     5  1858 -  1861      4
>> 0/ 1   2/  3     6 -  2039  6674           2034
>> 1/ 1   1/339     6 -    11  1868 -  1873      6
>> 1/ 1   2/339    12 -    17  1880 -  1885      6
>> < Continued like wise...>
>>
>> Comparing CASE2 with CASE1, still uninit extent "1-1" is present
>> within the extent tree.
>>
>> In function ext4_ext_remove_space(), there is call to function
>> ext4_ext_rm_leaf which is responsible for removal of this extent.
>> But this function is not getting called in CASE 2 :
>> if (i == depth) {
>>                        /* this is leaf block */
>>                        err = ext4_ext_rm_leaf(handle, inode, path,
>>                                        start, end);
>> /* root level has p_bh == NULL, brelse() eats this */
>>                        brelse(path[i].p_bh);
>>                        path[i].p_bh = NULL;
>>                        i--;
>>                        continue;
>>                }
>>
>> Varibale "i" does not become equals to "depth" because
>> ext4_ext_more_to_rm is returning "0" hence the following if condition
>> is turning out to be false for 1st extent index:
>> if (ext4_ext_more_to_rm(path + i)) {
>>
>> Looking at the defination of ext4_ext_more_to_rm :
>> /*
>> * ext4_ext_more_to_rm:
>> * returns 1 if current index has to be freed (even partial)
>> */
>> static int
>> ext4_ext_more_to_rm(struct ext4_ext_path *path)
>> {
>>        BUG_ON(path->p_idx == NULL);
>>        if (path->p_idx < EXT_FIRST_INDEX(path->p_hdr))
>>                return 0;
>>
>>        /*
>>         * if truncate on deeper level happened, it wasn't partial,
>>         * so we have to consider current index for truncation
>>         */
>>        if (le16_to_cpu(path->p_hdr->eh_entries) == path->p_block) <=
>> This condition is turning out to be true
>>                return 0;       <= The function is returning zero from
>> here.
>>        return 1;
>> }
>>
>> I could not understand the significance of the above mentioned if
>> condition check, if anyone could explain a little, it will be help.
>> --
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