linux-ext4 - Re: Punching hole using fallocate is not removing the uninit extent from extent tree

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Message-ID: <alpine.LFD.2.00.1206181300470.2394@dhcp-1-248.brq.redhat.com>
Date:	Mon, 18 Jun 2012 13:03:04 +0200 (CEST)
From:	Lukáš Czerner <lczerner@...hat.com>
To:	Ashish Sangwan <ashishsangwan2@...il.com>
cc:	linux-ext4@...r.kernel.org
Subject: Re: Punching hole using fallocate is not removing the uninit extent
 from extent tree

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 ?

Also what kernel version did try this on ?

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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