linux-ext4 - Re: [PATCH] e2fsck: Let end_blk to be the maximum value of u32.

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Message-ID: <4FB3C2BE.8080502@tao.ma>
Date:	Wed, 16 May 2012 23:07:42 +0800
From:	Tao Ma <tm@....ma>
To:	Eric Sandeen <sandeen@...hat.com>
CC:	linux-ext4@...r.kernel.org, Theodore Ts'o <tytso@....edu>
Subject: Re: [PATCH] e2fsck: Let end_blk to be the maximum value of u32.

On 05/16/2012 10:04 PM, Eric Sandeen wrote:
> On 5/16/12 3:50 AM, Tao Ma wrote:
>> From: Tao Ma <boyu.mt@...bao.com>
>>
>> Now we can use fallocate to create a large file while keep the size
>> to be small. It will cause the e2fsck complain about it. The test
>> script is simple and I have pasted it here.
>>
>> DEVICE=/dev/sdb1
>> mount -t ext4 $DEVICE /mnt/ext4
>> for((i=0;i<10;i++))do fallocate -n -o $[$i*8192] -l 4096 /mnt/ext4/a;done
>> umount $DEVICE
>> e2fsck -fn $DEVICE
> 
> Should this be put into an e2fsprogs regression test?
sure, but could you please tell me where I can find the repo?
> 
>> The error message will be like this:
>> e2fsck 1.42.3 (14-May-2012)
>> Pass 1: Checking inodes, blocks, and sizes
>> Inode 12 has zero length extent
>> 	(invalid logical block 0, physical block 32775)
>> Clear? no
>>
>> Inode 12, i_blocks is 88, should be 0.  Fix? no
>>
>> Pass 2: Checking directory structure
>> Pass 3: Checking directory connectivity
>> Pass 4: Checking reference counts
>> Pass 5: Checking group summary information
>> Block bitmap differences:  -(8231--8232) -(32770--32778)
>> Fix? no
>>
>> Now actually the end_blk can be any value which is less than
>> u32, so make end_blk be the maximum value of u32.
>>
>> Cc: Theodore Ts'o <tytso@....edu>
>> Signed-off-by: Tao Ma <boyu.mt@...bao.com>
>> ---
>>  lib/ext2fs/extent.c |    4 +---
>>  1 files changed, 1 insertions(+), 3 deletions(-)
>>
>> diff --git a/lib/ext2fs/extent.c b/lib/ext2fs/extent.c
>> index eb096d6..e2815c2 100644
>> --- a/lib/ext2fs/extent.c
>> +++ b/lib/ext2fs/extent.c
>> @@ -253,9 +253,7 @@ extern errcode_t ext2fs_extent_open2(ext2_filsys fs, ext2_ino_t ino,
>>  		ext2fs_le16_to_cpu(eh->eh_entries);
>>  	handle->path[0].max_entries = ext2fs_le16_to_cpu(eh->eh_max);
>>  	handle->path[0].curr = 0;
>> -	handle->path[0].end_blk =
>> -		(EXT2_I_SIZE(handle->inode) + fs->blocksize - 1) >>
>> -		 EXT2_BLOCK_SIZE_BITS(fs->super);
> 
> Hm, so this picked the actual last block of the file, whereas
No, it doesn't. With fallocate(FALLOC_FL_KEEP_SIZE), we have no idea of
what is the last block until we iterate the last leaf ext4_extent.
> 
>> +	handle->path[0].end_blk = ((((unsigned long long) 1) << 32) - 1);
> 
> this gives it an upper bound... why is that ok?  It's been a long time since
> I looked at this code, but some explanation in the commit and in code
> comments would be helpful.
> 
> If end_blk can be any value less than u32, what is its purpose?
As I have mentioned above, now there is no way for us to tell the end
block of a file at the very beginning of ext2fs_extent_open2, so
actually any value less than u32 could be OK if we have a sparse file
while the last block is fallocated near the end of u32 logical block
offset. Actually path[0]->end_blk is only used when we have no idea of
the length of the last ext4_extent_idx. See ext2fs_extent_get.

if (path->left > 0) {
	ix++;
        newpath->end_blk = ext2fs_le32_to_cpu(ix->ei_block);
} else
	newpath->end_blk = path->end_blk;

Having said that, I have to admit that I didn't think of the case of
ext3 and I am not sure whether this change will affect it or not.

Thanks
Tao
> 
> -Eric
> 
>>  	handle->path[0].visit_num = 1;
>>  	handle->level = 0;
>>  	handle->magic = EXT2_ET_MAGIC_EXTENT_HANDLE;
> 
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