[PATCH] ext4: correctly calculate number of blocks for fiemap

[Brad Figg]

On 04/01/2010 02:37 AM, Stefan Bader wrote:
> Ok, looks good to me now.
>
> Surbhi Palande wrote:
>> The following cherry-picked patch fixes the calculation of extents in fiemap.
>> Without this patch a user of fiemap could be misled into thinking that there
>> are no extents in a file when actually there are. This patch is necessary for
>> code such as ureadahead which uses fiemap to find out if the file extents have
>> increased.
>>
>> The commit id in the upstream code is: aca92ff6f57c000d1b4523e383c8bd6b8269b8b1
>> This patch fixes bug 474597 on LP. Do consider merging this in Lucid.
>>
>>  From dfaa33f0502a8bf9dff0e596783a40ce330c7a9b Mon Sep 17 00:00:00 2001
>> From: Leonard Michlmayr<leonard.michlmayr at gmail.com>
>> Date: Thu, 4 Mar 2010 17:07:28 -0500
>> Subject: [PATCH] ext4: correctly calculate number of blocks for fiemap
>>
>> ext4_fiemap() rounds the length of the requested range down to
>> blocksize, which is is not the true number of blocks that cover the
>> requested region.  This problem is especially impressive if the user
>> requests only the first byte of a file: not a single extent will be
>> reported.
>>
>> We fix this by calculating the last block of the region and then
>> subtract to find the number of blocks in the extents.
>>
>> Signed-off-by: Leonard Michlmayr<leonard.michlmayr at gmail.com>
>> Signed-off-by: "Theodore Ts'o"<tytso at mit.edu>
>> (cherry picked from commit aca92ff6f57c000d1b4523e383c8bd6b8269b8b1 upstream)
>>
>> BugLink: http://launchpad.net/bugs/474597
>> Signed-off-by: Surbhi Palande<surbhi.palande at canonical.com>
> Acked-by: Stefan Bader<stefan.bader at canonical.com>
>> ---
>>   fs/ext4/extents.c |    9 +++++++--
>>   1 files changed, 7 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
>> index 8b8bae4..f199d20 100644
>> --- a/fs/ext4/extents.c
>> +++ b/fs/ext4/extents.c
>> @@ -3711,7 +3711,6 @@ int ext4_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
>>   		__u64 start, __u64 len)
>>   {
>>   	ext4_lblk_t start_blk;
>> -	ext4_lblk_t len_blks;
>>   	int error = 0;
>>
>>   	/* fallback to generic here if not in extents fmt */
>> @@ -3725,8 +3724,14 @@ int ext4_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
>>   	if (fieinfo->fi_flags&  FIEMAP_FLAG_XATTR) {
>>   		error = ext4_xattr_fiemap(inode, fieinfo);
>>   	} else {
>> +		ext4_lblk_t len_blks;
>> +		__u64 last_blk;
>> +
>>   		start_blk = start>>  inode->i_sb->s_blocksize_bits;
>> -		len_blks = len>>  inode->i_sb->s_blocksize_bits;
>> +		last_blk = (start + len - 1)>>  inode->i_sb->s_blocksize_bits;
>> +		if (last_blk>= EXT_MAX_BLOCK)
>> +			last_blk = EXT_MAX_BLOCK-1;
>> +		len_blks = ((ext4_lblk_t) last_blk) - start_blk + 1;
>>
>>   		/*
>>   		 * Walk the extent tree gathering extent information.
>
>

Maybe I'm misreading this bug according to
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/474597/comments/7
ted applied a different patch to fix this issue.

Brad
-- 
Brad Figg brad.figg at canonical.com http://www.canonical.com