Re: [RFC] btrfs: Don't return extent in fiemap if we meet with a hole.

["jeff.liu" <jeff.liu@oracle.com>]

Tao Ma wrote:
> Recently, my colleague Jeff tried to add fiemap support to cp(1).
> http://www.mail-archive.com/bug-coreutils@gnu.org/msg19987.html
> 
> He just meet with a strange issue with following command:
> dd if=/dev/null of=/btrfs/sparse bs=1 seek=4096
> When we use fiemap to the file, btrfs returns an extent with len '4096'
> and flag 'unwritten' while actually there is no data allocated.
> 
> I just dived into this and to my surprise, it is done by btrfs
> intentionally. I checked other file systems which support fiemap.
> Actually with the file created by the script, ocfs2, ext3/4 and
> xfs all return zero extent. And according to the documentation file
> Documentation/filesystems/fiemap.txt, FIEMAP_EXTENT_UNWRITTEN should
> be used when the extent is allocated but it's data has not been
> initialized. So I think btrfs should work like other filesystems.
> 
> Cc: Jeff Liu <jeff.liu@oracle.com>
> Cc: Chris Mason <chris.mason@oracle.com>
> Signed-off-by: Tao Ma <tao.ma@oracle.com>
> ---
>  fs/btrfs/extent_io.c |   15 +++++++++------
>  1 files changed, 9 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> index b177ed3..7eb4d77 100644
> --- a/fs/btrfs/extent_io.c
> +++ b/fs/btrfs/extent_io.c
> @@ -2951,7 +2951,7 @@ int extent_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
>  	u32 flags = 0;
>  	u64 disko = 0;
>  	struct extent_map *em = NULL;
> -	int end = 0;
> +	int end = 0, hole = 0;
>  	u64 em_start = 0, em_len = 0;
>  	unsigned long emflags;
>  	ret = 0;
> @@ -2978,12 +2978,13 @@ int extent_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
>  
>  		disko = 0;
>  		flags = 0;
> +		hole = 0;
>  
>  		if (em->block_start == EXTENT_MAP_LAST_BYTE) {
>  			end = 1;
>  			flags |= FIEMAP_EXTENT_LAST;
>  		} else if (em->block_start == EXTENT_MAP_HOLE) {
> -			flags |= FIEMAP_EXTENT_UNWRITTEN;
> +			hole = 1;
>  		} else if (em->block_start == EXTENT_MAP_INLINE) {
>  			flags |= (FIEMAP_EXTENT_DATA_INLINE |
>  				  FIEMAP_EXTENT_NOT_ALIGNED);
> @@ -3015,10 +3016,12 @@ int extent_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
>  			end = 1;
>  		}
>  
> -		ret = fiemap_fill_next_extent(fieinfo, em_start, disko,
> -					em_len, flags);
> -		if (ret)
> -			goto out_free;
> +		if (!hole) {
> +			ret = fiemap_fill_next_extent(fieinfo, em_start, disko,
> +						      em_len, flags);
> +			if (ret)
> +				goto out_free;
> +		}
>  	}
>  out_free:
>  	free_extent_map(em);

I have just tried a simple test against this patch, it works for me.


Thanks,
-Jeff