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

[Stefan Bader]

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.