Re: [PATCH 4/6 v5] ext4: Correct large hole offset calcuation

[Eric Sandeen <sandeen@redhat.com>]

On 8/20/11 9:29 PM, Allison Henderson wrote:
> This bug was reported by Lukas Czerner while working on a
> new patch to add discard support for loop devices using
> punch hole.
> 
> The bug is happens because the data type for logical blocks is

s/is //

> not large enough to calculate the block offset for holes that are

Should that be "the _byte_ offset for holes ...?"

> very large.  This bug is resolved by casting the ext4_lblk_t
> to an loff_t before calculating the byte offset of the block.
> 
> Reviewed-and-Tested-by: Lukas Czerner <lczerner@redhat.com>
> 
> Signed-off-by: Allison Henderson <achender@linux.vnet.ibm.com>


I wonder if it'd be more straightforward to just use a mask
to compute these rather than the right shift / left shift, but
no big deal I guess esp. since we need the intermediate value
anyway?

Aside from the question about the commit message,

Reviewed-by: Eric Sandeen <sandeen@redhat.com>

> ---
> :100644 100644 0d7617d... b417e47... M	fs/ext4/extents.c
>  fs/ext4/extents.c |    4 ++--
>  1 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index 0d7617d..b417e47 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -4179,8 +4179,8 @@ int ext4_ext_punch_hole(struct file *file, loff_t offset, loff_t length)
>  		EXT4_BLOCK_SIZE_BITS(sb);
>  	last_block = (offset + length) >> EXT4_BLOCK_SIZE_BITS(sb);
>  
> -	first_block_offset = first_block << EXT4_BLOCK_SIZE_BITS(sb);
> -	last_block_offset = last_block << EXT4_BLOCK_SIZE_BITS(sb);
> +	first_block_offset = ((loff_t)first_block) << EXT4_BLOCK_SIZE_BITS(sb);
> +	last_block_offset = ((loff_t)last_block) << EXT4_BLOCK_SIZE_BITS(sb);
>  
>  	first_page = (offset + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;
>  	last_page = (offset + length) >> PAGE_CACHE_SHIFT;