Btrfs devel - Oct 2007 - A question about extent buffer codes

Hello

Some xxx_extent_buffer functions have codes like:
-- 
offset = dst_offset & ((unsigned long)PAGE_CACHE_SIZE - 1);
if (i == 0)
	offset += start_offset;

I can't understand why these is a test.  I feel the variable
'offset'
should be computed by code like:
--
 offset = (dst_offset + start_offset) & ((unsigned long)PAGE_CACHE_SIZE -
1);

Thanks
YZ

On Tue, 2007-10-16 at 20:32 +0800, Yan Zheng wrote:
> Hello
> 
> Some xxx_extent_buffer functions have codes like:
> -- 
> offset = dst_offset & ((unsigned long)PAGE_CACHE_SIZE - 1);
> if (i == 0)
>         offset += start_offset;
> 
> I can't understand why these is a test.  I feel the variable
'offset'
> should be computed by code like:
> --
>  offset = (dst_offset + start_offset) & ((unsigned
> long)PAGE_CACHE_SIZE - 1);

It could be offset = (dst->start + dst_offset) & PAGE_CACHE_SIZE - 1,
but start_offset is already the offset into the first page.  So, we only
want to add it on the first page.

-chris