On 28.11.18 г. 10:54 ч., Johannes Thumshirn wrote:
> Document why map_private_extent_buffer() cannot return '1' (i.e. the map
> spans two pages) for the csum_tree_block() case.
>
> The current algorithm for detecting a page boundary crossing in
> map_private_extent_buffer() will return a '1' *IFF* the extent buffer's
> offset in the page + the offset passed in by csum_tree_block() and the
> minimal length passed in by csum_tree_block() - 1 are bigger than
> PAGE_SIZE.
>
> We always pass BTRFS_CSUM_SIZE (32) as offset and a minimal length of 32
> and the current extent buffer allocator always guarantees page aligned
> extends, so the above condition can't be true.
>
> Signed-off-by: Johannes Thumshirn <jthumshirn@xxxxxxx>
Reviewed-by: Nikolay Borisov <nborisov@xxxxxxxx>
>
> ---
> Changes to v1:
> * Changed wording of the commit message according to Noah's suggestion
> ---
> fs/btrfs/disk-io.c | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
> index 4bc270ef29b4..14d355d0cb7a 100644
> --- a/fs/btrfs/disk-io.c
> +++ b/fs/btrfs/disk-io.c
> @@ -279,6 +279,12 @@ static int csum_tree_block(struct btrfs_fs_info *fs_info,
>
> len = buf->len - offset;
> while (len > 0) {
> + /*
> + * Note: we don't need to check for the err == 1 case here, as
> + * with the given combination of 'start = BTRFS_CSUM_SIZE (32)'
> + * and 'min_len = 32' and the currently implemented mapping
> + * algorithm we cannot cross a page boundary.
> + */
> err = map_private_extent_buffer(buf, offset, 32,
> &kaddr, &map_start, &map_len);
> if (err)
>
