On Fri, Sep 23, 2016 at 01:44:44PM -0700, Liu Bo wrote:
> @@ -3732,15 +3734,21 @@ static noinline_for_stack int write_one_eb(struct 
> extent_buffer *eb,
>       if (btrfs_header_owner(eb) == BTRFS_TREE_LOG_OBJECTID)
>               bio_flags = EXTENT_BIO_TREE_LOG;
>  
> -     /* set btree node beyond nritems with 0 to avoid stale content */
> +     /* set btree blocks beyond nritems with 0 to avoid stale content. */
> +     nritems = btrfs_header_nritems(eb);
>       if (btrfs_header_level(eb) > 0) {
> -             u32 nritems;
> -             unsigned long end;
> -
> -             nritems = btrfs_header_nritems(eb);
>               end = btrfs_node_key_ptr_offset(nritems);
>  
>               memset_extent_buffer(eb, 0, end, eb->len - end);
> +     } else {
> +             /*
> +              * leaf:
> +              * header 0 1 2 .. N ... data_N .. data_2 data_1 data_0
> +              */
> +             start = btrfs_item_nr_offset(nritems);

So, AFAICS this also works for empty root nodes (nritems == 0). It's
hidden in leaf_data_end that actually looks at nritems and decides.

Reviewed-by: David Sterba <dste...@suse.com>

> +             end = btrfs_leaf_data(eb) +
> +                   leaf_data_end(fs_info->tree_root, eb);
> +             memset_extent_buffer(eb, 0, start, end - start);
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