On 18.05.2018 16:12, Nikolay Borisov wrote:
> Here is a doc-only patch which tires to deobfuscate the terra-incognita
> that arguments for delayed refs are.
> 
> Signed-off-by: Nikolay Borisov <nbori...@suse.com>
> ---
> Hello, 
> 
> This patch needs revieweing since I'm not entirely sure I managed to capture 
> the semantics of the "parent" and "owner" arguments. Specifically, parent is 
> passed "ref->parent" only if we have a shared block, however looking at the 
> code where parent is passed to the add delayed tree/data refs, it seems that 
> parent is set to the eb->Start only if the tree where this extent comes from 
> is 
> the data_reloc_tree, i.e the code in 
> replace_path/replace_file_extents/do_relocation
> __btrfs_cow_block.
> 
> For the "owner" argument in case of data extents it'set to the ino, for 
> metadata
> extents it's a bit trickier, what I think it always contains for such extents 
> is the level of the parent block in the tree. 
> 
> 
> If this function is documented correctly then it wil be fairly trivial to 
> document btrfs_add_delayed(tree|data)_ref ones as well. 


Ping

> 
>  fs/btrfs/extent-tree.c | 29 +++++++++++++++++++++++++++++
>  1 file changed, 29 insertions(+)
> 
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index 2ce32f05812f..5a2f4a86dc71 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -2207,6 +2207,35 @@ int btrfs_inc_extent_ref(struct btrfs_trans_handle 
> *trans,
>       return ret;
>  }
>  
> +/*
> + * __btrfs_inc_extent_ref - insert backreference for a given extent
> + *
> + * @trans:       Handle of transaction
> + *
> + * @node:        The delayed ref node used to get the bytenr/length for
> + *               extent
> + *
> + * @parent:      If this is a shared extent (BTRFS_SHARED_DATA_REF_KEY/
> + *               BTRFS_SHARED_BLOCK_REF_KEY) then parent *may* hold the
> + *               logical bytenr of the parent block.
> + *
> + * @root_objectid: The id of the root where this modification has originated,
> + *               this can be either one of the well-known metadata trees or
> + *               the subvolume id which references this extent.
> + *
> + * @owner:       For data extents it is the inode number of the owning file.
> + *               For metadata extents this parameter holds the level in the
> + *               tree of the parent block.
> + *
> + * @offset:      For metadata extents this is always 0. For data extents it
> + *               is the fileoffset this extent belongs to.
> + *
> + * @refs_to_add    Number of references to add
> + *
> + * @extent_op      Pointer to a structure, holding information necessary when
> + *                 updating a tree block's flags
> + *
> + */
>  static int __btrfs_inc_extent_ref(struct btrfs_trans_handle *trans,
>                                 struct btrfs_delayed_ref_node *node,
>                                 u64 parent, u64 root_objectid,
> 
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