Re: [PATCH v3 1/9] btrfs: delayed-ref: Introduce better documented delayed ref structures

[Nikolay Borisov <nborisov@suse.com>]

On 11.02.19 г. 7:16 ч., Qu Wenruo wrote:
> Current delayed ref interface has several problems:
> - Longer and longer parameter lists
>   bytenr
>   num_bytes
>   parent
>   ---------- so far so good
>   ref_root
>   owner
>   offset
>   ---------- I don't feel good now
> 
> - Different interpretation for the same parameter
>   Above @owner for data ref is inode number (u64),
>   while for tree ref, it's level (int).
> 
>   They are even in different size range.
>   For level we only need 0~8, while for ino it's
>   BTRFS_FIRST_FREE_OBJECTID~BTRFS_LAST_FREE_OBJECTID.
> 
>   And @offset doesn't even makes sense for tree ref.
> 
>   Such parameter reuse may look clever as an hidden union, but it
>   destroys code readability.
> 
> To solve both problems, we introduce a new structure, btrfs_ref to solve
> them:
> 
> - Structure instead of long parameter list
>   This makes later expansion easier, and better documented.
> 
> - Use btrfs_ref::type to distinguish data and tree ref
> 
> - Use proper union to store data/tree ref specific structures.
> 
> - Use separate functions to fill data/tree ref data, with a common generic
>   function to fill common bytenr/num_bytes members.
> 
> All parameters will find its place in btrfs_ref, and an extra member,
> @real_root, inspired by ref-verify code, is newly introduced for later
> qgroup code, to record which tree is triggered this extent modification.
> 
> This patch doesn't touch any code, but provides the basis for incoming
> refactors.
> 
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> ---
>  fs/btrfs/delayed-ref.h | 116 +++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 116 insertions(+)
> 
> diff --git a/fs/btrfs/delayed-ref.h b/fs/btrfs/delayed-ref.h
> index d2af974f68a1..24addc5163bc 100644
> --- a/fs/btrfs/delayed-ref.h
> +++ b/fs/btrfs/delayed-ref.h
> @@ -187,6 +187,90 @@ struct btrfs_delayed_ref_root {
>  	u64 qgroup_to_skip;
>  };
>  
> +enum btrfs_ref_type {
> +	BTRFS_REF_NOT_SET,
> +	BTRFS_REF_DATA,
> +	BTRFS_REF_METADATA,
> +	BTRFS_REF_LAST,
> +};
> +
> +struct btrfs_data_ref {
> +	/* For EXTENT_DATA_REF */
> +
> +	/* Root who refers to this data extent */
nit: s/who/which/
> +	u64 ref_root;
> +
> +	/* Inode who refers to this data extent */
nit: DITTO
> +	u64 ino;
> +
> +	/*
> +	 * file_offset - extent_offset
> +	 *
> +	 * file_offset is the key.offset of the EXTENT_DATA key.
> +	 * extent_offset is btrfs_file_extent_offset() of the EXTENT_DATA data.
> +	 */

This needs rewording since it's rather cryptic now. Looking at the dev
docs and the description for 'offset' field in btrfs_file_extent_item I
can sort of deduce that this field will only be different than null if
this reference is for an extent which is shared between 2 snapshots.

So if file foo is shared between two snapshots, has 1 extent and in
snapshot2 this extent is partially changed then I'd expect extent_offset
to point to the start in the original (unchanged extent), correct?

> +	u64 offset;
> +};
> +
> +struct btrfs_tree_ref {
> +	/*
> +	 * Level of this tree block
> +	 *
> +	 * Shared for skinny (TREE_BLOCK_REF) and normal tree ref.

This sentence is also not very clear? You mean this level applies to
tree block refs (irrespective of whether they are shared or normal tree
block refs)?

> +	 */
> +	int level;
> +
> +	/*
> +	 * Root who refers to this tree block.

nit:s/who/which

> +	 *
> +	 * For TREE_BLOCK_REF (skinny metadata, either inline or keyed)
> +	 */
> +	u64 root;
> +
> +	/* For non-skinny metadata, no special member needed */
> +};
> +
> +struct btrfs_ref {
> +	enum btrfs_ref_type type;
> +	int action;
> +
> +	/*
> +	 * Only use parent pointers as backref (SHARED_BLOCK_REF or
> +	 * SHARED_DATA_REF) for this extent and its children.
> +	 * Set for reloc trees.
> +	 */
> +	bool only_backreferences:1;
> +
> +	/*
> +	 * Whether this extent should go through qgroup record.
> +	 *
> +	 * Normally false, but for certain case like delayed subtree scan,
> +	 * setting this flag can hugely reduce qgroup overhead.
> +	 */
> +	bool skip_qgroup:1;
> +
> +	/*
> +	 * Optional. To which root this modification is for.
> +	 * Mostly used for qgroup optimization.
> +	 *
> +	 * When unset, data/tree ref init code will populate it.
> +	 * In certain case, we're modifying reference for a different root.
> +	 * E.g. Cow fs tree blocks for balance.
> +	 * In that case, tree_ref::root will be fs tree, but we're doing this
> +	 * for reloc tree, then we should set @real_root to reloc tree.
> +	 */
> +	u64 real_root;
> +	u64 bytenr;
> +	u64 len;
> +
> +	/* Bytenr of the parent tree block */
> +	u64 parent;
> +	union {
> +		struct btrfs_data_ref data_ref;
> +		struct btrfs_tree_ref tree_ref;
> +	};
> +};
> +
>  extern struct kmem_cache *btrfs_delayed_ref_head_cachep;
>  extern struct kmem_cache *btrfs_delayed_tree_ref_cachep;
>  extern struct kmem_cache *btrfs_delayed_data_ref_cachep;
> @@ -195,6 +279,38 @@ extern struct kmem_cache *btrfs_delayed_extent_op_cachep;
>  int __init btrfs_delayed_ref_init(void);
>  void __cold btrfs_delayed_ref_exit(void);
>  
> +static inline void btrfs_init_generic_ref(struct btrfs_ref *generic_ref,
> +				int action, u64 bytenr, u64 len, u64 parent)
> +{
> +	generic_ref->action = action;
> +	generic_ref->bytenr = bytenr;
> +	generic_ref->len = len;
> +	generic_ref->parent = parent;
> +}
> +
> +static inline void btrfs_init_tree_ref(struct btrfs_ref *generic_ref,
> +				int level, u64 root)
> +{
> +	/* If @real_root not set, use @root as fallback */
> +	if (!generic_ref->real_root)
> +		generic_ref->real_root = root;
> +	generic_ref->tree_ref.level = level;
> +	generic_ref->tree_ref.root = root;
> +	generic_ref->type = BTRFS_REF_METADATA;
> +}
> +
> +static inline void btrfs_init_data_ref(struct btrfs_ref *generic_ref,
> +				u64 ref_root, u64 ino, u64 offset)
> +{
> +	/* If @real_root not set, use @root as fallback */
> +	if (!generic_ref->real_root)
> +		generic_ref->real_root = ref_root;
> +	generic_ref->data_ref.ref_root = ref_root;
> +	generic_ref->data_ref.ino = ino;
> +	generic_ref->data_ref.offset = offset;
> +	generic_ref->type = BTRFS_REF_DATA;
> +}
> +
>  static inline struct btrfs_delayed_extent_op *
>  btrfs_alloc_delayed_extent_op(void)
>  {
>