Re: [PATCH 1/8] btrfs: delayed-ref: Introduce better documented delayed ref structures

From: Nikolay Borisov <nborisov@suse.com>
To: Qu Wenruo <wqu@suse.com>, linux-btrfs@vger.kernel.org
Subject: Re: [PATCH 1/8] btrfs: delayed-ref: Introduce better documented delayed ref structures
Date: Mon, 10 Dec 2018 11:48:50 +0200	[thread overview]
Message-ID: <235ca34f-bcda-128d-dc3d-7465766271c7@suse.com> (raw)
In-Reply-To: <20181206065903.11343-2-wqu@suse.com>

On 6.12.18 г. 8:58 ч., Qu Wenruo wrote:
> Current delayed ref interface has several problems:
> - Longer and longer parameter lists
>   bytenr
>   num_bytes
>   parent
>   ref_root
>   owner
>   offset
>   for_reloc << Only qgroup code cares.
> 
> - Different interpretation for the same parameter
>   Above @owner for data ref is ino owning this extent,
>   while for tree ref, it's level. 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 | 109 +++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 109 insertions(+)
> 
> diff --git a/fs/btrfs/delayed-ref.h b/fs/btrfs/delayed-ref.h
> index d8fa12d3f2cc..e36d6b05d85e 100644
> --- a/fs/btrfs/delayed-ref.h
> +++ b/fs/btrfs/delayed-ref.h
> @@ -176,6 +176,81 @@ struct btrfs_delayed_ref_root {
>  	u64 qgroup_to_skip;
>  };
>  
> +enum btrfs_ref_type {
> +	BTRFS_REF_NOT_SET = 0,

No need for explicit initialisation when your enums start from 0.

> +	BTRFS_REF_DATA,
> +	BTRFS_REF_METADATA,
> +	BTRFS_REF_LAST,
> +};
> +
> +struct btrfs_data_ref {
> +	/*
> +	 * For EXTENT_DATA_REF
> +	 *
> +	 * @ref_root:	current owner of the extent.
> +	 * 		may differ from btrfs_ref::real_root.

Here 'owner' is a bit ambiguous. Isn't this the root of the owner

> +	 * @ino: 	inode number of the owner. 

And the owner is really the owning inode?

> +	 * @offset:	*CALCULATED* offset. Not EXTENT_DATA key offset.

What does calculated mean here?

> +	 *
> +	 */

Ugh, this is ugly, why not have a single /* */ line above each member
and document them like that?

> +	u64 ref_root;
> +	u64 ino;
> +	u64 offset;
> +};
> +
> +struct btrfs_tree_ref {
> +	/* Common for all sub types and skinny combination */
> +	int level;

Like you've done here. Mention that this is the level in the tree that
this reference is located at.

> +
> +	/*
> +	 * For TREE_BLOCK_REF (skinny metadata, either inline or keyed)
> +	 *
> +	 * root here may differ from btrfs_ref::real_root.
> +	 */

Make it refer to the documentation of the generic btrfs_ref structure
because if someone reads this comment and doesn't read the one in
btrfs_ref then it's not clear under what conditions they might differ.

> +	u64 root;
> +
> +	/* For non-skinny metadata, no special member needed */
> +};
> +
> +struct btrfs_ref {
> +	enum btrfs_ref_type type;
> +	int action;
> +
> +	/*
> +	 * Use full backref(SHARED_BLOCK_REF or SHARED_DATA_REF) for this
> +	 * extent and its children.
> +	 * Set for reloc trees.
> +	 */
> +	unsigned int use_fullback:1;

'fullback' is too terse, use_backreferences or something conveying more
information? Also please use explicit bool type:

bool xxxx:1

> +
> +	/*
> +	 * Whether this extent should go through qgroup record.
> +	 * Normally false, but for certain case like delayed subtree scan,
> +	 * this can hugely reduce qgroup overhead.
> +	 */
> +	unsigned int skip_qgroup:1;

again, explicit bool type please.

> +
> +	/*
> +	 * Who owns this reference modification, optional.
> +	 *
> +	 * One example:
> +	 * When creating reloc tree for source fs, it will increase tree block
> +	 * ref for children tree blocks.
> +	 * In that case, btrfs_ref::real_root = reloc tree,
> +	 * while btrfs_ref::tree_ref::root = fs tree.
> +	 */
> +	u64 real_root;
> +	u64 bytenr;
> +	u64 len;
> +
> +	/* Common @parent for SHARED_DATA_REF/SHARED_BLOCK_REF */

OK, it's common but what does it really contain? Either document it or
rename it to parent_block or something.

> +	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;
> @@ -184,6 +259,40 @@ 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 real_root,
> +				u64 parent)
> +{
> +	generic_ref->action = action;

IMO it makes sense in this series to have a patch which converts the
action defines to an enum and subsequently modify functions/structs to
actually be of enum type.

> +	generic_ref->bytenr = bytenr;
> +	generic_ref->len = len;
> +	generic_ref->real_root = real_root;
> +	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)
>  {
> 