On 2018/12/10 下午5:48, Nikolay Borisov wrote:
>
>
> 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 <w...@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.
Forgot that.
>
>> + 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
Well, the @ref_root is a little confusing.
Here it could mean "the current btrfs_header_owner(eb)".
I don't really know the correct term here.
>
>> + * @ino: inode number of the owner.
>
> And the owner is really the owning inode?
Yes, the owning inode.
>
>> + * @offset: *CALCULATED* offset. Not EXTENT_DATA key offset.
>
> What does calculated mean here?
Because btrfs data backref instead of using the offset of the owning
EXTENT_DATA key, it uses key->offset - extent_offset.
>
>> + *
>> + */
>
> Ugh, this is ugly, why not have a single /* */ line above each member
> and document them like that?
Because the complexity of @ref_root can't really go one line.
>
>> + 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?
@use_backreferences looks even stranger to me.
Here the point is, if this bit set, all backref will use
SHARED_BLOCK_REF or SHARED_DATA_REF subtype, and just leave a pointer to
its parent.
Any good idea for explaining this?
> Also please use explicit bool type:
>
> bool xxxx:1
Is this valid? Haven't seen such usage in kernel code IIRC.
>
>> +
>> + /*
>> + * 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.
Makes sense.
Thanks,
Qu
>
>> + 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)
>> {
>>
