On 2019/2/11 下午8:55, Nikolay Borisov wrote:
>
>
> 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 <w...@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.
It's cryptic due to the EXTENT_ITEM design from the very beginning.
I'm all ears to improve this description.
> 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.
Don't forget reflink and data CoW.
Like this:
item 6 key (257 EXTENT_DATA 0) itemoff 15813 itemsize 53
generation 6 type 1 (regular)
extent data disk byte 13631488 nr 1048576
extent data offset 0 nr 4096 ram 1048576
item 7 key (257 EXTENT_DATA 4096) itemoff 15760 itemsize 53
generation 7 type 1 (regular)
extent data disk byte 14680064 nr 4096
extent data offset 0 nr 4096 ram 4096
item 8 key (257 EXTENT_DATA 8192) itemoff 15707 itemsize 53
generation 6 type 1 (regular)
extent data disk byte 13631488 nr 1048576
extent data offset 8192 nr 1040384 ram 1048576
EXTENT_DATA items at 0 and 8K offset are original from one larger
extent, EXTENT_DATA item at 4K offset is newly written one.
But the current design makes EXTENT_ITEM inline data backref pretty clean:
item 0 key (13631488 EXTENT_ITEM 1048576) itemoff 16230 itemsize 53
refs 2 gen 6 flags DATA
extent data backref root FS_TREE objectid 257 offset 0
count 2
No need for an extra inline data backref, just increase the original
count from 1 to 2.
>
> 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?
As long as there is some new DATA_EXTENT pointing to the original
unchanged extent, then yes, the 'offset' will change.
Just like the EXTENT_DATA at 8K offset above.
>
>> + 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)?
This is for any keyed or inlined tree ref who uses skinny metadata
(level stored in key.offset, the common case now) or non-skinny
EXTENT_ITEM who uses btrfs_tree_block_info like:
item 7 key (30507008 EXTENT_ITEM 16384) itemoff 15956 itemsize 51
refs 1 gen 4 flags TREE_BLOCK
tree block key (0 UNKNOWN.0 0) level 0 <<< here.
tree block backref root UUID_TREE
It's possible for extent tree to not include above cases, like the
following case:
item 1 key (12648448 EXTENT_ITEM 16384) itemoff 16235 itemsize 24
refs 9 gen 7 flags TREE_BLOCK
item 2 key (12648448 SHARED_BLOCK_REF 4481024) itemoff 3461
itemsize 0
shared block backref
So I'm not sure how to describe such case clearly.
Thanks,
Qu
>
>> + */
>> + 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)
>> {
>>
