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

[David Sterba <dsterba@suse.cz>]

On Fri, Apr 05, 2019 at 09:18:24PM +0800, Qu Wenruo wrote:
> 
> 
> On 2019/4/5 下午8:09, David Sterba wrote:
> > On Thu, Apr 04, 2019 at 02:45:29PM +0800, 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 70606da440aa..8eb5b3576759 100644
> >> --- a/fs/btrfs/delayed-ref.h
> >> +++ b/fs/btrfs/delayed-ref.h
> >> @@ -176,6 +176,90 @@ 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 */
> >> +	u64 ref_root;
> >> +
> >> +	/* Inode who refers to this data extent */
> >> +	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.
> >> +	 */
> >> +	u64 offset;
> >> +};
> >> +
> >> +struct btrfs_tree_ref {
> >> +	/*
> >> +	 * Level of this tree block
> >> +	 *
> >> +	 * Shared for skinny (TREE_BLOCK_REF) and normal tree ref.
> >> +	 */
> >> +	int level;
> >> +
> >> +	/*
> >> +	 * Root who refers to this tree block.
> >> +	 *
> >> +	 * For TREE_BLOCK_REF (skinny metadata, either inline or keyed)
> >> +	 */
> >> +	u64 root;
> >> +
> >> +	/* For non-skinny metadata, no special member needed */
> >> +};
> >> +
> >> +struct btrfs_ref {
> > 
> > The structure name sounds a bit generic, but I think we can keep it
> > short. There are no other btrfs-specific references that could be
> > confused, there are 'backrefs', 'delayed-refs' all refering to the
> > b-tree references.
> > 
> >> +	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;
> > 
> > No bool bitfields please, wasn't this mentioned last time?
> > 
> Oh, I forgot that one.
> 
> Do I need to resend or just edit that commit in my github branch?

No need to resend, I'll edit that as it's a trivial change. I'll have
another look at the whole structure if the layout could be optimized,
eg. the enum takes 4 bytes but we'd be fine with a byte.