Re: [PATCH v2 06/10] btrfs: relocation: Use wrapper to replace open-coded edge linking

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

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On 2020/3/4 上午1:30, David Sterba wrote:
> On Mon, Mar 02, 2020 at 05:45:49PM +0800, Qu Wenruo wrote:
>> Since backref_edge is used to connect upper and lower backref nodes, and
>> need to access both nodes, some code can look pretty nasty:
>>
>> 		list_add_tail(&edge->list[LOWER], &cur->upper);
>>
>> The above code will link @cur to the LOWER side of the edge, while both
>> "LOWER" and "upper" words show up.
>> This can sometimes be very confusing for reader to grasp.
>>
>> This patch introduce a new wrapper, link_backref_edge(), to handle the
>> linking behavior.
>> Which also has extra ASSERT() to ensure caller won't pass wrong nodes
>> in.
>>
>> Also, this updates the comment of related lists of backref_node and
>> backref_edge, to make it more clear that each list points to what.
>>
>> Signed-off-by: Qu Wenruo <wqu@suse.com>
>> ---
>>  fs/btrfs/relocation.c | 53 ++++++++++++++++++++++++++++++-------------
>>  1 file changed, 37 insertions(+), 16 deletions(-)
>>
>> diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
>> index 04416489d87a..c76849409c81 100644
>> --- a/fs/btrfs/relocation.c
>> +++ b/fs/btrfs/relocation.c
>> @@ -91,10 +91,12 @@ struct backref_node {
>>  	u64 owner;
>>  	/* link to pending, changed or detached list */
>>  	struct list_head list;
>> -	/* list of upper level blocks reference this block */
>> +
>> +	/* List of upper level edges, which links this node to its parent(s) */
>>  	struct list_head upper;
>> -	/* list of child blocks in the cache */
>> +	/* List of lower level edges, which links this node to its child(ren) */
>>  	struct list_head lower;
>> +
>>  	/* NULL if this node is not tree root */
>>  	struct btrfs_root *root;
>>  	/* extent buffer got by COW the block */
>> @@ -123,17 +125,26 @@ struct backref_node {
>>  	unsigned int detached:1;
>>  };
>>  
>> +#define LOWER	0
>> +#define UPPER	1
>> +#define RELOCATION_RESERVED_NODES	256
>>  /*
>> - * present a block pointer in the backref cache
>> + * present an edge connecting upper and lower backref nodes.
>>   */
>>  struct backref_edge {
>> +	/*
>> +	 * list[LOWER] is linked to backref_node::upper of lower level node,
>> +	 * and list[UPPER] is linked to backref_node::lower of upper level node.
>> +	 *
>> +	 * Also, build_backref_tree() uses list[UPPER] for pending edges, before
>> +	 * linking list[UPPER] to its upper level nodes.
>> +	 */
>>  	struct list_head list[2];
>> +
>> +	/* Two related nodes */
>>  	struct backref_node *node[2];
>>  };
>>  
>> -#define LOWER	0
>> -#define UPPER	1
>> -#define RELOCATION_RESERVED_NODES	256
>>  
>>  struct backref_cache {
>>  	/* red black tree of all backref nodes in the cache */
>> @@ -332,6 +343,22 @@ static struct backref_edge *alloc_backref_edge(struct backref_cache *cache)
>>  	return edge;
>>  }
>>  
>> +#define		LINK_LOWER	(1 << 0)
>> +#define		LINK_UPPER	(1 << 1)
>> +static inline void link_backref_edge(struct backref_edge *edge,
>> +				     struct backref_node *lower,
>> +				     struct backref_node *upper,
>> +				     int link_which)
> 
> Again not a static inline, but plain static function.

Oh, I mixed my further plan with my current code...

It would be moved to backref.h, which have to be static inline.

Will remove the inline prefix in next version.

Thanks,
Qu
> 
>> +{
>> +	ASSERT(upper && lower && upper->level == lower->level + 1);
>> +	edge->node[LOWER] = lower;
>> +	edge->node[UPPER] = upper;
>> +	if (link_which & LINK_LOWER)
>> +		list_add_tail(&edge->list[LOWER], &lower->upper);
>> +	if (link_which & LINK_UPPER)
>> +		list_add_tail(&edge->list[UPPER], &upper->lower);
>> +}


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