[PATCH v3 4/5] maple_tree: refine mas_store_root() on storing NULL

[Liam R. Howlett]

* Wei Yang <richard.weiyang at gmail.com> [241017 22:40]:
> Currently, when storing NULL on mas_store_root(), the behavior could be
> improved.
> 
> For example possible cases are:
> 
>   * store NULL at any range result a new node
>   * store NULL at range [m, n] where m > 0 to a single entry tree result
>     a new node with range [m, n] set to NULL
>   * store NULL at range [m, n] where m > 0 to an empty tree result
>     consecutive NULL slot
> 
> This patch tries to improve in:
> 
>   * memory efficient by setting to empty tree instead of using a node

>   * remove the possibility of consecutive NULL slot which will prohibit
>     extended null in later operation

I don't understand this.  Do we actually store consecutive NULLs now?

This is a very odd change log for fixing an optimisation.  Maybe start
by explaining how we end up with a node with a single value now, then
state how this code changes that?

> 
> Signed-off-by: Wei Yang <richard.weiyang at gmail.com>
> CC: Liam R. Howlett <Liam.Howlett at Oracle.com>
> CC: Sidhartha Kumar <sidhartha.kumar at oracle.com>
> CC: Lorenzo Stoakes <lorenzo.stoakes at oracle.com>
> 
> ---
> v3: move change into mas_store_root()
> ---
>  lib/maple_tree.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/maple_tree.c b/lib/maple_tree.c
> index db8b89487c98..03fbee9880eb 100644
> --- a/lib/maple_tree.c
> +++ b/lib/maple_tree.c
> @@ -3439,7 +3439,11 @@ static inline void mas_root_expand(struct ma_state *mas, void *entry)
>  
>  static inline void mas_store_root(struct ma_state *mas, void *entry)
>  {
> -	if (likely((mas->last != 0) || (mas->index != 0)))
> +	if (!entry) {
> +		void *contents = mas_root_locked(mas);
> +
> +		if (!mas->index && contents)
> +			rcu_assign_pointer(mas->tree->ma_root, NULL);

You are changing what used to handle any range that wasn't 0 to handle
storing NULL.

This seems really broken.

> +	} else if (likely((mas->last != 0) || (mas->index != 0)))

Isn't this exactly what you have above in the if statement?

>  		mas_root_expand(mas, entry);
>  	else if (((unsigned long) (entry) & 3) == 2)
>  		mas_root_expand(mas, entry);
> -- 
> 2.34.1
>