Re: [PATCH 3/4] maple_tree: use the correct min to calculate split

["Liam R. Howlett" <Liam.Howlett@oracle.com>]

* Wei Yang <richard.weiyang@gmail.com> [241019 22:46]:
> The check in mab_calc_split() is to make sure the gap between
> [0, split] won't be too small. But we don't pass the correct min.
> 
> First we may encounter a pivot[split] smaller than min. For example:
> 
>        mt_init_flags(mt, 0);
>        for (count = 0; count <= 240; count++) {
>                mas_set(&mas, count);
>                mas_store(&mas, xa_mk_value(count));
>        }
> 
> On splitting root for storing 240, the pivot[split] is smaller than min.
> This result a huge (pivot[split] - min).

This is fine.

There is an open work item to make it more accurate at higher levels,
but it's not a problem as it is.

Each level upwards needs a better 'minimum span', meaning that the node
should have at least mas.min - mas.min + level * something.

It works today for leaves, somewhat.

> 
> Second prev_l_mas.min is not initialized for the first iteration. This
> means on splitting leaf node, this value is mostly taking no effect.

No, it is set to 0.  Not initialized would cause random data loss.

See MA_STATE() in the header.

> 
> Since we are now calculating the split of mas->node, we should use the
> mas->min instead of prev_l_mas.min.

This sounds reasonable, but considering what this number is used for, I
don't see how it is working as well as it is today.  I will need to look
deeper into this.

> 
> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
> CC: Liam R. Howlett <Liam.Howlett@Oracle.com>
> CC: Sidhartha Kumar <sidhartha.kumar@oracle.com>
> CC: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> ---
>  lib/maple_tree.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/lib/maple_tree.c b/lib/maple_tree.c
> index 894dc5e9436e..c2d4b188646c 100644
> --- a/lib/maple_tree.c
> +++ b/lib/maple_tree.c
> @@ -3357,7 +3357,7 @@ static void mas_split(struct ma_state *mas, struct maple_big_node *b_node)
>  		if (mas_push_data(mas, height, &mast, false))
>  			break;
>  
> -		split = mab_calc_split(mas, b_node, &mid_split, prev_l_mas.min);
> +		split = mab_calc_split(mas, b_node, &mid_split, mas->min);
>  		mast_split_data(&mast, mas, split);
>  		/*
>  		 * Usually correct, mab_mas_cp in the above call overwrites
> -- 
> 2.34.1
>