linux-kernel - Re: [PATCH 04/10] maple_tree: Simplify mas_is_span

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Message-ID: <20230515160644.lqhrclw3kcutsafc@revolver>
Date:   Mon, 15 May 2023 12:06:44 -0400
From:   "Liam R. Howlett" <Liam.Howlett@...cle.com>
To:     Peng Zhang <zhangpeng.00@...edance.com>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, maple-tree@...ts.infradead.org
Subject: Re: [PATCH 04/10] maple_tree: Simplify mas_is_span_wr()

* Peng Zhang <zhangpeng.00@...edance.com> [230515 09:18]:
> Make the code for detecting spanning writes more concise.
> 
> Signed-off-by: Peng Zhang <zhangpeng.00@...edance.com>
> ---
>  lib/maple_tree.c | 36 +++++++++---------------------------
>  1 file changed, 9 insertions(+), 27 deletions(-)
> 
> diff --git a/lib/maple_tree.c b/lib/maple_tree.c
> index 43a25d3042c1b..fbb6efc40e576 100644
> --- a/lib/maple_tree.c
> +++ b/lib/maple_tree.c
> @@ -3728,41 +3728,23 @@ static bool mas_is_span_wr(struct ma_wr_state *wr_mas)
>  {
>  	unsigned long max;
>  	unsigned long last = wr_mas->mas->last;
> -	unsigned long piv = wr_mas->r_max;
>  	enum maple_type type = wr_mas->type;
>  	void *entry = wr_mas->entry;
>  
> -	/* Contained in this pivot */
> -	if (piv > last)

I test this first since it is far more likely than the rest of the cases
in this function.  Most writes will land in a leaf node eventually, so
this will be true most of the time.  I'd like to keep it as a fast path.

> +	max = unlikely(ma_is_leaf(type)) ? wr_mas->mas->max : wr_mas->r_max;

Please expand this to multiple lines.  It is better to be more clear
than compact lines of code.

> +	if (last < max) {
> +		/* Contained in this pivot or this leaf node */
>  		return false;
> -
> -	max = wr_mas->mas->max;
> -	if (unlikely(ma_is_leaf(type))) {
> -		/* Fits in the node, but may span slots. */
> -		if (last < max)
> -			return false;
> -
> -		/* Writes to the end of the node but not null. */
> -		if ((last == max) && entry)
> -			return false;
> -
> +	} else if (last == max) {
>  		/*
> -		 * Writing ULONG_MAX is not a spanning write regardless of the
> -		 * value being written as long as the range fits in the node.
> +		 * The last entry of leaf node cannot be NULL unless it is the
> +		 * rightmost node (writing ULONG_MAX), otherwise it spans slots.
> +		 * If this is not leaf node, detect spanning store wr walk.
>  		 */
> -		if ((last == ULONG_MAX) && (last == max))
> -			return false;
> -	} else if (piv == last) {
> -		if (entry)
> -			return false;
> -
> -		/* Detect spanning store wr walk */
> -		if (last == ULONG_MAX)
> +		if (entry || last == ULONG_MAX)
>  			return false;
>  	}
> -
> -	trace_ma_write(__func__, wr_mas->mas, piv, entry);
> -
> +	trace_ma_write(__func__, wr_mas->mas, wr_mas->r_max, entry);
>  	return true;
>  }
>  
> -- 
> 2.20.1
>