linux-kernel - Re: [PATCH 2/2] maple tree: Add and fix some comments

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Message-ID: <bxhfvhmk4iyyz7l3nitsgf4xgsydraskligg56n4ww3fe5zlvr@xqzh44dk3q5b>
Date: Thu, 26 Jun 2025 16:04:55 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Dev Jain <dev.jain@....com>
Cc: akpm@...ux-foundation.org, richard.weiyang@...il.com,
        maple-tree@...ts.infradead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] maple tree: Add and fix some comments

* Dev Jain <dev.jain@....com> [250626 13:19]:
> Add comments explaining the fields for maple_metadata, since "end" is
> ambiguous and "gap" can be confused as the largest gap, whereas it
> is actually the offset of the largest gap.
> 
> MAPLE_ROOT_NODE is used for mt_mk_root() and mt_safe_root(), indicating
> that it is used to mark the node as root. So fix the comment.

That's not quite the entire story here.

The first pointer in the tree may not be a node at all, and may be an
entry.  So having that bit set tells us the root of the tree is a node,
so the comment is correct but maybe you have a better way of expressing
this information?


> 
> Add comment for mas_ascend() to explain, whose min and max we are
> trying to find. Explain that, for example, if we are already on offset
> zero, then the parent min is mas->min, otherwise we need to walk up
> to find the implied pivot min.
> 
> Signed-off-by: Dev Jain <dev.jain@....com>
> ---
>  include/linux/maple_tree.h | 4 ++--
>  lib/maple_tree.c           | 9 +++++++--
>  2 files changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/maple_tree.h b/include/linux/maple_tree.h
> index 9ef129038224..bafe143b1f78 100644
> --- a/include/linux/maple_tree.h
> +++ b/include/linux/maple_tree.h
> @@ -75,8 +75,8 @@
>   * searching for gaps or any other code that needs to find the end of the data.
>   */
>  struct maple_metadata {
> -	unsigned char end;
> -	unsigned char gap;
> +	unsigned char end;	/* end of data */
> +	unsigned char gap;	/* offset of largest gap */

Thanks.

>  };
>  
>  /*
> diff --git a/lib/maple_tree.c b/lib/maple_tree.c
> index 6c89e6790fb5..e4735ccd06f2 100644
> --- a/lib/maple_tree.c
> +++ b/lib/maple_tree.c
> @@ -338,7 +338,7 @@ static inline void mte_set_node_dead(struct maple_enode *mn)
>  	smp_wmb(); /* Needed for RCU */
>  }
>  
> -/* Bit 1 indicates the root is a node */
> +/* Bit 1 indicates the node is the root */
>  #define MAPLE_ROOT_NODE			0x02
>  /* maple_type stored bit 3-6 */
>  #define MAPLE_ENODE_TYPE_SHIFT		0x03
> @@ -1053,7 +1053,7 @@ static inline void mte_set_gap(const struct maple_enode *mn,
>   * mas_ascend() - Walk up a level of the tree.
>   * @mas: The maple state
>   *
> - * Sets the @mas->max and @mas->min to the correct values when walking up.  This
> + * Sets the @mas->max and @mas->min for the parent node of mas->node.  This
>   * may cause several levels of walking up to find the correct min and max.
>   * May find a dead node which will cause a premature return.
>   * Return: 1 on dead node, 0 otherwise
> @@ -1098,6 +1098,11 @@ static int mas_ascend(struct ma_state *mas)
>  
>  	min = 0;
>  	max = ULONG_MAX;
> +
> +	/*
> +	 * !mas->offset => parent node min == mas->min. mas->offset => need
> +	 * to walk up to find the implied pivot min.

The => arrows here are a bit hard to parse, especially mixed with ==.
Maybe use more words?

> +	 */
>  	if (!mas->offset) {
>  		min = mas->min;
>  		set_min = true;
> -- 
> 2.30.2
>