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

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Message-ID: <2d55c06a-f4a5-4728-b692-60d88a5fe692@arm.com>
Date: Sat, 28 Jun 2025 17:26:18 +0530
From: Dev Jain <dev.jain@....com>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>, 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


On 27/06/25 1:34 am, Liam R. Howlett wrote:
> * 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?

Hmm. Can you please correct me on my understanding - when we have an
empty tree, then we insert a root and can store a value there. Now when
we store the second entry, we allocate a node and make the root a node,
the root points to that node, and we store the values at offsets 0 and 1.

I am reading more to answer my own question.

>
>
>> 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?

Okay.

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