linux-kernel - [PATCH v2] maple tree: Add some comments

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Message-Id: <20250703063338.51509-1-dev.jain@arm.com>
Date: Thu,  3 Jul 2025 12:03:38 +0530
From: Dev Jain <dev.jain@....com>
To: akpm@...ux-foundation.org,
	Liam.Howlett@...cle.com
Cc: richard.weiyang@...il.com,
	maple-tree@...ts.infradead.org,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Dev Jain <dev.jain@....com>
Subject: [PATCH v2] maple tree: Add some comments

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.

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>
---

v1->v2:
 - Drop MAPLE_ROOT_NODE comment modification

v1:
 - https://lore.kernel.org/all/20250626171918.17261-2-dev.jain@arm.com/

 include/linux/maple_tree.h | 4 ++--
 lib/maple_tree.c           | 8 +++++++-
 2 files changed, 9 insertions(+), 3 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 */
 };
 
 /*
diff --git a/lib/maple_tree.c b/lib/maple_tree.c
index 0e85e92c5375..b4ee2d29d7a9 100644
--- a/lib/maple_tree.c
+++ b/lib/maple_tree.c
@@ -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 +1098,12 @@ static int mas_ascend(struct ma_state *mas)
 
 	min = 0;
 	max = ULONG_MAX;
+
+	/*
+	 * !mas->offset implies that parent node min == mas->min.
+	 * mas->offset > 0 implies that we need to walk up to find the
+	 * implied pivot min.
+	 */
 	if (!mas->offset) {
 		min = mas->min;
 		set_min = true;
-- 
2.30.2