linux-kernel - [RFC PATCH 1/4] XArray: Initialize nodes while splitting instead of while allocating

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Message-ID: <20251117224701.1279139-2-ackerleytng@google.com>
Date: Mon, 17 Nov 2025 14:46:58 -0800
From: Ackerley Tng <ackerleytng@...gle.com>
To: willy@...radead.org, akpm@...ux-foundation.org, 
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org
Cc: david@...hat.com, michael.roth@....com, vannapurve@...gle.com, 
	Ackerley Tng <ackerleytng@...gle.com>
Subject: [RFC PATCH 1/4] XArray: Initialize nodes while splitting instead of
 while allocating

Currently, newly allocated nodes in xas_split_alloc are initialized
immediately upon allocation. This occurs before the node's specific role,
shift, and offset within the splitting hierarchy are determined.

Move the call to __xas_init_node_for_split() from xas_split_alloc() to
xas_split(). This removes any dependence on "entry" during node allocation,
in preparation for extending xa_split* functions to support multi-level
splits, where intermediate nodes need not have their slots initialized.

Signed-off-by: Ackerley Tng <ackerleytng@...gle.com>
---

After moving the call to __xas_init_node_for_split() from
xas_split_alloc() to xas_split(), void *entry is unused and no longer
needs to be passed to xas_split_alloc().

I would like to get feedback on moving initialization into xas_split()
before making a full refactoring to remove void *entry as a parameter
to xas_split_alloc().

 lib/xarray.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/xarray.c b/lib/xarray.c
index edb877c2d51c..636edcf014f1 100644
--- a/lib/xarray.c
+++ b/lib/xarray.c
@@ -1060,7 +1060,6 @@ void xas_split_alloc(struct xa_state *xas, void *entry, unsigned int order,
 		if (!node)
 			goto nomem;

-		__xas_init_node_for_split(xas, node, entry);
 		RCU_INIT_POINTER(node->parent, xas->xa_alloc);
 		xas->xa_alloc = node;
 	} while (sibs-- > 0);
@@ -1103,6 +1102,7 @@ void xas_split(struct xa_state *xas, void *entry, unsigned int order)
 			struct xa_node *child = xas->xa_alloc;

 			xas->xa_alloc = rcu_dereference_raw(child->parent);
+			__xas_init_node_for_split(xas, child, entry);
 			child->shift = node->shift - XA_CHUNK_SHIFT;
 			child->offset = offset;
 			child->count = XA_CHUNK_SIZE;
--
2.52.0.rc1.455.g30608eb744-goog