linux-kernel - [PATCH v2] xarray: Add a BUG_ON() to ensure caller is not sibling

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Message-Id: <20250604041533.91198-1-dev.jain@arm.com>
Date: Wed,  4 Jun 2025 09:45:33 +0530
From: Dev Jain <dev.jain@....com>
To: akpm@...ux-foundation.org,
	willy@...radead.org
Cc: linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	david@...hat.com,
	anshuman.khandual@....com,
	ryan.roberts@....com,
	ziy@...dia.com,
	aneesh.kumar@...nel.org,
	Dev Jain <dev.jain@....com>
Subject: [PATCH v2] xarray: Add a BUG_ON() to ensure caller is not sibling

Suppose xas is pointing somewhere near the end of the multi-entry batch.
Then it may happen that the computed slot already falls beyond the batch,
thus breaking the loop due to !xa_is_sibling(), and computing the wrong
order. For example, suppose we have a shift-6 node having an order-9
entry => 8 - 1 = 7 siblings, so assume the slots are at offset 0 till 7 in
this node. If xas->xa_offset is 6, then the code will compute order as
1 + xas->xa_node->shift = 7. Therefore, the order computation must start
from the beginning of the multi-slot entries, that is, the non-sibling
entry. Thus ensure that the caller is aware of this by triggering a BUG
when the entry is a sibling entry. Note that this BUG_ON() is only
active while running selftests, so there is no overhead in a running
kernel.

Signed-off-by: Dev Jain <dev.jain@....com>
---
v1->v2:
 - Expand changelog, add comment

Based on Torvalds' master branch.

 lib/xarray.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/lib/xarray.c b/lib/xarray.c
index 76dde3a1cacf..ae3d80f4b4ee 100644
--- a/lib/xarray.c
+++ b/lib/xarray.c
@@ -1910,6 +1910,7 @@ EXPORT_SYMBOL(xa_store_range);
  * @xas: XArray operation state.
  *
  * Called after xas_load, the xas should not be in an error state.
+ * The xas should not be pointing to a sibling entry.
  *
  * Return: A number between 0 and 63 indicating the order of the entry.
  */
@@ -1920,6 +1921,8 @@ int xas_get_order(struct xa_state *xas)
 	if (!xas->xa_node)
 		return 0;

+	XA_NODE_BUG_ON(xas->xa_node, xa_is_sibling(xa_entry(xas->xa,
+		       xas->xa_node, xas->xa_offset)));
 	for (;;) {
 		unsigned int slot = xas->xa_offset + (1 << order);

-- 
2.30.2