linux-kernel - [PATCH v2] mm: khugepaged: fix memory leak in collapse

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Message-Id: <20251124161149.1302507-1-shardul.b@mpiricsoftware.com>
Date: Mon, 24 Nov 2025 21:41:49 +0530
From: Shardul Bankar <shardul.b@...ricsoftware.com>
To: linux-mm@...ck.org,
	dev.jain@....com,
	david@...nel.org
Cc: linux-kernel@...r.kernel.org,
	syzbot+a785d07959bc94837d51@...kaller.appspotmail.com,
	akpm@...ux-foundation.org,
	lorenzo.stoakes@...cle.com,
	ziy@...dia.com,
	baolin.wang@...ux.alibaba.com,
	Liam.Howlett@...cle.com,
	npache@...hat.com,
	ryan.roberts@....com,
	baohua@...nel.org,
	lance.yang@...ux.dev,
	janak@...ricsoftware.com,
	shardul.b@...ricsoftware.com,
	shardulsb08@...il.com
Subject: [PATCH v2] mm: khugepaged: fix memory leak in collapse_file xas retry loop

collapse_file() uses xas_create_range() in a retry loop that calls
xas_nomem() on -ENOMEM and then retries. xas_nomem() may allocate a
spare xa_node and store it in xas->xa_alloc.

If the lock is dropped after xas_nomem(), another thread can expand
the xarray tree in the meantime. On the next retry, xas_create_range()
can then succeed trivially without consuming the node stored in
xas->xa_alloc. If we then either succeed or give up and go to the
rollback path without calling xas_destroy(), that spare node leaks.

Fix this by calling xas_destroy(&xas) in both the success case
(!xas_error(&xas)) and the failure case where xas_nomem() returns
false and we abort. xas_destroy() will free any unused spare node in
xas->xa_alloc and is a no-op if there is nothing left to free.

Link: https://syzkaller.appspot.com/bug?id=a274d65fc733448ed518ad15481ed575669dd98c
Fixes: cae106dd67b9 ("mm/khugepaged: refactor collapse_file control flow")
Signed-off-by: Shardul Bankar <shardul.b@...ricsoftware.com>
---
 v2:
 - Call xas_destroy() on both success and failure
 - Explained retry semantics and xa_alloc / concurrency risk
 - Dropped cleanup_empty_nodes from previous proposal

 mm/khugepaged.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index abe54f0043c7..0794a99c807f 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1872,11 +1872,14 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 	do {
 		xas_lock_irq(&xas);
 		xas_create_range(&xas);
-		if (!xas_error(&xas))
+		if (!xas_error(&xas)) {
+			xas_destroy(&xas);
 			break;
+		}
 		xas_unlock_irq(&xas);
 		if (!xas_nomem(&xas, GFP_KERNEL)) {
 			result = SCAN_FAIL;
+			xas_destroy(&xas);
 			goto rollback;
 		}
 	} while (1);
-- 
2.34.1