linux-kernel - [RFC PATCH v1 2/5] mm: swap: Change SWAP_NEXT

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Date: Wed, 19 Jun 2024 00:26:42 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Andrew Morton <akpm@...ux-foundation.org>,
	Chris Li <chrisl@...nel.org>,
	Kairui Song <kasong@...cent.com>,
	"Huang, Ying" <ying.huang@...el.com>,
	Kalesh Singh <kaleshsingh@...gle.com>,
	Barry Song <baohua@...nel.org>,
	Hugh Dickins <hughd@...gle.com>,
	David Hildenbrand <david@...hat.com>
Cc: Ryan Roberts <ryan.roberts@....com>,
	linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: [RFC PATCH v1 2/5] mm: swap: Change SWAP_NEXT_INVALID to highest value

We are about to introduce a scanning mechanism that can present 0 as a
valid cluster offset to scan_swap_map_try_ssd_cluster(), so let's change
SWAP_NEXT_INVALID to UINT_MAX, which is always invalid as an offset in
practice.

Signed-off-by: Ryan Roberts <ryan.roberts@....com>
---
 include/linux/swap.h | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index bd450023b9a4..66566251ba31 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -261,12 +261,12 @@ struct swap_cluster_info {
 #define CLUSTER_FLAG_NEXT_NULL 2 /* This cluster has no next cluster */

 /*
- * The first page in the swap file is the swap header, which is always marked
- * bad to prevent it from being allocated as an entry. This also prevents the
- * cluster to which it belongs being marked free. Therefore 0 is safe to use as
- * a sentinel to indicate next is not valid in percpu_cluster.
+ * swap_info_struct::max is an unsigned int, so the maximum number of pages in
+ * the swap file is UINT_MAX. Therefore the highest legitimate index is
+ * UINT_MAX-1. Therefore UINT_MAX is safe to use as a sentinel to indicate next
+ * is not valid in percpu_cluster.
  */
-#define SWAP_NEXT_INVALID	0
+#define SWAP_NEXT_INVALID	UINT_MAX

 #ifdef CONFIG_THP_SWAP
 #define SWAP_NR_ORDERS		(PMD_ORDER + 1)
--
2.43.0