mm, swap: use an enum to define all cluster flags and wrap flags changes

Currently, we are only using flags to indicate which list the cluster is
on.  Using one bit for each list type might be a waste, as the list type
grows, we will consume too many bits.  Additionally, the current mixed
usage of '&' and '==' is a bit confusing.

Make it clean by using an enum to define all possible cluster statuses. 
Only an off-list cluster will have the NONE (0) flag.  And use a wrapper
to annotate and sanitize all flag settings and list movements.

Link: https://lkml.kernel.org/r/20250113175732.48099-9-ryncsn@gmail.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Suggested-by: Chris Li <chrisl@kernel.org>
Cc: Baoquan He <bhe@redhat.com>
Cc: Barry Song <v-songbaohua@oppo.com>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Hugh Dickens <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kalesh Singh <kaleshsingh@google.com>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

2 files changed, 53 insertions, 40 deletions

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 1651174959c8..0e59cb158b15 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -256,10 +256,19 @@ struct swap_cluster_info {
 	u8 order;
 	struct list_head list;
 };
-#define CLUSTER_FLAG_FREE 1 /* This cluster is free */
-#define CLUSTER_FLAG_NONFULL 2 /* This cluster is on nonfull list */
-#define CLUSTER_FLAG_FRAG 4 /* This cluster is on nonfull list */
-#define CLUSTER_FLAG_FULL 8 /* This cluster is on full list */
+
+/* All on-list cluster must have a non-zero flag. */
+enum swap_cluster_flags {
+	CLUSTER_FLAG_NONE = 0, /* For temporary off-list cluster */
+	CLUSTER_FLAG_FREE,
+	CLUSTER_FLAG_NONFULL,
+	CLUSTER_FLAG_FRAG,
+	/* Clusters with flags above are allocatable */
+	CLUSTER_FLAG_USABLE = CLUSTER_FLAG_FRAG,
+	CLUSTER_FLAG_FULL,
+	CLUSTER_FLAG_DISCARD,
+	CLUSTER_FLAG_MAX,
+};
 
 /*
  * The first page in the swap file is the swap header, which is always marked
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 3898576f947a..b754c9e16c3b 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -403,7 +403,7 @@ static void discard_swap_cluster(struct swap_info_struct *si,
 
 static inline bool cluster_is_free(struct swap_cluster_info *info)
 {
-	return info->flags & CLUSTER_FLAG_FREE;
+	return info->flags == CLUSTER_FLAG_FREE;
 }
 
 static inline unsigned int cluster_index(struct swap_info_struct *si,
@@ -434,6 +434,28 @@ static inline void unlock_cluster(struct swap_cluster_info *ci)
 	spin_unlock(&ci->lock);
 }
 
+static void move_cluster(struct swap_info_struct *si,
+			 struct swap_cluster_info *ci, struct list_head *list,
+			 enum swap_cluster_flags new_flags)
+{
+	VM_WARN_ON(ci->flags == new_flags);
+
+	BUILD_BUG_ON(1 << sizeof(ci->flags) * BITS_PER_BYTE < CLUSTER_FLAG_MAX);
+
+	if (ci->flags == CLUSTER_FLAG_NONE) {
+		list_add_tail(&ci->list, list);
+	} else {
+		if (ci->flags == CLUSTER_FLAG_FRAG) {
+			VM_WARN_ON(!si->frag_cluster_nr[ci->order]);
+			si->frag_cluster_nr[ci->order]--;
+		}
+		list_move_tail(&ci->list, list);
+	}
+	ci->flags = new_flags;
+	if (new_flags == CLUSTER_FLAG_FRAG)
+		si->frag_cluster_nr[ci->order]++;
+}
+
 /* Add a cluster to discard list and schedule it to do discard */
 static void swap_cluster_schedule_discard(struct swap_info_struct *si,
 		struct swap_cluster_info *ci)
@@ -447,10 +469,8 @@ static void swap_cluster_schedule_discard(struct swap_info_struct *si,
 	 */
 	memset(si->swap_map + idx * SWAPFILE_CLUSTER,
 			SWAP_MAP_BAD, SWAPFILE_CLUSTER);
-
-	VM_BUG_ON(ci->flags & CLUSTER_FLAG_FREE);
-	list_move_tail(&ci->list, &si->discard_clusters);
-	ci->flags = 0;
+	VM_BUG_ON(ci->flags == CLUSTER_FLAG_FREE);
+	move_cluster(si, ci, &si->discard_clusters, CLUSTER_FLAG_DISCARD);
 	schedule_work(&si->discard_work);
 }
 
@@ -458,12 +478,7 @@ static void __free_cluster(struct swap_info_struct *si, struct swap_cluster_info
 {
 	lockdep_assert_held(&si->lock);
 	lockdep_assert_held(&ci->lock);
-
-	if (ci->flags)
-		list_move_tail(&ci->list, &si->free_clusters);
-	else
-		list_add_tail(&ci->list, &si->free_clusters);
-	ci->flags = CLUSTER_FLAG_FREE;
+	move_cluster(si, ci, &si->free_clusters, CLUSTER_FLAG_FREE);
 	ci->order = 0;
 }
 
@@ -479,6 +494,8 @@ static void swap_do_scheduled_discard(struct swap_info_struct *si)
 	while (!list_empty(&si->discard_clusters)) {
 		ci = list_first_entry(&si->discard_clusters, struct swap_cluster_info, list);
 		list_del(&ci->list);
+		/* Must clear flag when taking a cluster off-list */
+		ci->flags = CLUSTER_FLAG_NONE;
 		idx = cluster_index(si, ci);
 		spin_unlock(&si->lock);
 
@@ -519,9 +536,6 @@ static void free_cluster(struct swap_info_struct *si, struct swap_cluster_info *
 	lockdep_assert_held(&si->lock);
 	lockdep_assert_held(&ci->lock);
 
-	if (ci->flags & CLUSTER_FLAG_FRAG)
-		si->frag_cluster_nr[ci->order]--;
-
 	/*
 	 * If the swap is discardable, prepare discard the cluster
 	 * instead of free it immediately. The cluster will be freed
@@ -573,13 +587,9 @@ static void dec_cluster_info_page(struct swap_info_struct *si,
 		return;
 	}
 
-	if (!(ci->flags & CLUSTER_FLAG_NONFULL)) {
-		VM_BUG_ON(ci->flags & CLUSTER_FLAG_FREE);
-		if (ci->flags & CLUSTER_FLAG_FRAG)
-			si->frag_cluster_nr[ci->order]--;
-		list_move_tail(&ci->list, &si->nonfull_clusters[ci->order]);
-		ci->flags = CLUSTER_FLAG_NONFULL;
-	}
+	if (ci->flags != CLUSTER_FLAG_NONFULL)
+		move_cluster(si, ci, &si->nonfull_clusters[ci->order],
+			     CLUSTER_FLAG_NONFULL);
 }
 
 static bool cluster_reclaim_range(struct swap_info_struct *si,
@@ -663,11 +673,13 @@ static bool cluster_alloc_range(struct swap_info_struct *si, struct swap_cluster
 	if (!(si->flags & SWP_WRITEOK))
 		return false;
 
+	VM_BUG_ON(ci->flags == CLUSTER_FLAG_NONE);
+	VM_BUG_ON(ci->flags > CLUSTER_FLAG_USABLE);
+
 	if (cluster_is_free(ci)) {
-		if (nr_pages < SWAPFILE_CLUSTER) {
-			list_move_tail(&ci->list, &si->nonfull_clusters[order]);
-			ci->flags = CLUSTER_FLAG_NONFULL;
-		}
+		if (nr_pages < SWAPFILE_CLUSTER)
+			move_cluster(si, ci, &si->nonfull_clusters[order],
+				     CLUSTER_FLAG_NONFULL);
 		ci->order = order;
 	}
 
@@ -675,14 +687,8 @@ static bool cluster_alloc_range(struct swap_info_struct *si, struct swap_cluster
 	swap_range_alloc(si, nr_pages);
 	ci->count += nr_pages;
 
-	if (ci->count == SWAPFILE_CLUSTER) {
-		VM_BUG_ON(!(ci->flags &
-			  (CLUSTER_FLAG_FREE | CLUSTER_FLAG_NONFULL | CLUSTER_FLAG_FRAG)));
-		if (ci->flags & CLUSTER_FLAG_FRAG)
-			si->frag_cluster_nr[ci->order]--;
-		list_move_tail(&ci->list, &si->full_clusters);
-		ci->flags = CLUSTER_FLAG_FULL;
-	}
+	if (ci->count == SWAPFILE_CLUSTER)
+		move_cluster(si, ci, &si->full_clusters, CLUSTER_FLAG_FULL);
 
 	return true;
 }
@@ -821,9 +827,7 @@ new_cluster:
 		while (!list_empty(&si->nonfull_clusters[order])) {
 			ci = list_first_entry(&si->nonfull_clusters[order],
 					      struct swap_cluster_info, list);
-			list_move_tail(&ci->list, &si->frag_clusters[order]);
-			ci->flags = CLUSTER_FLAG_FRAG;
-			si->frag_cluster_nr[order]++;
+			move_cluster(si, ci, &si->frag_clusters[order], CLUSTER_FLAG_FRAG);
 			offset = alloc_swap_scan_cluster(si, cluster_offset(si, ci),
 							 &found, order, usage);
 			frags++;