swap: change block allocation algorithm for SSD

I'm using a fast SSD to do swap. scan_swap_map() sometimes uses up to 20~30% CPU time (when cluster is hard to find, the CPU time can be up to 80%), which becomes a bottleneck. scan_swap_map() scans a byte array to search a 256 page cluster, which is very slow.

Here I introduced a simple algorithm to search cluster. Since we only care about 256 pages cluster, we can just use a counter to track if a cluster is free. Every 256 pages use one int to store the counter. If the counter of a cluster is 0, the cluster is free. All free clusters will be added to a list, so searching cluster is very efficient. With this, scap_swap_map() overhead disappears.

This might help low end SD card swap too. Because if the cluster is aligned, SD firmware can do flash erase more efficiently.

We only enable the algorithm for SSD. Hard disk swap isn't fast enough and has downside with the algorithm which might introduce regression (see below).

The patch slightly changes which cluster is choosen. It always adds free cluster to list tail. This can help wear leveling for low end SSD too. And if no cluster found, the scan_swap_map() will do search from the end of last cluster. So if no cluster found, the scan_swap_map() will do search from the end of last free cluster, which is random. For SSD, this isn't a problem at all.

Another downside is the cluster must be aligned to 256 pages, which will reduce the chance to find a cluster. I would expect this isn't a big problem for SSD because of the non-seek penality. (And this is the reason I only enable the algorithm for SSD).

Changes from last post:
1. refresh to laster kernel
2. use bitfield as suggested by Andrew, which does cut code size a little bit.

Signed-off-by Shaohua Li <[email protected]>

---
  include/linux/swap.h |   14 ++
  mm/swapfile.c        |  281 +++++++++++++++++++++++++++++++++++++++++++--------
  2 files changed, 256 insertions(+), 39 deletions(-)

Index: linux/include/linux/swap.h
===================================================================

include/linux/swap.h +14 -0

@@ -182,6 +182,17 @@ enum {
  #define SWAP_MAP_SHMEM	0xbf	/* Owned by shmem/tmpfs, in first swap_map */
 
  /*
+ * the data field stores next cluster if the cluster is free or cluster counter
+ * otherwise
+ */
+struct swap_cluster_info {
+	unsigned int data:24;
+	unsigned int flags:8;
+};
+#define CLUSTER_FLAG_FREE 1 /* This cluster is free */
+#define CLUSTER_FLAG_NEXT_NULL 2 /* This cluster has no next cluster */
+
+/*
   * The in-memory structure used to track swap areas.
   */
  struct swap_info_struct {

@@ -191,6 +202,9 @@ struct swap_info_struct {
  	signed char	next;		/* next type on the swap list */
  	unsigned int	max;		/* extent of the swap_map */
  	unsigned char *swap_map;	/* vmalloc'ed array of usage counts */
+	struct swap_cluster_info *cluster_info; /* cluster info. Only for SSD */
+	struct swap_cluster_info free_cluster_head; /* free cluster list head */
+	struct swap_cluster_info free_cluster_tail; /* free cluster list tail */
  	unsigned int lowest_bit;	/* index of first free in swap_map */
  	unsigned int highest_bit;	/* index of last free in swap_map */
  	unsigned int pages;		/* total of usable pages of swap */

Index: linux/mm/swapfile.c
===================================================================

mm/swapfile.c +70 -0

@@ -184,6 +184,125 @@ static int wait_for_discard(void *word)
  #define SWAPFILE_CLUSTER	256
  #define LATENCY_LIMIT		256
 
+static inline void cluster_set_flag(struct swap_cluster_info *info,
+	unsigned int flag)
+{
+	info->flags = flag;
+}
+
+static inline unsigned int cluster_count(struct swap_cluster_info *info)
+{
+	return info->data;
+}
+
+static inline void cluster_set_count(struct swap_cluster_info *info,
+				     unsigned int c)
+{
+	info->data = c;
+}
+
+static inline void cluster_set_count_flag(struct swap_cluster_info *info,
+					 unsigned int c, unsigned int f)
+{
+	info->flags = f;
+	info->data = c;
+}
+
+static inline unsigned int cluster_next(struct swap_cluster_info *info)
+{
+	return info->data;
+}
+
+static inline void cluster_set_next(struct swap_cluster_info *info,
+				    unsigned int n)
+{
+	info->data = n;
+}
+
+static inline void cluster_set_next_flag(struct swap_cluster_info *info,
+					 unsigned int n, unsigned int f)
+{
+	info->flags = f;
+	info->data = n;
+}
+
+static inline bool cluster_is_free(struct swap_cluster_info *info)
+{
+	return info->flags & CLUSTER_FLAG_FREE;
+}
+
+static inline bool cluster_is_null(struct swap_cluster_info *info)
+{
+	return info->flags & CLUSTER_FLAG_NEXT_NULL;
+}
+
+static inline void cluster_set_null(struct swap_cluster_info *info)
+{
+	info->flags = CLUSTER_FLAG_NEXT_NULL;
+	info->data = 0;
+}
+
+static void inc_cluster_info_page(struct swap_info_struct *p,
+	struct swap_cluster_info *cluster_info, unsigned long page_nr)
+{
+	unsigned long idx = page_nr / SWAPFILE_CLUSTER;
+
+	if (!cluster_info)
+		return;
+	if (cluster_is_free(&cluster_info[idx])) {
+		VM_BUG_ON(cluster_next(&p->free_cluster_head) != idx);
+		cluster_set_next_flag(&p->free_cluster_head,
+			cluster_next(&cluster_info[idx]), 0);
+		if (cluster_next(&p->free_cluster_tail) == idx) {

mm/swapfile.c +15 -0

+
+static void dec_cluster_info_page(struct swap_info_struct *p,
+	struct swap_cluster_info *cluster_info, unsigned long page_nr)
+{
+	unsigned long idx = page_nr / SWAPFILE_CLUSTER;
+
+	if (!cluster_info)
+		return;
+
+	VM_BUG_ON(cluster_count(&cluster_info[idx]) == 0);
+	cluster_set_count(&cluster_info[idx],
+		cluster_count(&cluster_info[idx]) - 1);
+
+	if (cluster_count(&cluster_info[idx]) == 0) {
+		cluster_set_flag(&cluster_info[idx], CLUSTER_FLAG_FREE);

mm/swapfile.c +76 -3

+
+/*
+ * It's possible scan_swap_map() uses a free cluster in the middle of free
+ * cluster list. Avoiding such abuse to avoid list corruption.
+ */
+static inline bool scan_swap_map_recheck_cluster(struct swap_info_struct *si,
+	unsigned long offset)
+{
+	offset /= SWAPFILE_CLUSTER;
+	return !cluster_is_null(&si->free_cluster_head) &&
+		offset != cluster_next(&si->free_cluster_head) &&
+		cluster_is_free(&si->cluster_info[offset]);
+}
+
  static unsigned long scan_swap_map(struct swap_info_struct *si,
  				   unsigned char usage)
  {

@@ -225,6 +344,25 @@ static unsigned long scan_swap_map(struc
  			si->lowest_alloc = si->max;
  			si->highest_alloc = 0;
  		}
+check_cluster:
+		if (!cluster_is_null(&si->free_cluster_head)) {
+			offset = cluster_next(&si->free_cluster_head) *
+						SWAPFILE_CLUSTER;
+			last_in_cluster = offset + SWAPFILE_CLUSTER - 1;
+			si->cluster_next = offset;
+			si->cluster_nr = SWAPFILE_CLUSTER - 1;
+			found_free_cluster = 1;
+			goto checks;
+		} else if (si->cluster_info) {
+			/*
+			 * Checking free cluster is fast enough, we can do the
+			 * check every time
+			 */
+			si->cluster_nr = 0;
+			si->lowest_alloc = 0;
+			goto checks;
+		}
+
  		spin_unlock(&si->lock);
 
  		/*

@@ -285,6 +423,8 @@ static unsigned long scan_swap_map(struc
  	}
 
  checks:
+	if (scan_swap_map_recheck_cluster(si, offset))
+		goto check_cluster;
  	if (!(si->flags & SWP_WRITEOK))
  		goto no_page;
  	if (!si->highest_bit)

@@ -317,6 +457,7 @@ checks:
  		si->highest_bit = 0;
  	}
  	si->swap_map[offset] = usage;
+	inc_cluster_info_page(si, si->cluster_info, offset);
  	si->cluster_next = offset + 1;
  	si->flags -= SWP_SCANNING;
 

@@ -600,6 +741,7 @@ static unsigned char swap_entry_free(str
 
  	/* free if no reference */
  	if (!usage) {
+		dec_cluster_info_page(p, p->cluster_info, offset);
  		if (offset < p->lowest_bit)
  			p->lowest_bit = offset;
  		if (offset > p->highest_bit)

@@ -1509,7 +1651,8 @@ static int setup_swap_extents(struct swa
  }
 
  static void _enable_swap_info(struct swap_info_struct *p, int prio,
-				unsigned char *swap_map)
+				unsigned char *swap_map,
+				struct swap_cluster_info *cluster_info)
  {
  	int i, prev;
 

@@ -1518,6 +1661,7 @@ static void _enable_swap_info(struct swa
  	else
  		p->prio = --least_priority;
  	p->swap_map = swap_map;
+	p->cluster_info = cluster_info;
  	p->flags |= SWP_WRITEOK;
  	atomic_long_add(p->pages, &nr_swap_pages);
  	total_swap_pages += p->pages;

@@ -1538,12 +1682,13 @@ static void _enable_swap_info(struct swa
 
  static void enable_swap_info(struct swap_info_struct *p, int prio,
  				unsigned char *swap_map,
+				struct swap_cluster_info *cluster_info,
  				unsigned long *frontswap_map)
  {
  	frontswap_init(p->type, frontswap_map);
  	spin_lock(&swap_lock);
  	spin_lock(&p->lock);
-	 _enable_swap_info(p, prio, swap_map);
+	 _enable_swap_info(p, prio, swap_map, cluster_info);
  	spin_unlock(&p->lock);
  	spin_unlock(&swap_lock);
  }

@@ -1552,7 +1697,7 @@ static void reinsert_swap_info(struct sw
  {
  	spin_lock(&swap_lock);
  	spin_lock(&p->lock);
-	_enable_swap_info(p, p->prio, p->swap_map);
+	_enable_swap_info(p, p->prio, p->swap_map, p->cluster_info);
  	spin_unlock(&p->lock);
  	spin_unlock(&swap_lock);
  }

@@ -1561,6 +1706,7 @@ SYSCALL_DEFINE1(swapoff, const char __us
  {
  	struct swap_info_struct *p = NULL;
  	unsigned char *swap_map;
+	struct swap_cluster_info *cluster_info;
  	unsigned long *frontswap_map;
  	struct file *swap_file, *victim;
  	struct address_space *mapping;

@@ -1660,6 +1806,8 @@ SYSCALL_DEFINE1(swapoff, const char __us
  	p->max = 0;
  	swap_map = p->swap_map;
  	p->swap_map = NULL;
+	cluster_info = p->cluster_info;
+	p->cluster_info = NULL;
  	p->flags = 0;
  	frontswap_map = frontswap_map_get(p);
  	frontswap_map_set(p, NULL);

@@ -1668,6 +1816,7 @@ SYSCALL_DEFINE1(swapoff, const char __us
  	frontswap_invalidate_area(type);
  	mutex_unlock(&swapon_mutex);
  	vfree(swap_map);
+	vfree(cluster_info);
  	vfree(frontswap_map);
  	/* Destroy swap account informatin */
  	swap_cgroup_swapoff(type);

@@ -1980,15 +2129,21 @@ static unsigned long read_swap_header(st
  static int setup_swap_map_and_extents(struct swap_info_struct *p,
  					union swap_header *swap_header,
  					unsigned char *swap_map,
+					struct swap_cluster_info *cluster_info,
  					unsigned long maxpages,
  					sector_t *span)
  {
  	int i;
  	unsigned int nr_good_pages;
  	int nr_extents;
+	unsigned long nr_clusters = DIV_ROUND_UP(maxpages, SWAPFILE_CLUSTER);
+	unsigned long idx = p->cluster_next / SWAPFILE_CLUSTER;
 
  	nr_good_pages = maxpages - 1;	/* omit header page */
 
+	cluster_set_null(&p->free_cluster_head);
+	cluster_set_null(&p->free_cluster_tail);
+
  	for (i = 0; i < swap_header->info.nr_badpages; i++) {
  		unsigned int page_nr = swap_header->info.badpages[i];
  		if (page_nr == 0 || page_nr > swap_header->info.last_page)

@@ -1996,11 +2151,25 @@ static int setup_swap_map_and_extents(st
  		if (page_nr < maxpages) {
  			swap_map[page_nr] = SWAP_MAP_BAD;
  			nr_good_pages--;
+			/*
+			 * Haven't marked the cluster free yet, no list
+			 * operation involved
+			 */
+			inc_cluster_info_page(p, cluster_info, page_nr);
  		}
  	}
 
+	/* Haven't marked the cluster free yet, no list operation involved */
+	for (i = maxpages; i < round_up(maxpages, SWAPFILE_CLUSTER); i++)
+		inc_cluster_info_page(p, cluster_info, i);
+
  	if (nr_good_pages) {
  		swap_map[0] = SWAP_MAP_BAD;
+		/*
+		 * Not mark the cluster free yet, no list
+		 * operation involved
+		 */
+		inc_cluster_info_page(p, cluster_info, 0);
  		p->max = maxpages;
  		p->pages = nr_good_pages;
  		nr_extents = setup_swap_extents(p, span);

@@ -2013,6 +2182,30 @@ static int setup_swap_map_and_extents(st
  		return -EINVAL;
  	}
 
+	if (!cluster_info)
+		return nr_extents;
+
+	for (i = 0; i < nr_clusters; i++) {
+		if (!cluster_count(&cluster_info[idx])) {
+			cluster_set_flag(&cluster_info[idx], CLUSTER_FLAG_FREE);
+			if (cluster_is_null(&p->free_cluster_head)) {
+				cluster_set_next_flag(&p->free_cluster_head,
+								idx, 0);

mm/swapfile.c +52 -37

+			}
+		}
+		idx++;
+		if (idx == nr_clusters)
+			idx = 0;
+	}
  	return nr_extents;
  }

@@ -2044,6 +2237,7 @@ SYSCALL_DEFINE2(swapon, const char __use
  	sector_t span;
  	unsigned long maxpages;
  	unsigned char *swap_map = NULL;
+	struct swap_cluster_info *cluster_info = NULL;
  	unsigned long *frontswap_map = NULL;
  	struct page *page = NULL;
  	struct inode *inode = NULL;

@@ -2117,13 +2311,28 @@ SYSCALL_DEFINE2(swapon, const char __use
  		error = -ENOMEM;
  		goto bad_swap;
  	}
+	if (p->bdev && blk_queue_nonrot(bdev_get_queue(p->bdev))) {
+		p->flags |= SWP_SOLIDSTATE;
+		/*
+		 * select a random position to start with to help wear leveling
+		 * SSD
+		 */
+		p->cluster_next = 1 + (prandom_u32() % p->highest_bit);
+
+		cluster_info = vzalloc(DIV_ROUND_UP(maxpages,
+			SWAPFILE_CLUSTER) * sizeof(*cluster_info));
+		if (!cluster_info) {
+			error = -ENOMEM;
+			goto bad_swap;
+		}
+	}
 
  	error = swap_cgroup_swapon(p->type, maxpages);
  	if (error)
  		goto bad_swap;
 
  	nr_extents = setup_swap_map_and_extents(p, swap_header, swap_map,
-		maxpages, &span);
+		cluster_info, maxpages, &span);
  	if (unlikely(nr_extents < 0)) {
  		error = nr_extents;
  		goto bad_swap;

@@ -2132,41 +2341,34 @@ SYSCALL_DEFINE2(swapon, const char __use
  	if (frontswap_enabled)
  		frontswap_map = vzalloc(BITS_TO_LONGS(maxpages) * sizeof(long));
 
-	if (p->bdev) {
-		if (blk_queue_nonrot(bdev_get_queue(p->bdev))) {
-			p->flags |= SWP_SOLIDSTATE;
-			p->cluster_next = 1 + (prandom_u32() % p->highest_bit);
-		}
-
-		if ((swap_flags & SWAP_FLAG_DISCARD) && swap_discardable(p)) {
-			/*
-			 * When discard is enabled for swap with no particular
-			 * policy flagged, we set all swap discard flags here in
-			 * order to sustain backward compatibility with older
-			 * swapon(8) releases.
-			 */
-			p->flags |= (SWP_DISCARDABLE | SWP_AREA_DISCARD |
-				     SWP_PAGE_DISCARD);
+	if (p->bdev &&(swap_flags & SWAP_FLAG_DISCARD) && swap_discardable(p)) {
+		/*
+		 * When discard is enabled for swap with no particular
+		 * policy flagged, we set all swap discard flags here in
+		 * order to sustain backward compatibility with older
+		 * swapon(8) releases.
+		 */
+		p->flags |= (SWP_DISCARDABLE | SWP_AREA_DISCARD |
+			     SWP_PAGE_DISCARD);
 
-			/*
-			 * By flagging sys_swapon, a sysadmin can tell us to
-			 * either do single-time area discards only, or to just
-			 * perform discards for released swap page-clusters.
-			 * Now it's time to adjust the p->flags accordingly.
-			 */
-			if (swap_flags & SWAP_FLAG_DISCARD_ONCE)
-				p->flags &= ~SWP_PAGE_DISCARD;
-			else if (swap_flags & SWAP_FLAG_DISCARD_PAGES)
-				p->flags &= ~SWP_AREA_DISCARD;
-
-			/* issue a swapon-time discard if it's still required */
-			if (p->flags & SWP_AREA_DISCARD) {
-				int err = discard_swap(p);
-				if (unlikely(err))
-					printk(KERN_ERR
-					       "swapon: discard_swap(%p): %d\n",
-						p, err);
-			}
+		/*
+		 * By flagging sys_swapon, a sysadmin can tell us to
+		 * either do single-time area discards only, or to just
+		 * perform discards for released swap page-clusters.
+		 * Now it's time to adjust the p->flags accordingly.
+		 */
+		if (swap_flags & SWAP_FLAG_DISCARD_ONCE)
+			p->flags &= ~SWP_PAGE_DISCARD;
+		else if (swap_flags & SWAP_FLAG_DISCARD_PAGES)
+			p->flags &= ~SWP_AREA_DISCARD;
+
+		/* issue a swapon-time discard if it's still required */
+		if (p->flags & SWP_AREA_DISCARD) {
+			int err = discard_swap(p);
+			if (unlikely(err))
+				printk(KERN_ERR
+				       "swapon: discard_swap(%p): %d\n",
+					p, err);
  		}
  	}
 

@@ -2175,7 +2377,7 @@ SYSCALL_DEFINE2(swapon, const char __use
  	if (swap_flags & SWAP_FLAG_PREFER)
  		prio =
  		  (swap_flags & SWAP_FLAG_PRIO_MASK) >> SWAP_FLAG_PRIO_SHIFT;
-	enable_swap_info(p, prio, swap_map, frontswap_map);
+	enable_swap_info(p, prio, swap_map, cluster_info, frontswap_map);
 
  	printk(KERN_INFO "Adding %uk swap on %s.  "
  			"Priority:%d extents:%d across:%lluk %s%s%s%s%s\n",

@@ -2207,6 +2409,7 @@ bad_swap:
  	p->flags = 0;
  	spin_unlock(&swap_lock);
  	vfree(swap_map);
+	vfree(cluster_info);
  	if (swap_file) {
  		if (inode && S_ISREG(inode->i_mode)) {
  			mutex_unlock(&inode->i_mutex);
 
--

To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to [email protected]. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"[email protected]"> [email protected] </a>

I'm using a fast SSD to do swap. scan_swap_map() sometimes uses up to 20~30% CPU time (when cluster is hard to find, the CPU time can be up to 80%), which becomes a bottleneck. scan_swap_map() scans a byte array to search a 256 page cluster, which is very slow.

Here I introduced a simple algorithm to search cluster. Since we only care about 256 pages cluster, we can just use a counter to track if a cluster is free. Every 256 pages use one int to store the counter. If the counter of a cluster is 0, the cluster is free. All free clusters will be added to a list, so searching cluster is very efficient. With this, scap_swap_map() overhead disappears.

This might help low end SD card swap too. Because if the cluster is aligned, SD firmware can do flash erase more efficiently.

We only enable the algorithm for SSD. Hard disk swap isn't fast enough and has downside with the algorithm which might introduce regression (see below).

The patch slightly changes which cluster is choosen. It always adds free cluster to list tail. This can help wear leveling for low end SSD too. And if no cluster found, the scan_swap_map() will do search from the end of last cluster. So if no cluster found, the scan_swap_map() will do search from the end of last free cluster, which is random. For SSD, this isn't a problem at all.

Another downside is the cluster must be aligned to 256 pages, which will reduce the chance to find a cluster. I would expect this isn't a big problem for SSD because of the non-seek penality. (And this is the reason I only enable the algorithm for SSD).

include/linux/swap.h +2 -0

+#define CLUSTER_FLAG_FREE 1 /* This cluster is free */
+#define CLUSTER_FLAG_NEXT_NULL 2 /* This cluster has no next cluster */
 
...

@@ -2117,13 +2311,28 @@ SYSCALL_DEFINE2(swapon, const char __use
 		error = -ENOMEM;
 		goto bad_swap;
 	}

Quoted 25 lines · On Tue, 16 Jul 2013 04:43:20 +0800 Shaohua Li <[email protected]> wrote:

I'm using a fast SSD to do swap. scan_swap_map() sometimes uses up to 20~30% CPU time (when cluster is hard to find, the CPU time can be up to 80%), which becomes a bottleneck. scan_swap_map() scans a byte array to search a 256 page cluster, which is very slow.

Here I introduced a simple algorithm to search cluster. Since we only care about 256 pages cluster, we can just use a counter to track if a cluster is free. Every 256 pages use one int to store the counter. If the counter of a cluster is 0, the cluster is free. All free clusters will be added to a list, so searching cluster is very efficient. With this, scap_swap_map() overhead disappears.

This might help low end SD card swap too. Because if the cluster is aligned, SD firmware can do flash erase more efficiently.

We only enable the algorithm for SSD. Hard disk swap isn't fast enough and has downside with the algorithm which might introduce regression (see below).

The patch slightly changes which cluster is choosen. It always adds free cluster to list tail. This can help wear leveling for low end SSD too. And if no cluster found, the scan_swap_map() will do search from the end of last cluster. So if no cluster found, the scan_swap_map() will do search from the end of last free cluster, which is random. For SSD, this isn't a problem at all.

Quoted 4 lines

Another downside is the cluster must be aligned to 256 pages, which will reduce the chance to find a cluster. I would expect this isn't a big problem for SSD because of the non-seek penality. (And this is the reason I only enable the algorithm for SSD).

I have to agree with Will here - the patch adds a significant new design/algorithm into core MM but there wasn't even an attempt to describe it within the code.

The changelog provdes a reasonable overview, most notably the second paragraph. Could you please find a way to flesh that part out a bit then integrate it into a code comment? And yes, the major functions should have their own comments explaining how they serve the overall scheme.

include/linux/swap.h enum { · line 191

Quoted 13 lines

include/linux/swap.h +7 -0

@@ -182,6 +182,17 @@ enum {
 #define SWAP_MAP_SHMEM	0xbf	/* Owned by shmem/tmpfs, in first swap_map */
 
 /*
+ * the data field stores next cluster if the cluster is free or cluster counter
+ * otherwise
+ */
+struct swap_cluster_info {
+	unsigned int data:24;
+	unsigned int flags:8;
+};

If I'm understanding it correctly, the code and data structures which this patch adds are all protected by swap_info_struct.lock, yes? This is also worth mentioning in a comment, perhaps at the swap_cluster_info definition site

Quoted 9 lines

include/linux/swap.h +2 -0

+#define CLUSTER_FLAG_FREE 1 /* This cluster is free */
+#define CLUSTER_FLAG_NEXT_NULL 2 /* This cluster has no next cluster */
 
...

@@ -2117,13 +2311,28 @@ SYSCALL_DEFINE2(swapon, const char __use
 		error = -ENOMEM;
 		goto bad_swap;
 	}

include/linux/swap.h SYSCALL_DEFINE2(swapon, const char __use · line 2323

Quoted 10 lines

include/linux/swap.h +10 -0

+	if (p->bdev && blk_queue_nonrot(bdev_get_queue(p->bdev))) {
+		p->flags |= SWP_SOLIDSTATE;
+		/*
+		 * select a random position to start with to help wear leveling
+		 * SSD
+		 */
+		p->cluster_next = 1 + (prandom_u32() % p->highest_bit);
+
+		cluster_info = vzalloc(DIV_ROUND_UP(maxpages,
+			SWAPFILE_CLUSTER) * sizeof(*cluster_info));

OK, what is the upper bound on the size of this allocation?

A failure here would be bad - perhaps a list is needed, rather than a flat array.