[PATCH v4 17/21] swapfile: remove zeromap bitmap

[Nhat Pham <nphamcs@gmail.com>]

Zero swap entries are now treated as a separate, decoupled backend in
the virtual swap layer. The zeromap bitmap of physical swapfile is no
longer used - remove it. This does not have any behavioral change, and
save 1 bit per swap page in terms of memory overhead.

Signed-off-by: Nhat Pham <nphamcs@gmail.com>
---
 include/linux/swap.h |  1 -
 mm/swapfile.c        | 30 +++++-------------------------
 2 files changed, 5 insertions(+), 26 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index fa73696733744..cc1ca4ac2946d 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -260,7 +260,6 @@ struct swap_info_struct {
 	signed char	type;		/* strange name for an index */
 	unsigned int	max;		/* extent of the swap_map */
 	unsigned char *swap_map;	/* vmalloc'ed array of usage counts */
-	unsigned long *zeromap;		/* kvmalloc'ed bitmap to track zero pages */
 	struct swap_cluster_info *cluster_info; /* cluster info. Only for SSD */
 	struct list_head free_clusters; /* free clusters list */
 	struct list_head full_clusters; /* full clusters list */
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 32aa080d96a4d..aeb3575df8a0b 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -2320,8 +2320,7 @@ static int setup_swap_extents(struct swap_info_struct *sis, sector_t *span)
 
 static void setup_swap_info(struct swap_info_struct *si, int prio,
 			    unsigned char *swap_map,
-			    struct swap_cluster_info *cluster_info,
-			    unsigned long *zeromap)
+			    struct swap_cluster_info *cluster_info)
 {
 	si->prio = prio;
 	/*
@@ -2332,7 +2331,6 @@ static void setup_swap_info(struct swap_info_struct *si, int prio,
 	si->avail_list.prio = -si->prio;
 	si->swap_map = swap_map;
 	si->cluster_info = cluster_info;
-	si->zeromap = zeromap;
 }
 
 static void _enable_swap_info(struct swap_info_struct *si)
@@ -2350,12 +2348,11 @@ static void _enable_swap_info(struct swap_info_struct *si)
 
 static void enable_swap_info(struct swap_info_struct *si, int prio,
 				unsigned char *swap_map,
-				struct swap_cluster_info *cluster_info,
-				unsigned long *zeromap)
+				struct swap_cluster_info *cluster_info)
 {
 	spin_lock(&swap_lock);
 	spin_lock(&si->lock);
-	setup_swap_info(si, prio, swap_map, cluster_info, zeromap);
+	setup_swap_info(si, prio, swap_map, cluster_info);
 	spin_unlock(&si->lock);
 	spin_unlock(&swap_lock);
 	/*
@@ -2373,7 +2370,7 @@ static void reinsert_swap_info(struct swap_info_struct *si)
 {
 	spin_lock(&swap_lock);
 	spin_lock(&si->lock);
-	setup_swap_info(si, si->prio, si->swap_map, si->cluster_info, si->zeromap);
+	setup_swap_info(si, si->prio, si->swap_map, si->cluster_info);
 	_enable_swap_info(si);
 	spin_unlock(&si->lock);
 	spin_unlock(&swap_lock);
@@ -2444,7 +2441,6 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 {
 	struct swap_info_struct *p = NULL;
 	unsigned char *swap_map;
-	unsigned long *zeromap;
 	struct swap_cluster_info *cluster_info;
 	struct file *swap_file, *victim;
 	struct address_space *mapping;
@@ -2539,8 +2535,6 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 	p->swap_file = NULL;
 	swap_map = p->swap_map;
 	p->swap_map = NULL;
-	zeromap = p->zeromap;
-	p->zeromap = NULL;
 	maxpages = p->max;
 	cluster_info = p->cluster_info;
 	p->max = 0;
@@ -2552,7 +2546,6 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 	kfree(p->global_cluster);
 	p->global_cluster = NULL;
 	vfree(swap_map);
-	kvfree(zeromap);
 	free_cluster_info(cluster_info, maxpages);
 
 	inode = mapping->host;
@@ -3016,7 +3009,6 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 	sector_t span;
 	unsigned long maxpages;
 	unsigned char *swap_map = NULL;
-	unsigned long *zeromap = NULL;
 	struct swap_cluster_info *cluster_info = NULL;
 	struct folio *folio = NULL;
 	struct inode *inode = NULL;
@@ -3122,17 +3114,6 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 	if (error)
 		goto bad_swap_unlock_inode;
 
-	/*
-	 * Use kvmalloc_array instead of bitmap_zalloc as the allocation order might
-	 * be above MAX_PAGE_ORDER incase of a large swap file.
-	 */
-	zeromap = kvmalloc_array(BITS_TO_LONGS(maxpages), sizeof(long),
-				    GFP_KERNEL | __GFP_ZERO);
-	if (!zeromap) {
-		error = -ENOMEM;
-		goto bad_swap_unlock_inode;
-	}
-
 	if (si->bdev && bdev_stable_writes(si->bdev))
 		si->flags |= SWP_STABLE_WRITES;
 
@@ -3199,7 +3180,7 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 	prio = DEF_SWAP_PRIO;
 	if (swap_flags & SWAP_FLAG_PREFER)
 		prio = swap_flags & SWAP_FLAG_PRIO_MASK;
-	enable_swap_info(si, prio, swap_map, cluster_info, zeromap);
+	enable_swap_info(si, prio, swap_map, cluster_info);
 
 	pr_info("Adding %uk swap on %s.  Priority:%d extents:%d across:%lluk %s%s%s%s\n",
 		K(si->pages), name->name, si->prio, nr_extents,
@@ -3227,7 +3208,6 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 	si->flags = 0;
 	spin_unlock(&swap_lock);
 	vfree(swap_map);
-	kvfree(zeromap);
 	if (cluster_info)
 		free_cluster_info(cluster_info, maxpages);
 	if (inced_nr_rotate_swap)
-- 
2.52.0