[PATCH v6 20/22] swapfile: replace the swap map with bitmaps

Nhat Pham
2026-05-05T15:39:43Z

Now that we have moved the swap count state to virtual swap layer, each
swap map entry only has 3 possible states: free, allocated, and bad.
Replace the swap map with 2 bitmaps (one for allocated state and one for
bad state), saving 6 bits per swap entry.

Signed-off-by: Nhat Pham <nphamcs@gmail.com>
---
include/linux/swap.h | 3 +-
mm/swapfile.c | 81 +++++++++++++++++++++++---------------------
2 files changed, 44 insertions(+), 40 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index ad5f59c807c6..aac9971633f2 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -259,7 +259,8 @@ struct swap_info_struct {
struct plist_node list; /* entry in swap_active_head */
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 *swap_map; /* bitmap for allocated state */
+ unsigned long *bad_map; /* bitmap for bad state */
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 701bc80bc381..b5b126904d20 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -767,25 +767,19 @@ static bool cluster_reclaim_range(struct swap_info_struct *si,
struct swap_cluster_info *ci,
unsigned long start, unsigned long end)
{
- unsigned char *map = si->swap_map;
unsigned long offset = start;
int nr_reclaim;

spin_unlock(&ci->lock);
do {
- switch (READ_ONCE(map[offset])) {
- case 0:
+ if (!test_bit(offset, si->swap_map)) {
offset++;
- break;
- case SWAP_MAP_ALLOCATED:
+ } else {
nr_reclaim = __try_to_reclaim_swap(si, offset, TTRS_ANYWAY);
if (nr_reclaim > 0)
offset += nr_reclaim;
else
goto out;
- break;
- default:
- goto out;
}
} while (offset < end);
out:
@@ -794,11 +788,7 @@ static bool cluster_reclaim_range(struct swap_info_struct *si,
* Recheck the range no matter reclaim succeeded or not, the slot
* could have been be freed while we are not holding the lock.
*/
- for (offset = start; offset < end; offset++)
- if (READ_ONCE(map[offset]))
- return false;
-
- return true;
+ return find_next_bit(si->swap_map, end, start) >= end;
}

static bool cluster_scan_range(struct swap_info_struct *si,
@@ -807,15 +797,16 @@ static bool cluster_scan_range(struct swap_info_struct *si,
bool *need_reclaim)
{
unsigned long offset, end = start + nr_pages;
- unsigned char *map = si->swap_map;
- unsigned char count;

if (cluster_is_empty(ci))
return true;

for (offset = start; offset < end; offset++) {
- count = READ_ONCE(map[offset]);
- if (!count)
+ /* Bad slots cannot be used for allocation */
+ if (test_bit(offset, si->bad_map))
+ return false;
+
+ if (!test_bit(offset, si->swap_map))
continue;

if (swap_cache_only(si, offset)) {
@@ -848,7 +839,7 @@ static bool cluster_alloc_range(struct swap_info_struct *si, struct swap_cluster
if (cluster_is_empty(ci))
ci->order = order;

- memset(si->swap_map + start, usage, nr_pages);
+ bitmap_set(si->swap_map, start, nr_pages);
swap_range_alloc(si, nr_pages);
ci->count += nr_pages;

@@ -1414,7 +1405,7 @@ static struct swap_info_struct *_swap_info_get(swp_slot_t slot)
offset = swp_slot_offset(slot);
if (offset >= si->max)
goto bad_offset;
- if (data_race(!si->swap_map[swp_slot_offset(slot)]))
+ if (data_race(!test_bit(offset, si->swap_map)))
goto bad_free;
return si;

@@ -1528,8 +1519,7 @@ static void swap_slots_free(struct swap_info_struct *si,
swp_slot_t slot, unsigned int nr_pages)
{
unsigned long offset = swp_slot_offset(slot);
- unsigned char *map = si->swap_map + offset;
- unsigned char *map_end = map + nr_pages;
+ unsigned long end = offset + nr_pages;

/* It should never free entries across different clusters */
VM_BUG_ON(ci != __swap_offset_to_cluster(si, offset + nr_pages - 1));
@@ -1537,10 +1527,8 @@ static void swap_slots_free(struct swap_info_struct *si,
VM_BUG_ON(ci->count < nr_pages);

ci->count -= nr_pages;
- do {
- VM_BUG_ON(!swap_is_last_ref(*map));
- *map = 0;
- } while (++map < map_end);
+ VM_BUG_ON(find_next_zero_bit(si->swap_map, end, offset) < end);
+ bitmap_clear(si->swap_map, offset, nr_pages);

swap_range_free(si, offset, nr_pages);

@@ -1700,9 +1688,7 @@ unsigned int count_swap_pages(int type, int free)
static bool swap_slot_allocated(struct swap_info_struct *si,
unsigned long offset)
{
- unsigned char count = READ_ONCE(si->swap_map[offset]);
-
- return count && swap_count(count) != SWAP_MAP_BAD;
+ return test_bit(offset, si->swap_map);
}

/*
@@ -2023,7 +2009,7 @@ 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,
+ unsigned long *swap_map,
struct swap_cluster_info *cluster_info)
{
si->prio = prio;
@@ -2051,7 +2037,7 @@ 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,
+ unsigned long *swap_map,
struct swap_cluster_info *cluster_info)
{
spin_lock(&swap_lock);
@@ -2144,7 +2130,8 @@ static void flush_percpu_swap_cluster(struct swap_info_struct *si)
SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
{
struct swap_info_struct *p = NULL;
- unsigned char *swap_map;
+ unsigned long *swap_map;
+ unsigned long *bad_map;
struct swap_cluster_info *cluster_info;
struct file *swap_file, *victim;
struct address_space *mapping;
@@ -2239,6 +2226,8 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
p->swap_file = NULL;
swap_map = p->swap_map;
p->swap_map = NULL;
+ bad_map = p->bad_map;
+ p->bad_map = NULL;
maxpages = p->max;
cluster_info = p->cluster_info;
p->max = 0;
@@ -2249,7 +2238,8 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
mutex_unlock(&swapon_mutex);
kfree(p->global_cluster);
p->global_cluster = NULL;
- vfree(swap_map);
+ kvfree(swap_map);
+ kvfree(bad_map);
free_cluster_info(cluster_info, maxpages);

inode = mapping->host;
@@ -2597,18 +2587,20 @@ static unsigned long read_swap_header(struct swap_info_struct *si,

static int setup_swap_map(struct swap_info_struct *si,
union swap_header *swap_header,
- unsigned char *swap_map,
+ unsigned long *swap_map,
+ unsigned long *bad_map,
unsigned long maxpages)
{
unsigned long i;

- swap_map[0] = SWAP_MAP_BAD; /* omit header page */
+ set_bit(0, bad_map); /* omit header page */
+
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)
return -EINVAL;
if (page_nr < maxpages) {
- swap_map[page_nr] = SWAP_MAP_BAD;
+ set_bit(page_nr, bad_map);
si->pages--;
}
}
@@ -2712,7 +2704,7 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
int nr_extents;
sector_t span;
unsigned long maxpages;
- unsigned char *swap_map = NULL;
+ unsigned long *swap_map = NULL, *bad_map = NULL;
struct swap_cluster_info *cluster_info = NULL;
struct folio *folio = NULL;
struct inode *inode = NULL;
@@ -2808,16 +2800,24 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
maxpages = si->max;

/* OK, set up the swap map and apply the bad block list */
- swap_map = vzalloc(maxpages);
+ swap_map = kvcalloc(BITS_TO_LONGS(maxpages), sizeof(long), GFP_KERNEL);
if (!swap_map) {
error = -ENOMEM;
goto bad_swap_unlock_inode;
}

- error = setup_swap_map(si, swap_header, swap_map, maxpages);
+ bad_map = kvcalloc(BITS_TO_LONGS(maxpages), sizeof(long), GFP_KERNEL);
+ if (!bad_map) {
+ error = -ENOMEM;
+ goto bad_swap_unlock_inode;
+ }
+
+ error = setup_swap_map(si, swap_header, swap_map, bad_map, maxpages);
if (error)
goto bad_swap_unlock_inode;

+ si->bad_map = bad_map;
+
if (si->bdev && bdev_stable_writes(si->bdev))
si->flags |= SWP_STABLE_WRITES;

@@ -2911,7 +2911,10 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
si->swap_file = NULL;
si->flags = 0;
spin_unlock(&swap_lock);
- vfree(swap_map);
+ if (swap_map)
+ kvfree(swap_map);
+ if (bad_map)
+ kvfree(bad_map);
if (cluster_info)
free_cluster_info(cluster_info, maxpages);
if (inced_nr_rotate_swap)
--
2.52.0