2 files changed, 138 insertions, 50 deletions

diff --git a/include/linux/swap.h b/include/linux/swap.h
index c0d49dad7a4b..16dcf8bd1a4e 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -306,7 +306,7 @@ struct swap_info_struct {
 					/* list of cluster that are fragmented or contented */
 	unsigned int frag_cluster_nr[SWAP_NR_ORDERS];
 	unsigned int pages;		/* total of usable pages of swap */
-	unsigned int inuse_pages;	/* number of those currently in use */
+	atomic_long_t inuse_pages;	/* number of those currently in use */
 	struct percpu_cluster __percpu *percpu_cluster; /* per cpu's swap location */
 	struct rb_root swap_extent_root;/* root of the swap extent rbtree */
 	struct block_device *bdev;	/* swap device or bdev of swap file */
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 2686032d3510..91faf2073006 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -128,6 +128,26 @@ static inline unsigned char swap_count(unsigned char ent)
 	return ent & ~SWAP_HAS_CACHE;	/* may include COUNT_CONTINUED flag */
 }
 
+/*
+ * Use the second highest bit of inuse_pages counter as the indicator
+ * if one swap device is on the available plist, so the atomic can
+ * still be updated arithmetically while having special data embedded.
+ *
+ * inuse_pages counter is the only thing indicating if a device should
+ * be on avail_lists or not (except swapon / swapoff). By embedding the
+ * off-list bit in the atomic counter, updates no longer need any lock
+ * to check the list status.
+ *
+ * This bit will be set if the device is not on the plist and not
+ * usable, will be cleared if the device is on the plist.
+ */
+#define SWAP_USAGE_OFFLIST_BIT (1UL << (BITS_PER_TYPE(atomic_t) - 2))
+#define SWAP_USAGE_COUNTER_MASK (~SWAP_USAGE_OFFLIST_BIT)
+static long swap_usage_in_pages(struct swap_info_struct *si)
+{
+	return atomic_long_read(&si->inuse_pages) & SWAP_USAGE_COUNTER_MASK;
+}
+
 /* Reclaim the swap entry anyway if possible */
 #define TTRS_ANYWAY		0x1
 /*
@@ -717,7 +737,7 @@ static void swap_reclaim_full_clusters(struct swap_info_struct *si, bool force)
 	int nr_reclaim;
 
 	if (force)
-		to_scan = si->inuse_pages / SWAPFILE_CLUSTER;
+		to_scan = swap_usage_in_pages(si) / SWAPFILE_CLUSTER;
 
 	while (!list_empty(&si->full_clusters)) {
 		ci = list_first_entry(&si->full_clusters, struct swap_cluster_info, list);
@@ -872,42 +892,128 @@ done:
 	return found;
 }
 
-static void __del_from_avail_list(struct swap_info_struct *si)
+/* SWAP_USAGE_OFFLIST_BIT can only be set by this helper. */
+static void del_from_avail_list(struct swap_info_struct *si, bool swapoff)
 {
 	int nid;
+	unsigned long pages;
+
+	spin_lock(&swap_avail_lock);
+
+	if (swapoff) {
+		/*
+		 * Forcefully remove it. Clear the SWP_WRITEOK flags for
+		 * swapoff here so it's synchronized by both si->lock and
+		 * swap_avail_lock, to ensure the result can be seen by
+		 * add_to_avail_list.
+		 */
+		lockdep_assert_held(&si->lock);
+		si->flags &= ~SWP_WRITEOK;
+		atomic_long_or(SWAP_USAGE_OFFLIST_BIT, &si->inuse_pages);
+	} else {
+		/*
+		 * If not called by swapoff, take it off-list only if it's
+		 * full and SWAP_USAGE_OFFLIST_BIT is not set (strictly
+		 * si->inuse_pages == pages), any concurrent slot freeing,
+		 * or device already removed from plist by someone else
+		 * will make this return false.
+		 */
+		pages = si->pages;
+		if (!atomic_long_try_cmpxchg(&si->inuse_pages, &pages,
+					     pages | SWAP_USAGE_OFFLIST_BIT))
+			goto skip;
+	}
 
-	assert_spin_locked(&si->lock);
 	for_each_node(nid)
 		plist_del(&si->avail_lists[nid], &swap_avail_heads[nid]);
+
+skip:
+	spin_unlock(&swap_avail_lock);
 }
 
-static void del_from_avail_list(struct swap_info_struct *si)
+/* SWAP_USAGE_OFFLIST_BIT can only be cleared by this helper. */
+static void add_to_avail_list(struct swap_info_struct *si, bool swapon)
 {
+	int nid;
+	long val;
+	unsigned long pages;
+
 	spin_lock(&swap_avail_lock);
-	__del_from_avail_list(si);
+
+	/* Corresponding to SWP_WRITEOK clearing in del_from_avail_list */
+	if (swapon) {
+		lockdep_assert_held(&si->lock);
+		si->flags |= SWP_WRITEOK;
+	} else {
+		if (!(READ_ONCE(si->flags) & SWP_WRITEOK))
+			goto skip;
+	}
+
+	if (!(atomic_long_read(&si->inuse_pages) & SWAP_USAGE_OFFLIST_BIT))
+		goto skip;
+
+	val = atomic_long_fetch_and_relaxed(~SWAP_USAGE_OFFLIST_BIT, &si->inuse_pages);
+
+	/*
+	 * When device is full and device is on the plist, only one updater will
+	 * see (inuse_pages == si->pages) and will call del_from_avail_list. If
+	 * that updater happen to be here, just skip adding.
+	 */
+	pages = si->pages;
+	if (val == pages) {
+		/* Just like the cmpxchg in del_from_avail_list */
+		if (atomic_long_try_cmpxchg(&si->inuse_pages, &pages,
+					    pages | SWAP_USAGE_OFFLIST_BIT))
+			goto skip;
+	}
+
+	for_each_node(nid)
+		plist_add(&si->avail_lists[nid], &swap_avail_heads[nid]);
+
+skip:
 	spin_unlock(&swap_avail_lock);
 }
 
-static void swap_range_alloc(struct swap_info_struct *si,
-			     unsigned int nr_entries)
+/*
+ * swap_usage_add / swap_usage_sub of each slot are serialized by ci->lock
+ * within each cluster, so the total contribution to the global counter should
+ * always be positive and cannot exceed the total number of usable slots.
+ */
+static bool swap_usage_add(struct swap_info_struct *si, unsigned int nr_entries)
 {
-	WRITE_ONCE(si->inuse_pages, si->inuse_pages + nr_entries);
-	if (si->inuse_pages == si->pages) {
-		del_from_avail_list(si);
+	long val = atomic_long_add_return_relaxed(nr_entries, &si->inuse_pages);
 
-		if (vm_swap_full())
-			schedule_work(&si->reclaim_work);
+	/*
+	 * If device is full, and SWAP_USAGE_OFFLIST_BIT is not set,
+	 * remove it from the plist.
+	 */
+	if (unlikely(val == si->pages)) {
+		del_from_avail_list(si, false);
+		return true;
 	}
+
+	return false;
 }
 
-static void add_to_avail_list(struct swap_info_struct *si)
+static void swap_usage_sub(struct swap_info_struct *si, unsigned int nr_entries)
 {
-	int nid;
+	long val = atomic_long_sub_return_relaxed(nr_entries, &si->inuse_pages);
 
-	spin_lock(&swap_avail_lock);
-	for_each_node(nid)
-		plist_add(&si->avail_lists[nid], &swap_avail_heads[nid]);
-	spin_unlock(&swap_avail_lock);
+	/*
+	 * If device is not full, and SWAP_USAGE_OFFLIST_BIT is set,
+	 * remove it from the plist.
+	 */
+	if (unlikely(val & SWAP_USAGE_OFFLIST_BIT))
+		add_to_avail_list(si, false);
+}
+
+static void swap_range_alloc(struct swap_info_struct *si,
+			     unsigned int nr_entries)
+{
+	if (swap_usage_add(si, nr_entries)) {
+		if (vm_swap_full())
+			schedule_work(&si->reclaim_work);
+	}
 }
 
 static void swap_range_free(struct swap_info_struct *si, unsigned long offset,
@@ -925,8 +1031,6 @@ static void swap_range_free(struct swap_info_struct *si, unsigned long offset,
 	for (i = 0; i < nr_entries; i++)
 		clear_bit(offset + i, si->zeromap);
 
-	if (si->inuse_pages == si->pages)
-		add_to_avail_list(si);
 	if (si->flags & SWP_BLKDEV)
 		swap_slot_free_notify =
 			si->bdev->bd_disk->fops->swap_slot_free_notify;
@@ -946,7 +1050,7 @@ static void swap_range_free(struct swap_info_struct *si, unsigned long offset,
 	 */
 	smp_wmb();
 	atomic_long_add(nr_entries, &nr_swap_pages);
-	WRITE_ONCE(si->inuse_pages, si->inuse_pages - nr_entries);
+	swap_usage_sub(si, nr_entries);
 }
 
 static int cluster_alloc_swap(struct swap_info_struct *si,
@@ -1036,19 +1140,6 @@ start_over:
 		plist_requeue(&si->avail_lists[node], &swap_avail_heads[node]);
 		spin_unlock(&swap_avail_lock);
 		spin_lock(&si->lock);
-		if ((si->inuse_pages == si->pages) || !(si->flags & SWP_WRITEOK)) {
-			spin_lock(&swap_avail_lock);
-			if (plist_node_empty(&si->avail_lists[node])) {
-				spin_unlock(&si->lock);
-				goto nextsi;
-			}
-			WARN(!(si->flags & SWP_WRITEOK),
-			     "swap_info %d in list but !SWP_WRITEOK\n",
-			     si->type);
-			__del_from_avail_list(si);
-			spin_unlock(&si->lock);
-			goto nextsi;
-		}
 		n_ret = scan_swap_map_slots(si, SWAP_HAS_CACHE,
 					    n_goal, swp_entries, order);
 		spin_unlock(&si->lock);
@@ -1057,7 +1148,6 @@ start_over:
 		cond_resched();
 
 		spin_lock(&swap_avail_lock);
-nextsi:
 		/*
 		 * if we got here, it's likely that si was almost full before,
 		 * and since scan_swap_map_slots() can drop the si->lock,
@@ -1789,7 +1879,7 @@ unsigned int count_swap_pages(int type, int free)
 		if (sis->flags & SWP_WRITEOK) {
 			n = sis->pages;
 			if (free)
-				n -= sis->inuse_pages;
+				n -= swap_usage_in_pages(sis);
 		}
 		spin_unlock(&sis->lock);
 	}
@@ -2124,7 +2214,7 @@ static int try_to_unuse(unsigned int type)
 	swp_entry_t entry;
 	unsigned int i;
 
-	if (!READ_ONCE(si->inuse_pages))
+	if (!swap_usage_in_pages(si))
 		goto success;
 
 retry:
@@ -2137,7 +2227,7 @@ retry:
 
 	spin_lock(&mmlist_lock);
 	p = &init_mm.mmlist;
-	while (READ_ONCE(si->inuse_pages) &&
+	while (swap_usage_in_pages(si) &&
 	       !signal_pending(current) &&
 	       (p = p->next) != &init_mm.mmlist) {
 
@@ -2165,7 +2255,7 @@ retry:
 	mmput(prev_mm);
 
 	i = 0;
-	while (READ_ONCE(si->inuse_pages) &&
+	while (swap_usage_in_pages(si) &&
 	       !signal_pending(current) &&
 	       (i = find_next_to_unuse(si, i)) != 0) {
 
@@ -2200,7 +2290,7 @@ retry:
 	 * folio_alloc_swap(), temporarily hiding that swap.  It's easy
 	 * and robust (though cpu-intensive) just to keep retrying.
 	 */
-	if (READ_ONCE(si->inuse_pages)) {
+	if (swap_usage_in_pages(si)) {
 		if (!signal_pending(current))
 			goto retry;
 		return -EINTR;
@@ -2227,7 +2317,7 @@ static void drain_mmlist(void)
 	unsigned int type;
 
 	for (type = 0; type < nr_swapfiles; type++)
-		if (swap_info[type]->inuse_pages)
+		if (swap_usage_in_pages(swap_info[type]))
 			return;
 	spin_lock(&mmlist_lock);
 	list_for_each_safe(p, next, &init_mm.mmlist)
@@ -2406,7 +2496,6 @@ static void setup_swap_info(struct swap_info_struct *si, int prio,
 
 static void _enable_swap_info(struct swap_info_struct *si)
 {
-	si->flags |= SWP_WRITEOK;
 	atomic_long_add(si->pages, &nr_swap_pages);
 	total_swap_pages += si->pages;
 
@@ -2423,9 +2512,8 @@ static void _enable_swap_info(struct swap_info_struct *si)
 	 */
 	plist_add(&si->list, &swap_active_head);
 
-	/* add to available list if swap device is not full */
-	if (si->inuse_pages < si->pages)
-		add_to_avail_list(si);
+	/* Add back to available list */
+	add_to_avail_list(si, true);
 }
 
 static void enable_swap_info(struct swap_info_struct *si, int prio,
@@ -2523,7 +2611,7 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 		goto out_dput;
 	}
 	spin_lock(&p->lock);
-	del_from_avail_list(p);
+	del_from_avail_list(p, true);
 	if (p->prio < 0) {
 		struct swap_info_struct *si = p;
 		int nid;
@@ -2541,7 +2629,6 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 	plist_del(&p->list, &swap_active_head);
 	atomic_long_sub(p->pages, &nr_swap_pages);
 	total_swap_pages -= p->pages;
-	p->flags &= ~SWP_WRITEOK;
 	spin_unlock(&p->lock);
 	spin_unlock(&swap_lock);
 
@@ -2721,7 +2808,7 @@ static int swap_show(struct seq_file *swap, void *v)
 	}
 
 	bytes = K(si->pages);
-	inuse = K(READ_ONCE(si->inuse_pages));
+	inuse = K(swap_usage_in_pages(si));
 
 	file = si->swap_file;
 	len = seq_file_path(swap, file, " \t\n\\");
@@ -2838,6 +2925,7 @@ static struct swap_info_struct *alloc_swap_info(void)
 	}
 	spin_lock_init(&p->lock);
 	spin_lock_init(&p->cont_lock);
+	atomic_long_set(&p->inuse_pages, SWAP_USAGE_OFFLIST_BIT);
 	init_completion(&p->comp);
 
 	return p;
@@ -3335,7 +3423,7 @@ void si_swapinfo(struct sysinfo *val)
 		struct swap_info_struct *si = swap_info[type];
 
 		if ((si->flags & SWP_USED) && !(si->flags & SWP_WRITEOK))
-			nr_to_be_unused += READ_ONCE(si->inuse_pages);
+			nr_to_be_unused += swap_usage_in_pages(si);
 	}
 	val->freeswap = atomic_long_read(&nr_swap_pages) + nr_to_be_unused;
 	val->totalswap = total_swap_pages + nr_to_be_unused;