[RFC PATCH v3 13/18] mm/swap: drop swap active plist

[Lian Wang <lianux.mm@gmail.com>]

From: Kairui Song <kasong@tencent.com>

Remove swap_active_head and the associated plist_node. Device identity
lookups can use the writable swap_info iterator without ordering.

Keep priority ordering for the block-cgroup swap throttle target by
walking the priority queue under the topology reader. Choose the first
unmasked, writable, non-offlist device with a backing block device.
Check WRITEOK and OFFLIST as well as the pointer tag because disable
and full transitions can temporarily leave an unmasked entry.

This read-only lookup does not consume per-CPU allocation quotas or
change retry cursors. It avoids making the throttle disk depend on
swapon's swap-type assignment order when devices have different
priorities.

Signed-off-by: Kairui Song <kasong@tencent.com>
Signed-off-by: Lian Wang <lianux.mm@gmail.com>
---
 include/linux/swap.h |  1 -
 mm/swapfile.c        | 59 ++++++++++++++++++++++++--------------------
 2 files changed, 32 insertions(+), 28 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 17b8ec654ee2..bfca46e1fbc6 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -261,7 +261,6 @@ struct swap_info_struct {
 	struct percpu_ref users;	/* indicate and keep swap device valid. */
 	unsigned long	flags;		/* SWP_USED etc: see above */
 	signed short	prio;		/* swap priority of this type */
-	struct plist_node list;		/* entry in swap_active_head */
 	signed char	type;		/* strange name for an index */
 	unsigned int	max;		/* size of this swap device */
 	struct swap_cluster_info *cluster_info; /* array, one entry per cluster */
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 5c26682c0bab..7b3520ce1e41 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -41,7 +41,6 @@
 #include <linux/completion.h>
 #include <linux/suspend.h>
 #include <linux/zswap.h>
-#include <linux/plist.h>
 
 #include <asm/tlbflush.h>
 #include <linux/leafops.h>
@@ -90,12 +89,6 @@ bool swap_migration_ad_supported;
 static const char Bad_file[] = "Bad swap file entry ";
 static const char Bad_offset[] = "Bad swap offset entry ";
 
-/*
- * all active swap_info_structs
- * protected with swapon_rwsem, and ordered by priority.
- */
-static PLIST_HEAD(swap_active_head);
-
 static inline struct swap_info_struct *__swap_iter(int *i, unsigned long flag)
 {
 	lockdep_assert_held(&swapon_rwsem);
@@ -3423,7 +3416,6 @@ static void __swap_device_enable(struct swap_info_struct *si)
 	spin_unlock(&swap_queue_update_lock);
 	atomic_long_add(si->pages, &nr_swap_pages);
 	total_swap_pages += si->pages;
-	plist_add(&si->list, &swap_active_head);
 	add_to_avail_list(si);
 }
 
@@ -3443,9 +3435,8 @@ static int swap_device_disable(struct address_space *mapping,
 	int err = -EINVAL;
 
 	percpu_down_write(&swapon_rwsem);
-	plist_for_each_entry(si, &swap_active_head, list) {
-		if ((si->flags & SWP_WRITEOK) &&
-		    si->swap_file->f_mapping == mapping) {
+	for_each_avail_swap(si) {
+		if (si->swap_file->f_mapping == mapping) {
 			err = 0;
 			break;
 		}
@@ -3470,7 +3461,6 @@ static int swap_device_disable(struct address_space *mapping,
 	spin_lock(&swap_queue_update_lock);
 	si->flags &= ~SWP_WRITEOK;
 	spin_unlock(&swap_queue_update_lock);
-	plist_del(&si->list, &swap_active_head);
 	total_swap_pages -= si->pages;
 	atomic_long_sub(si->pages, &nr_swap_pages);
 
@@ -3801,7 +3791,6 @@ static struct swap_info_struct *alloc_swap_info(void)
 		 */
 	}
 	p->swap_extent_root = RB_ROOT;
-	plist_node_init(&p->list, 0);
 	p->flags = SWP_USED;
 	percpu_up_write(&swapon_rwsem);
 	if (defer) {
@@ -4211,12 +4200,7 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 	if (swap_flags & SWAP_FLAG_PREFER)
 		prio = swap_flags & SWAP_FLAG_PRIO_MASK;
 
-	/*
-	 * The plist prio is negated because plist ordering is
-	 * low-to-high, while swap ordering is high-to-low
-	 */
 	si->prio = prio;
-	si->list.prio = -si->prio;
 
 	/*
 	 * Publish swap_file before making the percpu ref live, then add the device
@@ -4342,7 +4326,33 @@ int swap_dup_entry_direct(swp_entry_t entry)
 #if defined(CONFIG_MEMCG) && defined(CONFIG_BLK_CGROUP)
 static bool __has_usable_swap(void)
 {
-	return !plist_head_empty(&swap_active_head);
+	return READ_ONCE(total_swap_pages) > 0;
+}
+
+static struct swap_info_struct *swap_queue_first_usable_bdev(void)
+{
+	struct swap_prio_ring *ring;
+	struct swap_info_struct *si;
+	unsigned int ring_idx, dev_idx;
+
+	lockdep_assert_held(&swapon_rwsem);
+	for (ring_idx = 0; ring_idx < swap_queue_len; ring_idx++) {
+		ring = swap_queue[ring_idx];
+		for (dev_idx = 0; dev_idx < ring->size; dev_idx++) {
+			si = READ_ONCE(ring->dev[dev_idx]);
+			if (swap_device_masked(si))
+				continue;
+			si = swap_device_unmask_ptr(si);
+			if (!(READ_ONCE(si->flags) & SWP_WRITEOK))
+				continue;
+			if (atomic_long_read(&si->inuse_pages) &
+			    SWAP_USAGE_OFFLIST_BIT)
+				continue;
+			if (si->bdev)
+				return si;
+		}
+	}
+	return NULL;
 }
 
 void __folio_throttle_swaprate(struct folio *folio, gfp_t gfp)
@@ -4366,14 +4376,9 @@ void __folio_throttle_swaprate(struct folio *folio, gfp_t gfp)
 		return;
 
 	percpu_down_read(&swapon_rwsem);
-	plist_for_each_entry(si, &swap_active_head, list) {
-		if ((si->flags & SWP_WRITEOK) &&
-		    !(atomic_long_read(&si->inuse_pages) &
-		      SWAP_USAGE_OFFLIST_BIT) && si->bdev) {
-			blkcg_schedule_throttle(si->bdev->bd_disk, true);
-			break;
-		}
-	}
+	si = swap_queue_first_usable_bdev();
+	if (si)
+		blkcg_schedule_throttle(si->bdev->bd_disk, true);
 	percpu_up_read(&swapon_rwsem);
 }
 #endif