[RESEND RFC PATCH v2 00/13] mm/swap: introduce per-priority allocation queues

[RESEND RFC PATCH v2 00/13] mm/swap: introduce per-priority allocation queues

[Lian Wang (ProcessMission)]

Hi all,

RESEND: Only the first cover letter was sent, with an outdated sender
display name.  No patches from the series were sent.  This resend uses
the corrected sender identity and starts the complete thread.  There are
no code or content changes from the previously reviewed v2 draft.

This is v2 of Kairui's swap priority queue RFC [1].  With Kairui's
agreement, I am carrying this revision while preserving the original
authorship of his patches and Youngjun's per-device percpu-cluster patch.

My substantive code changes are concentrated in patches 3, 9 and 11.
Patches 5, 10 and 12 contain the dependent adaptations needed to keep
the stack coherent and bisectable.  The code patch-ids of patches 1, 2,
4, 6, 7, 8 and 13 are unchanged from v1.  My Signed-off-by on the full
series records the handoff and DCO chain; Co-developed-by is limited to
the three patches with substantive changes.

The current swap allocator rotates swap_avail_head with plist_requeue()
when it moves between devices of the same priority.  That couples device
selection to the global cluster cache and serializes cluster transitions
on swap_avail_lock.

This series first restores per-device percpu clusters, then replaces the
allocation-time plist rotation with a priority-ordered queue.  Each
priority has a mostly immutable ring of devices.  Per-CPU readers rotate
within a ring after a fixed allocation quota, so the allocator preserves
strict priority ordering without contending on one global rotation point.

The queue is also intended to separate device selection policy from the
per-device cluster allocator.  Youngjun's swap-tier series [2] remains
under separate review.  This RFC does not introduce a tier ABI or assume
that the tier series has landed: a ring is keyed by the existing swap
priority.  If tiers land first, the same queue can become an in-tier
allocation policy rather than a competing tier definition.

Changes since v1
================

The core allocator and lifetime changes are:

- keep a task-local cursor stable across retries, so a retry walk visits
  each same-priority peer exactly once even if another task rotates the
  shared per-CPU reader;
- disable task migration across the queue walk, keeping queue accounting
  and per-CPU cluster allocation on the same CPU while leaving the loop
  sleepable;
- retry every same-priority peer for a large folio before returning
  -E2BIG, without falling through to a lower priority merely because one
  device is fragmented;
- initialize the per-CPU quota consistently;
- represent full/disabled devices with tagged ring entries, serialize tag
  writers, and use READ_ONCE()/WRITE_ONCE() for lockless readers;
- publish swap_file, the live percpu reference, queue membership and
  SWP_WRITEOK in one swapon writer section, and tighten swapoff disable
  and teardown ordering;
- fix the transitional available-list locking and publication issues
  reported by Jihan;
- bound allocation-path synchronous discard to one cluster, while the
  background worker can continue draining the list; and
- remove the transitional available/active plists and the now-unused
  plist_requeue() API after the queue becomes authoritative.

These changes address the public review on patch 9 from Youngjun and
Jihan [3][4].

Open questions
==============

Two performance-policy questions from the v1 discussion remain open:

- A one-device ring, which is likely the most common configuration,
  still follows the common reader path and takes the local lock.  A
  size-one fast path could return ring->dev[0] directly while
  swapon_rwsem keeps the ring stable.  This revision keeps the tested
  common path unchanged; feedback on whether the special case is worth
  adding would be welcome.
- Full devices remain as masked entries in the static ring.  Readers skip
  them rather than rebuilding the ring on each full/available transition.
  This keeps writers simple, but a workload with many full devices in a
  higher-priority ring still pays the masked-entry checks before falling
  through.  It is not clear that extra per-ring availability state would
  justify its synchronization complexity.

Testing
=======

Kunwu ran an A/B/C/D/E matrix on a Kunpeng 920 server with 256 CPUs,
249 GiB RAM and an aarch64 kernel.  The workload built a fixed kernel
source with "make -j96 defconfig Image modules" under 2 GiB and 3 GiB
memory cgroups, using eight equal-priority ZRAM devices.  Every arm used
one warm-up followed by 12 measured runs.

A/B/C share the v1 base and reproduce the progression reported in v1:

  A: before the series
  B: after patch 8, with per-device percpu clusters but the old plist
  C: after patch 13, with the complete priority queue

D/E share the tested v2 base and isolate the current series:

  D: before the series
  E: after all 13 v2 patches

1. Reproducing the v1 performance shape

Average system time over 12 measured kernel builds:

                              2 GiB       3 GiB
  Before (A)                40899.14s    22621.86s
  After patch 8 (B)         88712.52s    53244.70s
  After patch 13 (C)        41140.76s    22925.48s

Patch 8 alone increased system time by 116.9% in the 2 GiB workload
and 135.4% in the 3 GiB workload.  Adding the priority queue returned
system time to within 0.59% and 1.34% of A, respectively.  The absolute
times differ from the v1 machine, but the expected performance shape is
reproduced: moving the cluster cache back to device scope is expensive
while allocation still rotates the plist, and the queue removes that
contention.

2. Isolating the v2 delta

Average system time over 12 measured runs:

                              2 GiB       3 GiB
  Base (D)                  39643.77s    22139.66s
  Full v2 (E)              40336.91s    22984.62s
  Delta                       +1.75%       +3.82%

Average elapsed time, with sample standard deviation:

                              2 GiB       3 GiB
  Base (D)              518.97 +/- 5.87s  334.07 +/- 4.01s
  Full v2 (E)           527.24 +/- 5.21s  342.98 +/- 2.78s
  Delta                       +1.59%       +2.67%

The arms were run sequentially rather than interleaved, so these small
D/E deltas may include temporal drift.  They are reported as observed
instead of being described as zero regression.

3. Correctness and stress

All 120 measured builds completed with build_exit=0, oom_kill=0,
pswpout>0 and a clean dmesg delta.  Arm E also passed tests covering:

- same-priority distribution and lower-priority isolation;
- concurrent swapon/swapoff and /proc/swaps readers;
- full-device mask, unmask and peer refill;
- same-ring large-folio peer retry without fallback; and
- dmesg cleanliness and complete cleanup.

A 600-second stress run concurrently migrated the memory-pressure tasks
across CPUs and inserted and removed a device from the priority ring.  It
completed with live workers, swapout progress and a clean dmesg delta.

For the 48 D/E measured runs, all eight ZRAM devices had nonzero recorded
peak usage.  On E, the per-sample largest/smallest peak ratio averaged
1.021 for 2 GiB and 1.031 for 3 GiB; the worst observed ratio was 1.049.

4. Scope and posting base

The full server matrix was run from commit 94f9b3980dd4
("mm/page_reporting: Add page_reporting_delay_ms module parameter") to
commit d5c8964cf19f ("lib/plist.c: remove requeue function").  The series
below is rebased onto current mm-unstable; range-diff shows all 13 patches
are patch-identical.

The original 48 GiB BRD workload and a real multi-SSD workload were not
rerun, so this revision makes no BRD or hardware-scaling claim.

Link: https://lore.kernel.org/20260714-swap-pcp-priq-v1-0-de9b164ed419@tencent.com [1]
Link: https://lore.kernel.org/20260713025644.170839-1-youngjun.park@lge.com [2]
Link: https://lore.kernel.org/alZ7UBXweuuOX4qz@yjaykim-PowerEdge-T330 [3]
Link: https://lore.kernel.org/77d6da3d-10af-49a1-a356-72aa8b462e85@gmail.com [4]

Thanks to Kairui for the original design and handoff, Youngjun and Jihan
for the v1 review, and Kunwu for the server validation.

Signed-off-by: Lian Wang (ProcessMission) <lianux.mm@gmail.com>
---

Kairui Song (12):
  mm/swap: remove unused parameter for reading swap header
  mm/swap: slightly cleanup the code for hibernation error handling
  mm/swap: cleanup and document swap device availability flag usage
  mm/swap: introduce swap device iteration helper
  mm/swap: change the swapon lock into a percpu rwsem
  mm/swap: remove swapon mutex and update proc reader
  mm/swap: consolidate swap inuse accounting helpers
  mm/swap: add priority queue for swap device allocation
  mm/swap: remove available list
  mm/swap: bound synchronous discard during allocation
  mm/swap: drop swap active plist
  lib/plist.c: remove requeue function

Youngjun Park (1):
  mm/swap: change back to use each swap device's percpu cluster

 include/linux/plist.h |    2 -
 include/linux/swap.h  |   43 +-
 lib/plist.c           |   64 --
 mm/swap.h             |   12 +-
 mm/swapfile.c         | 1301 +++++++++++++++++++++++++----------------
 5 files changed, 817 insertions(+), 605 deletions(-)


base-commit: aeddb4d52acfcc5ce5e988acd48f2906fe966ca3
-- 
2.55.0

[RESEND RFC PATCH v2 01/13] mm/swap: remove unused parameter for reading swap header

[Lian Wang (ProcessMission)]

From: Kairui Song <kasong@tencent.com>

No feature change, just a minor cleanup.

Signed-off-by: Kairui Song <kasong@tencent.com>
Signed-off-by: Lian Wang (ProcessMission) <lianux.mm@gmail.com>
Tested-by: Kunwu Chan <kunwu.chan@linux.dev>
---
 mm/swapfile.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 53bf01d5f7f1..0f962cdfa5c0 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -3467,9 +3467,8 @@ __weak unsigned long arch_max_swapfile_size(void)
 	return generic_max_swapfile_size();
 }
 
-static unsigned long read_swap_header(struct swap_info_struct *si,
-					union swap_header *swap_header,
-					struct inode *inode)
+static unsigned long read_swap_header(union swap_header *swap_header,
+				      struct inode *inode)
 {
 	int i;
 	unsigned long maxpages;
@@ -3700,7 +3699,7 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 	}
 	swap_header = kmap_local_folio(folio, 0);
 
-	maxpages = read_swap_header(si, swap_header, inode);
+	maxpages = read_swap_header(swap_header, inode);
 	if (unlikely(!maxpages)) {
 		error = -EINVAL;
 		goto bad_swap_unlock_inode;

base-commit: aeddb4d52acfcc5ce5e988acd48f2906fe966ca3
-- 
2.55.0

[RESEND RFC PATCH v2 02/13] mm/swap: slightly cleanup the code for hibernation error handling

[Lian Wang (ProcessMission)]

From: Kairui Song <kasong@tencent.com>

Restructure the error paths in pin_hibernation_swap_type() using a
goto out pattern to avoid repeated spin_unlock() calls and simplify
the return flow.  Also simplify unpin_hibernation_swap_type() by
making the si NULL check inline, and clean up find_first_swap() to
avoid an early return inside the loop.

Signed-off-by: Kairui Song <kasong@tencent.com>
Signed-off-by: Lian Wang (ProcessMission) <lianux.mm@gmail.com>
Tested-by: Kunwu Chan <kunwu.chan@linux.dev>
---
 mm/swapfile.c | 39 +++++++++++++++++----------------------
 1 file changed, 17 insertions(+), 22 deletions(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 0f962cdfa5c0..46772d0e3e68 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -2260,21 +2260,18 @@ static int __find_hibernation_swap_type(dev_t device, sector_t offset)
  */
 int pin_hibernation_swap_type(dev_t device, sector_t offset)
 {
-	int type;
+	int ret;
 	struct swap_info_struct *si;
 
 	spin_lock(&swap_lock);
+	ret = __find_hibernation_swap_type(device, offset);
+	if (ret < 0)
+		goto out;
 
-	type = __find_hibernation_swap_type(device, offset);
-	if (type < 0) {
-		spin_unlock(&swap_lock);
-		return type;
-	}
-
-	si = swap_type_to_info(type);
+	si = swap_type_to_info(ret);
 	if (WARN_ON_ONCE(!si)) {
-		spin_unlock(&swap_lock);
-		return -ENODEV;
+		ret = -ENODEV;
+		goto out;
 	}
 
 	/*
@@ -2283,14 +2280,15 @@ int pin_hibernation_swap_type(dev_t device, sector_t offset)
 	 * the same session.
 	 */
 	if (WARN_ON_ONCE(si->flags & SWP_HIBERNATION)) {
-		spin_unlock(&swap_lock);
-		return -EBUSY;
+		ret = -EBUSY;
+		goto out;
 	}
 
 	si->flags |= SWP_HIBERNATION;
 
+out:
 	spin_unlock(&swap_lock);
-	return type;
+	return ret;
 }
 
 /**
@@ -2309,11 +2307,8 @@ void unpin_hibernation_swap_type(int type)
 
 	spin_lock(&swap_lock);
 	si = swap_type_to_info(type);
-	if (!si) {
-		spin_unlock(&swap_lock);
-		return;
-	}
-	si->flags &= ~SWP_HIBERNATION;
+	if (si)
+		si->flags &= ~SWP_HIBERNATION;
 	spin_unlock(&swap_lock);
 }
 
@@ -2348,7 +2343,7 @@ int find_hibernation_swap_type(dev_t device, sector_t offset)
 
 int find_first_swap(dev_t *device)
 {
-	int type;
+	int type, ret = -ENODEV;
 
 	spin_lock(&swap_lock);
 	for (type = 0; type < nr_swapfiles; type++) {
@@ -2357,11 +2352,11 @@ int find_first_swap(dev_t *device)
 		if (!(sis->flags & SWP_WRITEOK))
 			continue;
 		*device = sis->bdev->bd_dev;
-		spin_unlock(&swap_lock);
-		return type;
+		ret = type;
+		break;
 	}
 	spin_unlock(&swap_lock);
-	return -ENODEV;
+	return ret;
 }
 
 /*
-- 
2.55.0

[RESEND RFC PATCH v2 03/13] mm/swap: cleanup and document swap device availability flag usage

[Lian Wang (ProcessMission)]

From: Kairui Song <kasong@tencent.com>

Rework swap device flag and metadata handling and swapon/swapoff to
be cleaner and better documented, in preparation for locking cleanup.

Consolidate existing routines and introduce swap_device_enable() and
swap_device_disable() as the clean boundary of exposing or isolating a
swap device. swap_device_enable() sets the proper flags and exposes the
device as an allocation candidate, while swap_device_disable() clears
related flags and ensures no more allocations will happen.

Keep the mapping lookup and disable transition in the same swap_lock
critical section. Otherwise, a concurrent swapoff can release the
selected slot and swapon can reuse it for a different device before the
first syscall disables the swap_info_struct it found. Drain the cluster
allocators in the same helper after dropping swap_lock, so the identity
transition remains atomic without holding the lock over all cluster
locks.

Add comment blocks documenting the lifetime and locking rules for swap
device flags and their locking conventions.

Apart from closing that lifecycle race, the remaining changes are code
rearrangement and documentation.

Signed-off-by: Kairui Song <kasong@tencent.com>
Co-developed-by: Lian Wang (ProcessMission) <lianux.mm@gmail.com>
Signed-off-by: Lian Wang (ProcessMission) <lianux.mm@gmail.com>
Tested-by: Kunwu Chan <kunwu.chan@linux.dev>
---
 include/linux/swap.h |  27 ++++--
 mm/swapfile.c        | 207 ++++++++++++++++++++-----------------------
 2 files changed, 115 insertions(+), 119 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 5658a1634b85..d9e535cd07c5 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -194,6 +194,22 @@ struct swap_extent {
 	((offsetof(union swap_header, magic.magic) - \
 	  offsetof(union swap_header, info.badpages)) / sizeof(int))
 
+/*
+ * Swap device flags, except the ones documented below, all are immutable
+ * after exposed by swap_device_enable, and until the device is freed again
+ * (SWP_USED unset). The exceptions:
+ * - SWP_USED: Protected by swap_lock. Indicates the device is inuse. Once
+ *   set, won't be cleared unless all reference to this device is freed and
+ *   swapoff finished.
+ * - SWP_WRITEOK: Protected by both swap_lock and swap_avail_lock, clearing
+ *   this flag also waits for all current cluster lock users to exit so
+ *   checking this flag while holding any of these locks ensures the device
+ *   is safe to use at the moment. Note: clearing this flag doesn't affect
+ *   pending IO or async requests, it only prevents further entry allocation
+ *   or new async request (e.g. discard) from initiating.
+ * - SWP_HIBERNATION: Protected by swap_lock. Indicates if the device
+ *   is pinned for hibernation.
+ */
 enum {
 	SWP_USED	= (1 << 0),	/* is slot in swap_info[] used? */
 	SWP_WRITEOK	= (1 << 1),	/* ok to write to this swap?	*/
@@ -262,14 +278,9 @@ struct swap_info_struct {
 	struct file *swap_file;		/* seldom referenced */
 	struct completion comp;		/* seldom referenced */
 	spinlock_t lock;		/*
-					 * protect map scan related fields like
-					 * inuse_pages and all cluster lists.
-					 * Other fields are only changed
-					 * at swapon/swapoff, so are protected
-					 * by swap_lock. changing flags need
-					 * hold this lock and swap_lock. If
-					 * both locks need hold, hold swap_lock
-					 * first.
+					 * Protect cluster lists. Other fields
+					 * are only changed at swapon/swapoff,
+					 * so are protected by swap_lock.
 					 */
 	struct work_struct discard_work; /* discard worker */
 	struct work_struct reclaim_work; /* reclaim worker */
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 46772d0e3e68..d7115b9195a6 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1199,28 +1199,20 @@ static void del_from_avail_list(struct swap_info_struct *si, bool swapoff)
 
 	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.
-		 */
+	/*
+	 * Force remove it only for swapoff. Else, take it off-list only if
+	 * it's full and SWAP_USAGE_OFFLIST_BIT is not set (strictly
+	 * si->inuse_pages == pages), so concurrent slot freeing, or
+	 * concurrent list removal will make the cmpxchg fail and skip
+	 * the removal.
+	 */
+	if (!swapoff) {
 		pages = si->pages;
 		if (!atomic_long_try_cmpxchg(&si->inuse_pages, &pages,
 					     pages | SWAP_USAGE_OFFLIST_BIT))
 			goto skip;
+	} else {
+		atomic_long_or(SWAP_USAGE_OFFLIST_BIT, &si->inuse_pages);
 	}
 
 	plist_del(&si->avail_list, &swap_avail_head);
@@ -1230,21 +1222,21 @@ static void del_from_avail_list(struct swap_info_struct *si, bool swapoff)
 }
 
 /* SWAP_USAGE_OFFLIST_BIT can only be cleared by this helper. */
-static void add_to_avail_list(struct swap_info_struct *si, bool swapon)
+static void add_to_avail_list(struct swap_info_struct *si)
 {
 	long val;
 	unsigned long pages;
 
 	spin_lock(&swap_avail_lock);
 
-	/* 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;
-	}
+	/*
+	 * Add the device to the avail list if SWP_WRITEOK is set and
+	 * SWAP_USAGE_OFFLIST_BIT is still set. Swapoff clears
+	 * SWP_WRITEOK first, so the device won't be re-added after
+	 * swapoff starts unless swap_device_enable resurrects it.
+	 */
+	if (!(si->flags & SWP_WRITEOK))
+		goto skip;
 
 	if (!(atomic_long_read(&si->inuse_pages) & SWAP_USAGE_OFFLIST_BIT))
 		goto skip;
@@ -1300,7 +1292,7 @@ static void swap_usage_sub(struct swap_info_struct *si, unsigned int nr_entries)
 	 * add it to the plist.
 	 */
 	if (unlikely(val & SWAP_USAGE_OFFLIST_BIT))
-		add_to_avail_list(si, false);
+		add_to_avail_list(si);
 }
 
 static void swap_range_alloc(struct swap_info_struct *si,
@@ -1354,7 +1346,7 @@ static bool get_swap_device_info(struct swap_info_struct *si)
 	 * up to dated.
 	 *
 	 * Paired with the spin_unlock() after setup_swap_info() in
-	 * enable_swap_info(), and smp_wmb() in swapoff.
+	 * swap_device_enable(), and smp_wmb() in swapoff.
 	 */
 	smp_rmb();
 	return true;
@@ -2977,58 +2969,87 @@ static int setup_swap_extents(struct swap_info_struct *sis,
 	return generic_swapfile_activate(sis, swap_file, span);
 }
 
-static void _enable_swap_info(struct swap_info_struct *si)
+/*
+ * Mark a fully initialized swap device writable and expose it to the
+ * allocator. The caller must have resurrected its percpu ref first.
+ */
+static void swap_device_enable(struct swap_info_struct *si)
 {
-	atomic_long_add(si->pages, &nr_swap_pages);
-	total_swap_pages += si->pages;
+	spin_lock(&swap_lock);
 
-	assert_spin_locked(&swap_lock);
+	spin_lock(&swap_avail_lock);
+	si->flags |= SWP_WRITEOK;
+	spin_unlock(&swap_avail_lock);
 
+	atomic_long_add(si->pages, &nr_swap_pages);
+	total_swap_pages += si->pages;
 	plist_add(&si->list, &swap_active_head);
+	spin_unlock(&swap_lock);
 
-	/* Add back to available list */
-	add_to_avail_list(si, true);
+	add_to_avail_list(si);
 }
 
-/*
- * Called after the swap device is ready, resurrect its percpu ref, it's now
- * safe to reference it. Add it to the list to expose it to the allocator.
- */
-static void enable_swap_info(struct swap_info_struct *si)
+static int swap_device_disable(struct address_space *mapping,
+			       struct swap_info_struct **swap_info)
 {
-	percpu_ref_resurrect(&si->users);
-	spin_lock(&swap_lock);
-	spin_lock(&si->lock);
-	_enable_swap_info(si);
-	spin_unlock(&si->lock);
-	spin_unlock(&swap_lock);
-}
+	struct swap_info_struct *si;
+	struct swap_cluster_info *ci;
+	unsigned long offset, end;
+	int err = -EINVAL;
 
-static void reinsert_swap_info(struct swap_info_struct *si)
-{
 	spin_lock(&swap_lock);
-	spin_lock(&si->lock);
-	_enable_swap_info(si);
-	spin_unlock(&si->lock);
-	spin_unlock(&swap_lock);
-}
+	plist_for_each_entry(si, &swap_active_head, list) {
+		if ((si->flags & SWP_WRITEOK) &&
+		    si->swap_file->f_mapping == mapping) {
+			err = 0;
+			break;
+		}
+	}
+	if (err)
+		goto unlock;
 
-/*
- * Called after clearing SWP_WRITEOK, ensures cluster_alloc_range
- * see the updated flags, so there will be no more allocations.
- */
-static void wait_for_allocation(struct swap_info_struct *si)
-{
-	unsigned long offset;
-	unsigned long end = ALIGN(si->max, SWAPFILE_CLUSTER);
-	struct swap_cluster_info *ci;
+	/*
+	 * Refuse swapoff while the device is pinned for hibernation.
+	 */
+	if (si->flags & SWP_HIBERNATION) {
+		err = -EBUSY;
+		goto unlock;
+	}
 
-	BUG_ON(si->flags & SWP_WRITEOK);
+	if (security_vm_enough_memory_mm(current->mm, si->pages)) {
+		err = -ENOMEM;
+		goto unlock;
+	}
+	vm_unacct_memory(si->pages);
 
+	spin_lock(&swap_avail_lock);
+	si->flags &= ~SWP_WRITEOK;
+	spin_unlock(&swap_avail_lock);
+
+	plist_del(&si->list, &swap_active_head);
+	total_swap_pages -= si->pages;
+	atomic_long_sub(si->pages, &nr_swap_pages);
+
+	end = ALIGN(si->max, SWAPFILE_CLUSTER);
+unlock:
+	spin_unlock(&swap_lock);
+	if (err)
+		return err;
+
+	del_from_avail_list(si, true);
+
+	/*
+	 * The swap allocator doesn't take swap_lock. Looping through every
+	 * cluster lock after clearing SWP_WRITEOK ensures that allocators see
+	 * the updated flag and that no allocation remains in flight.
+	 */
 	for (offset = 0; offset < end; offset += SWAPFILE_CLUSTER) {
 		ci = swap_cluster_lock(si, offset);
 		swap_cluster_unlock(ci);
 	}
+
+	*swap_info = si;
+	return 0;
 }
 
 static void free_swap_cluster_info(struct swap_cluster_info *cluster_info,
@@ -3073,7 +3094,6 @@ static void flush_percpu_swap_cluster(struct swap_info_struct *si)
 	}
 }
 
-
 SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 {
 	struct swap_info_struct *p = NULL;
@@ -3082,7 +3102,7 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 	struct address_space *mapping;
 	struct inode *inode;
 	unsigned int maxpages;
-	int err, found = 0;
+	int err;
 
 	if (!capable(CAP_SYS_ADMIN))
 		return -EPERM;
@@ -3095,44 +3115,11 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 		return PTR_ERR(victim);
 
 	mapping = victim->f_mapping;
-	spin_lock(&swap_lock);
-	plist_for_each_entry(p, &swap_active_head, list) {
-		if (p->flags & SWP_WRITEOK) {
-			if (p->swap_file->f_mapping == mapping) {
-				found = 1;
-				break;
-			}
-		}
-	}
-	if (!found) {
-		err = -EINVAL;
-		spin_unlock(&swap_lock);
-		goto out_dput;
-	}
-
-	/* Refuse swapoff while the device is pinned for hibernation */
-	if (p->flags & SWP_HIBERNATION) {
-		err = -EBUSY;
-		spin_unlock(&swap_lock);
-		goto out_dput;
-	}
-
-	if (!security_vm_enough_memory_mm(current->mm, p->pages))
-		vm_unacct_memory(p->pages);
-	else {
-		err = -ENOMEM;
-		spin_unlock(&swap_lock);
-		goto out_dput;
-	}
-	spin_lock(&p->lock);
-	del_from_avail_list(p, true);
-	plist_del(&p->list, &swap_active_head);
-	atomic_long_sub(p->pages, &nr_swap_pages);
-	total_swap_pages -= p->pages;
-	spin_unlock(&p->lock);
-	spin_unlock(&swap_lock);
+	err = swap_device_disable(mapping, &p);
+	filp_close(victim, NULL);
 
-	wait_for_allocation(p);
+	if (err)
+		return err;
 
 	set_current_oom_origin();
 	err = try_to_unuse(p->type);
@@ -3140,8 +3127,8 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 
 	if (err) {
 		/* re-insert swap space back into swap_list */
-		reinsert_swap_info(p);
-		goto out_dput;
+		swap_device_enable(p);
+		return err;
 	}
 
 	/*
@@ -3201,13 +3188,10 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 	p->flags = 0;
 	spin_unlock(&swap_lock);
 
-	err = 0;
 	atomic_inc(&proc_poll_event);
 	wake_up_interruptible(&proc_poll_wait);
 
-out_dput:
-	filp_close(victim, NULL);
-	return err;
+	return 0;
 }
 
 #ifdef CONFIG_PROC_FS
@@ -3629,7 +3613,7 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 	/*
 	 * Allocate or reuse existing !SWP_USED swap_info. The returned
 	 * si will stay in a dying status, so nothing will access its content
-	 * until enable_swap_info resurrects its percpu ref and expose it.
+	 * until swap_device_enable resurrects its percpu ref and expose it.
 	 */
 	si = alloc_swap_info();
 	if (IS_ERR(si))
@@ -3794,7 +3778,8 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 	si->swap_file = swap_file;
 
 	/* Sets SWP_WRITEOK, resurrect the percpu ref, expose the swap device */
-	enable_swap_info(si);
+	percpu_ref_resurrect(&si->users);
+	swap_device_enable(si);
 
 	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,
-- 
2.55.0

[RESEND RFC PATCH v2 04/13] mm/swap: introduce swap device iteration helper

[Lian Wang (ProcessMission)]

From: Kairui Song <kasong@tencent.com>

There are many users that access the swap info array just to iterate
through the swap devices to find usable ones. Introduce a generic helper
for this to prepare for dropping the lock convention.

Also slightly adjust __swap_type_to_info to allow access of swapped off
devices by moving the sanity check into its only current caller, and this
way makes more sense too: the only caller is __swap_entry_to_info, where
we should never see an entry referring to a dead device.

Signed-off-by: Kairui Song <kasong@tencent.com>
Signed-off-by: Lian Wang (ProcessMission) <lianux.mm@gmail.com>
Tested-by: Kunwu Chan <kunwu.chan@linux.dev>
---
 mm/swap.h     | 12 ++++----
 mm/swapfile.c | 77 +++++++++++++++++++++++++++++++--------------------
 2 files changed, 53 insertions(+), 36 deletions(-)

diff --git a/mm/swap.h b/mm/swap.h
index 90a551a88df6..403574a7fa8b 100644
--- a/mm/swap.h
+++ b/mm/swap.h
@@ -107,16 +107,16 @@ static inline unsigned int swp_cluster_offset(swp_entry_t entry)
  */
 static inline struct swap_info_struct *__swap_type_to_info(int type)
 {
-	struct swap_info_struct *si;
-
-	si = READ_ONCE(swap_info[type]); /* rcu_dereference() */
-	VM_WARN_ON_ONCE(percpu_ref_is_zero(&si->users)); /* race with swapoff */
-	return si;
+	return READ_ONCE(swap_info[type]); /* rcu_dereference() */
 }
 
 static inline struct swap_info_struct *__swap_entry_to_info(swp_entry_t entry)
 {
-	return __swap_type_to_info(swp_type(entry));
+	struct swap_info_struct *si;
+
+	si = __swap_type_to_info(swp_type(entry)); /* rcu_dereference() */
+	VM_WARN_ON_ONCE(percpu_ref_is_zero(&si->users)); /* race with swapoff */
+	return si;
 }
 
 static inline struct swap_cluster_info *__swap_offset_to_cluster(
diff --git a/mm/swapfile.c b/mm/swapfile.c
index d7115b9195a6..3847717f4d1d 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -106,6 +106,34 @@ static DEFINE_SPINLOCK(swap_avail_lock);
 
 struct swap_info_struct *swap_info[MAX_SWAPFILES];
 
+static inline struct swap_info_struct *__swap_iter(int *i, unsigned long flag)
+{
+	lockdep_assert_held(&swap_lock);
+	while (*i < nr_swapfiles) {
+		struct swap_info_struct *si = __swap_type_to_info(*i);
+
+		VM_WARN_ON(!si);
+		(*i)++;
+		if (flag && !((si->flags & flag) == flag))
+			continue;
+		return si;
+	}
+	return NULL;
+}
+
+#define __for_each_swap(si, flag)			\
+	for (int __i = 0; ((si) = __swap_iter(&__i, flag));)
+
+/*
+ * for_each_swap - iterate through all allocated and inuse swap devices
+ * @si: the iterator
+ *
+ * Context: The caller must hold swap_lock. The lock may be dropped during
+ * the loop body but must be re-acquired before the next iteration.
+ */
+#define for_each_swap(si) __for_each_swap(si, SWP_USED)
+#define for_each_avail_swap(si) __for_each_swap(si, SWP_USED | SWP_WRITEOK)
+
 static struct kmem_cache *swap_table_cachep;
 
 /* Protects si->swap_file for /proc/swaps usage */
@@ -2210,26 +2238,18 @@ void swap_free_hibernation_slot(swp_entry_t entry)
 
 static int __find_hibernation_swap_type(dev_t device, sector_t offset)
 {
-	int type;
-
-	lockdep_assert_held(&swap_lock);
+	struct swap_info_struct *si;
 
 	if (!device)
 		return -EINVAL;
 
-	for (type = 0; type < nr_swapfiles; type++) {
-		struct swap_info_struct *sis = swap_info[type];
-
-		if (!(sis->flags & SWP_WRITEOK))
-			continue;
-
-		if (device == sis->bdev->bd_dev) {
-			struct swap_extent *se = first_se(sis);
-
-			if (se->start_block == offset)
-				return type;
+	for_each_avail_swap(si) {
+		if (device == si->bdev->bd_dev) {
+			if (first_se(si)->start_block == offset)
+				return si->type;
 		}
 	}
+
 	return -ENODEV;
 }
 
@@ -2335,16 +2355,13 @@ int find_hibernation_swap_type(dev_t device, sector_t offset)
 
 int find_first_swap(dev_t *device)
 {
-	int type, ret = -ENODEV;
+	int ret = -ENODEV;
+	struct swap_info_struct *si;
 
 	spin_lock(&swap_lock);
-	for (type = 0; type < nr_swapfiles; type++) {
-		struct swap_info_struct *sis = swap_info[type];
-
-		if (!(sis->flags & SWP_WRITEOK))
-			continue;
-		*device = sis->bdev->bd_dev;
-		ret = type;
+	for_each_avail_swap(si) {
+		*device = si->bdev->bd_dev;
+		ret = si->type;
 		break;
 	}
 	spin_unlock(&swap_lock);
@@ -2830,12 +2847,14 @@ static int try_to_unuse(unsigned int type)
  */
 static void drain_mmlist(void)
 {
+	struct swap_info_struct *si;
 	struct list_head *p, *next;
-	unsigned int type;
 
-	for (type = 0; type < nr_swapfiles; type++)
-		if (swap_usage_in_pages(swap_info[type]))
+	for_each_swap(si) {
+		if (swap_usage_in_pages(si))
 			return;
+	}
+
 	spin_lock(&mmlist_lock);
 	list_for_each_safe(p, next, &init_mm.mmlist)
 		list_del_init(p);
@@ -3827,14 +3846,12 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 
 void si_swapinfo(struct sysinfo *val)
 {
-	unsigned int type;
+	struct swap_info_struct *si;
 	unsigned long nr_to_be_unused = 0;
 
 	spin_lock(&swap_lock);
-	for (type = 0; type < nr_swapfiles; type++) {
-		struct swap_info_struct *si = swap_info[type];
-
-		if ((si->flags & SWP_USED) && !(si->flags & SWP_WRITEOK))
+	for_each_swap(si) {
+		if (!(si->flags & SWP_WRITEOK))
 			nr_to_be_unused += swap_usage_in_pages(si);
 	}
 	val->freeswap = atomic_long_read(&nr_swap_pages) + nr_to_be_unused;
-- 
2.55.0

[RESEND RFC PATCH v2 05/13] mm/swap: change the swapon lock into a percpu rwsem

[Lian Wang (ProcessMission)]

From: Kairui Song <kasong@tencent.com>

Reading swap metadata is a common hot path, while swapon and swapoff
are costly and very rare.  Convert the existing swap_lock spinlock
into a percpu rwsem so that readers can run concurrently without
contention.  This is a first step toward cleaning up the swap lock
model and we might already see a performance gain for existing
readers.

Also update the comments in swap_info_struct to reflect the lock
name change.

Signed-off-by: Kairui Song <kasong@tencent.com>
Signed-off-by: Lian Wang (ProcessMission) <lianux.mm@gmail.com>
Tested-by: Kunwu Chan <kunwu.chan@linux.dev>
---
 include/linux/swap.h |  10 ++--
 mm/swapfile.c        | 115 +++++++++++++++++++++++--------------------
 2 files changed, 66 insertions(+), 59 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index d9e535cd07c5..7b8ba0d1903f 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -197,17 +197,17 @@ struct swap_extent {
 /*
  * Swap device flags, except the ones documented below, all are immutable
  * after exposed by swap_device_enable, and until the device is freed again
- * (SWP_USED unset). The exceptions:
- * - SWP_USED: Protected by swap_lock. Indicates the device is inuse. Once
+ * (SWP_USED unset). The exceptions are all protected by swapon_rwsem:
+ * - SWP_USED: Protected by swapon_rwsem. Indicates the device is inuse. Once
  *   set, won't be cleared unless all reference to this device is freed and
  *   swapoff finished.
- * - SWP_WRITEOK: Protected by both swap_lock and swap_avail_lock, clearing
+ * - SWP_WRITEOK: Protected by both swapon_rwsem and swap_avail_lock, clearing
  *   this flag also waits for all current cluster lock users to exit so
  *   checking this flag while holding any of these locks ensures the device
  *   is safe to use at the moment. Note: clearing this flag doesn't affect
  *   pending IO or async requests, it only prevents further entry allocation
  *   or new async request (e.g. discard) from initiating.
- * - SWP_HIBERNATION: Protected by swap_lock. Indicates if the device
+ * - SWP_HIBERNATION: Protected by swapon_rwsem. Indicates if the device
  *   is pinned for hibernation.
  */
 enum {
@@ -280,7 +280,7 @@ struct swap_info_struct {
 	spinlock_t lock;		/*
 					 * Protect cluster lists. Other fields
 					 * are only changed at swapon/swapoff,
-					 * so are protected by swap_lock.
+					 * so are protected by swapon_rwsem.
 					 */
 	struct work_struct discard_work; /* discard worker */
 	struct work_struct reclaim_work; /* reclaim worker */
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 3847717f4d1d..173375a7a215 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -57,23 +57,29 @@ static void move_cluster(struct swap_info_struct *si,
 			 enum swap_cluster_flags new_flags);
 
 /*
- * Protects the swap_info array, and the SWP_USED flag. swap_info contains
- * lazily allocated & freed swap device info struts, and SWP_USED indicates
- * which device is used, ~SWP_USED devices and can be reused.
- *
- * Also protects swap_active_head total_swap_pages, and the SWP_WRITEOK flag.
+ * Serializes swapon/swapoff (writers) and protects the swap_info
+ * array, nr_swapfiles, total_swap_pages, and part of swap device
+ * info content (see comment of swap_info_struct).  Readers
+ * (allocation, /proc/swaps, etc.) take percpu_down_read() which
+ * is cheap as the hot path.
+ */
+DEFINE_STATIC_PERCPU_RWSEM(swapon_rwsem);
+/*
+ * swap_info contains lazily allocated swap device info structs, and
+ * SWP_USED indicates which device is used, ~SWP_USED devices can be
+ * reused. Protected by swapon_rwsem, but reading could be lockless.
  */
-static DEFINE_SPINLOCK(swap_lock);
+struct swap_info_struct *swap_info[MAX_SWAPFILES];
 static unsigned int nr_swapfiles;
-atomic_long_t nr_swap_pages;
+long total_swap_pages;
+
 /*
  * Some modules use swappable objects and may try to swap them out under
  * memory pressure (via the shrinker). Before doing so, they may wish to
  * check to see if any swap space is available.
  */
+atomic_long_t nr_swap_pages;
 EXPORT_SYMBOL_GPL(nr_swap_pages);
-/* protected with swap_lock. reading in vm_swap_full() doesn't need lock */
-long total_swap_pages;
 #define DEF_SWAP_PRIO  -1
 unsigned long swapfile_maximum_size;
 #ifdef CONFIG_MIGRATION
@@ -85,7 +91,7 @@ static const char Bad_offset[] = "Bad swap offset entry ";
 
 /*
  * all active swap_info_structs
- * protected with swap_lock, and ordered by priority.
+ * protected with swapon_rwsem, and ordered by priority.
  */
 static PLIST_HEAD(swap_active_head);
 
@@ -95,20 +101,18 @@ static PLIST_HEAD(swap_active_head);
  * This is used by folio_alloc_swap() instead of swap_active_head
  * because swap_active_head includes all swap_info_structs,
  * but folio_alloc_swap() doesn't need to look at full ones.
- * This uses its own lock instead of swap_lock because when a
+ * This uses its own lock instead of swapon_rwsem because when a
  * swap_info_struct changes between not-full/full, it needs to
  * add/remove itself to/from this list, but the swap_info_struct->lock
- * is held and the locking order requires swap_lock to be taken
+ * is held and the locking order requires swapon_rwsem to be taken
  * before any swap_info_struct->lock.
  */
 static PLIST_HEAD(swap_avail_head);
 static DEFINE_SPINLOCK(swap_avail_lock);
 
-struct swap_info_struct *swap_info[MAX_SWAPFILES];
-
 static inline struct swap_info_struct *__swap_iter(int *i, unsigned long flag)
 {
-	lockdep_assert_held(&swap_lock);
+	lockdep_assert_held(&swapon_rwsem);
 	while (*i < nr_swapfiles) {
 		struct swap_info_struct *si = __swap_type_to_info(*i);
 
@@ -128,7 +132,7 @@ static inline struct swap_info_struct *__swap_iter(int *i, unsigned long flag)
  * for_each_swap - iterate through all allocated and inuse swap devices
  * @si: the iterator
  *
- * Context: The caller must hold swap_lock. The lock may be dropped during
+ * Context: The caller must hold swapon_rwsem. The lock may be dropped during
  * the loop body but must be re-acquired before the next iteration.
  */
 #define for_each_swap(si) __for_each_swap(si, SWP_USED)
@@ -1371,10 +1375,10 @@ static bool get_swap_device_info(struct swap_info_struct *si)
 	/*
 	 * Guarantee the si->users are checked before accessing other
 	 * fields of swap_info_struct, and si->flags (SWP_WRITEOK) is
-	 * up to dated.
+	 * up to date.
 	 *
-	 * Paired with the spin_unlock() after setup_swap_info() in
-	 * swap_device_enable(), and smp_wmb() in swapoff.
+	 * Paired with percpu_up_write() in swap_device_enable(), and
+	 * smp_wmb() after clearing SWP_WRITEOK in swapoff.
 	 */
 	smp_rmb();
 	return true;
@@ -1459,10 +1463,10 @@ static bool swap_sync_discard(void)
 	bool ret = false;
 	struct swap_info_struct *si, *next;
 
-	spin_lock(&swap_lock);
+	percpu_down_read(&swapon_rwsem);
 start_over:
 	plist_for_each_entry_safe(si, next, &swap_active_head, list) {
-		spin_unlock(&swap_lock);
+		percpu_up_read(&swapon_rwsem);
 		if (get_swap_device_info(si)) {
 			if (si->flags & SWP_PAGE_DISCARD)
 				ret = swap_do_scheduled_discard(si);
@@ -1471,11 +1475,11 @@ static bool swap_sync_discard(void)
 		if (ret)
 			return true;
 
-		spin_lock(&swap_lock);
+		percpu_down_read(&swapon_rwsem);
 		if (plist_node_empty(&next->list))
 			goto start_over;
 	}
-	spin_unlock(&swap_lock);
+	percpu_up_read(&swapon_rwsem);
 
 	return false;
 }
@@ -2275,7 +2279,7 @@ int pin_hibernation_swap_type(dev_t device, sector_t offset)
 	int ret;
 	struct swap_info_struct *si;
 
-	spin_lock(&swap_lock);
+	percpu_down_write(&swapon_rwsem);
 	ret = __find_hibernation_swap_type(device, offset);
 	if (ret < 0)
 		goto out;
@@ -2299,7 +2303,7 @@ int pin_hibernation_swap_type(dev_t device, sector_t offset)
 	si->flags |= SWP_HIBERNATION;
 
 out:
-	spin_unlock(&swap_lock);
+	percpu_up_write(&swapon_rwsem);
 	return ret;
 }
 
@@ -2317,11 +2321,11 @@ void unpin_hibernation_swap_type(int type)
 {
 	struct swap_info_struct *si;
 
-	spin_lock(&swap_lock);
+	percpu_down_write(&swapon_rwsem);
 	si = swap_type_to_info(type);
 	if (si)
 		si->flags &= ~SWP_HIBERNATION;
-	spin_unlock(&swap_lock);
+	percpu_up_write(&swapon_rwsem);
 }
 
 /**
@@ -2346,9 +2350,9 @@ int find_hibernation_swap_type(dev_t device, sector_t offset)
 {
 	int type;
 
-	spin_lock(&swap_lock);
+	percpu_down_read(&swapon_rwsem);
 	type = __find_hibernation_swap_type(device, offset);
-	spin_unlock(&swap_lock);
+	percpu_up_read(&swapon_rwsem);
 
 	return type;
 }
@@ -2358,13 +2362,13 @@ int find_first_swap(dev_t *device)
 	int ret = -ENODEV;
 	struct swap_info_struct *si;
 
-	spin_lock(&swap_lock);
+	percpu_down_read(&swapon_rwsem);
 	for_each_avail_swap(si) {
 		*device = si->bdev->bd_dev;
 		ret = si->type;
 		break;
 	}
-	spin_unlock(&swap_lock);
+	percpu_up_read(&swapon_rwsem);
 	return ret;
 }
 
@@ -2393,7 +2397,7 @@ unsigned int count_swap_pages(int type, int free)
 {
 	unsigned int n = 0;
 
-	spin_lock(&swap_lock);
+	percpu_down_read(&swapon_rwsem);
 	if ((unsigned int)type < nr_swapfiles) {
 		struct swap_info_struct *sis = swap_info[type];
 
@@ -2405,7 +2409,7 @@ unsigned int count_swap_pages(int type, int free)
 		}
 		spin_unlock(&sis->lock);
 	}
-	spin_unlock(&swap_lock);
+	percpu_up_read(&swapon_rwsem);
 	return n;
 }
 #endif /* CONFIG_HIBERNATION */
@@ -2717,10 +2721,10 @@ static unsigned int find_next_to_unuse(struct swap_info_struct *si,
 	unsigned long swp_tb;
 
 	/*
-	 * No need for swap_lock here: we're just looking
+	 * No need for swapon_rwsem here: we're just looking
 	 * for whether an entry is in use, not modifying it; false
 	 * hits are okay, and sys_swapoff() has already prevented new
-	 * allocations from this area (while holding swap_lock).
+	 * allocations from this area (while holding swapon_rwsem).
 	 */
 	for (i = prev + 1; i < si->max; i++) {
 		swp_tb = swap_table_get(__swap_offset_to_cluster(si, i),
@@ -2841,8 +2845,8 @@ static int try_to_unuse(unsigned int type)
 
 /*
  * After a successful try_to_unuse, if no swap is now in use, we know
- * we can empty the mmlist.  swap_lock must be held on entry and exit.
- * Note that mmlist_lock nests inside swap_lock, and an mm must be
+ * we can empty the mmlist. swapon_rwsem must be held on entry and exit.
+ * Note that mmlist_lock nests inside swapon_rwsem, and an mm must be
  * added to the mmlist just after page_duplicate - before would be racy.
  */
 static void drain_mmlist(void)
@@ -2994,8 +2998,7 @@ static int setup_swap_extents(struct swap_info_struct *sis,
  */
 static void swap_device_enable(struct swap_info_struct *si)
 {
-	spin_lock(&swap_lock);
-
+	percpu_down_write(&swapon_rwsem);
 	spin_lock(&swap_avail_lock);
 	si->flags |= SWP_WRITEOK;
 	spin_unlock(&swap_avail_lock);
@@ -3003,7 +3006,7 @@ static void swap_device_enable(struct swap_info_struct *si)
 	atomic_long_add(si->pages, &nr_swap_pages);
 	total_swap_pages += si->pages;
 	plist_add(&si->list, &swap_active_head);
-	spin_unlock(&swap_lock);
+	percpu_up_write(&swapon_rwsem);
 
 	add_to_avail_list(si);
 }
@@ -3016,7 +3019,7 @@ static int swap_device_disable(struct address_space *mapping,
 	unsigned long offset, end;
 	int err = -EINVAL;
 
-	spin_lock(&swap_lock);
+	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) {
@@ -3051,14 +3054,14 @@ static int swap_device_disable(struct address_space *mapping,
 
 	end = ALIGN(si->max, SWAPFILE_CLUSTER);
 unlock:
-	spin_unlock(&swap_lock);
+	percpu_up_write(&swapon_rwsem);
 	if (err)
 		return err;
 
 	del_from_avail_list(si, true);
 
 	/*
-	 * The swap allocator doesn't take swap_lock. Looping through every
+	 * The swap allocator doesn't take swapon_rwsem. Looping through every
 	 * cluster lock after clearing SWP_WRITEOK ensures that allocators see
 	 * the updated flag and that no allocation remains in flight.
 	 */
@@ -3172,7 +3175,7 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 		atomic_dec(&nr_rotate_swap);
 
 	mutex_lock(&swapon_mutex);
-	spin_lock(&swap_lock);
+	percpu_down_write(&swapon_rwsem);
 	spin_lock(&p->lock);
 	drain_mmlist();
 
@@ -3183,7 +3186,7 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 	p->max = 0;
 	p->cluster_info = NULL;
 	spin_unlock(&p->lock);
-	spin_unlock(&swap_lock);
+	percpu_up_write(&swapon_rwsem);
 	arch_swap_invalidate_area(p->type);
 	zswap_swapoff(p->type);
 	mutex_unlock(&swapon_mutex);
@@ -3202,10 +3205,14 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 	 * Clear the SWP_USED flag after all resources are freed so that swapon
 	 * can reuse this swap_info in alloc_swap_info() safely.  It is ok to
 	 * not hold p->lock after we cleared its SWP_WRITEOK.
+	 *
+	 * The write lock ensures the flag clear is visible to lockless
+	 * readers of swap_type_to_info() before alloc_swap_info() reuses
+	 * this slot.
 	 */
-	spin_lock(&swap_lock);
+	percpu_down_write(&swapon_rwsem);
 	p->flags = 0;
-	spin_unlock(&swap_lock);
+	percpu_up_write(&swapon_rwsem);
 
 	atomic_inc(&proc_poll_event);
 	wake_up_interruptible(&proc_poll_wait);
@@ -3365,13 +3372,13 @@ static struct swap_info_struct *alloc_swap_info(void)
 		return ERR_PTR(-ENOMEM);
 	}
 
-	spin_lock(&swap_lock);
+	percpu_down_write(&swapon_rwsem);
 	for (type = 0; type < nr_swapfiles; type++) {
 		if (!(swap_info[type]->flags & SWP_USED))
 			break;
 	}
 	if (type >= MAX_SWAPFILES) {
-		spin_unlock(&swap_lock);
+		percpu_up_write(&swapon_rwsem);
 		percpu_ref_exit(&p->users);
 		kvfree(p);
 		return ERR_PTR(-EPERM);
@@ -3396,7 +3403,7 @@ static struct swap_info_struct *alloc_swap_info(void)
 	plist_node_init(&p->list, 0);
 	plist_node_init(&p->avail_list, 0);
 	p->flags = SWP_USED;
-	spin_unlock(&swap_lock);
+	percpu_up_write(&swapon_rwsem);
 	if (defer) {
 		percpu_ref_exit(&defer->users);
 		kvfree(defer);
@@ -3829,9 +3836,9 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 	 * Clear the SWP_USED flag after all resources are freed so
 	 * alloc_swap_info can reuse this si safely.
 	 */
-	spin_lock(&swap_lock);
+	percpu_down_write(&swapon_rwsem);
 	si->flags = 0;
-	spin_unlock(&swap_lock);
+	percpu_up_write(&swapon_rwsem);
 	if (inced_nr_rotate_swap)
 		atomic_dec(&nr_rotate_swap);
 	if (swap_file)
@@ -3849,14 +3856,14 @@ void si_swapinfo(struct sysinfo *val)
 	struct swap_info_struct *si;
 	unsigned long nr_to_be_unused = 0;
 
-	spin_lock(&swap_lock);
+	percpu_down_read(&swapon_rwsem);
 	for_each_swap(si) {
 		if (!(si->flags & SWP_WRITEOK))
 			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;
-	spin_unlock(&swap_lock);
+	percpu_up_read(&swapon_rwsem);
 }
 
 /*
-- 
2.55.0

[RESEND RFC PATCH v2 06/13] mm/swap: remove swapon mutex and update proc reader

[Lian Wang (ProcessMission)]

From: Kairui Song <kasong@tencent.com>

Now that swapon_rwsem protects all swapon/swapoff-sensitive data,
the old swapon_mutex is redundant.  Remove it and convert the
/proc/swaps seq_file reader to use the rwsem instead.  Also convert
the swap_start iterator to use for_each_swap() for consistency.

Also adapt swap_next, it needs an explicit SWP_USED check. swap_file is
not reliably protected by swapon_rwsem, so checking SWP_USED is more
accurate and stable for filtering in-use devices.

Signed-off-by: Kairui Song <kasong@tencent.com>
Signed-off-by: Lian Wang (ProcessMission) <lianux.mm@gmail.com>
Tested-by: Kunwu Chan <kunwu.chan@linux.dev>
---
 mm/swapfile.c | 19 +++++--------------
 1 file changed, 5 insertions(+), 14 deletions(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 173375a7a215..6809c099eecc 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -140,9 +140,6 @@ static inline struct swap_info_struct *__swap_iter(int *i, unsigned long flag)
 
 static struct kmem_cache *swap_table_cachep;
 
-/* Protects si->swap_file for /proc/swaps usage */
-static DEFINE_MUTEX(swapon_mutex);
-
 static DECLARE_WAIT_QUEUE_HEAD(proc_poll_wait);
 /* Activity counter to indicate that a swapon or swapoff has occurred */
 static atomic_t proc_poll_event = ATOMIC_INIT(0);
@@ -3174,7 +3171,6 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 	if (!(p->flags & SWP_SOLIDSTATE))
 		atomic_dec(&nr_rotate_swap);
 
-	mutex_lock(&swapon_mutex);
 	percpu_down_write(&swapon_rwsem);
 	spin_lock(&p->lock);
 	drain_mmlist();
@@ -3189,7 +3185,6 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 	percpu_up_write(&swapon_rwsem);
 	arch_swap_invalidate_area(p->type);
 	zswap_swapoff(p->type);
-	mutex_unlock(&swapon_mutex);
 	kfree(p->global_cluster);
 	p->global_cluster = NULL;
 	free_swap_cluster_info(cluster_info, maxpages);
@@ -3235,20 +3230,18 @@ static __poll_t swaps_poll(struct file *file, poll_table *wait)
 	return EPOLLIN | EPOLLRDNORM;
 }
 
-/* iterator */
 static void *swap_start(struct seq_file *swap, loff_t *pos)
 {
 	struct swap_info_struct *si;
-	int type;
 	loff_t l = *pos;
 
-	mutex_lock(&swapon_mutex);
+	percpu_down_read(&swapon_rwsem);
 
 	if (!l)
 		return SEQ_START_TOKEN;
 
-	for (type = 0; (si = swap_type_to_info(type)); type++) {
-		if (!(si->swap_file))
+	for_each_swap(si) {
+		if (!si->swap_file)
 			continue;
 		if (!--l)
 			return si;
@@ -3269,7 +3262,7 @@ static void *swap_next(struct seq_file *swap, void *v, loff_t *pos)
 
 	++(*pos);
 	for (; (si = swap_type_to_info(type)); type++) {
-		if (!(si->swap_file))
+		if (!(si->flags & SWP_USED) || !(si->swap_file))
 			continue;
 		return si;
 	}
@@ -3279,7 +3272,7 @@ static void *swap_next(struct seq_file *swap, void *v, loff_t *pos)
 
 static void swap_stop(struct seq_file *swap, void *v)
 {
-	mutex_unlock(&swapon_mutex);
+	percpu_up_read(&swapon_rwsem);
 }
 
 static int swap_show(struct seq_file *swap, void *v)
@@ -3789,7 +3782,6 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 		goto free_swap_zswap;
 	}
 
-	mutex_lock(&swapon_mutex);
 	prio = DEF_SWAP_PRIO;
 	if (swap_flags & SWAP_FLAG_PREFER)
 		prio = swap_flags & SWAP_FLAG_PRIO_MASK;
@@ -3815,7 +3807,6 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 		(si->flags & SWP_AREA_DISCARD) ? "s" : "",
 		(si->flags & SWP_PAGE_DISCARD) ? "c" : "");
 
-	mutex_unlock(&swapon_mutex);
 	atomic_inc(&proc_poll_event);
 	wake_up_interruptible(&proc_poll_wait);
 
-- 
2.55.0

[RESEND RFC PATCH v2 07/13] mm/swap: consolidate swap inuse accounting helpers

[Lian Wang (ProcessMission)]

From: Kairui Song <kasong@tencent.com>

Inline the swap_usage_add/sub helpers into their only callers and
rename the callers to better reflect what they actually do: track
per-device inuse page counts.

No functional change.

Signed-off-by: Kairui Song <kasong@tencent.com>
Signed-off-by: Lian Wang (ProcessMission) <lianux.mm@gmail.com>
Tested-by: Kunwu Chan <kunwu.chan@linux.dev>
---
 mm/swapfile.c | 64 ++++++++++++++++++++++-----------------------------
 1 file changed, 27 insertions(+), 37 deletions(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 6809c099eecc..1fa6945d6415 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -49,8 +49,8 @@
 #include "internal.h"
 #include "swap.h"
 
-static void swap_range_alloc(struct swap_info_struct *si,
-			     unsigned int nr_entries);
+static void swap_device_inuse_add(struct swap_info_struct *si,
+				  unsigned int nr_entries);
 static bool folio_swapcache_freeable(struct folio *folio);
 static void move_cluster(struct swap_info_struct *si,
 			 struct swap_cluster_info *ci, struct list_head *list,
@@ -984,7 +984,7 @@ static bool __swap_cluster_alloc_entries(struct swap_info_struct *si,
 	if (cluster_is_empty(ci))
 		ci->order = order;
 	ci->count += nr_pages;
-	swap_range_alloc(si, nr_pages);
+	swap_device_inuse_add(si, nr_pages);
 
 	return true;
 }
@@ -1292,53 +1292,36 @@ static void add_to_avail_list(struct swap_info_struct *si)
 }
 
 /*
- * 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.
+ * swap_device_inuse_add / swap_device_inuse_sub are called within cluster
+ * update critical sections and 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)
+static void swap_device_inuse_add(struct swap_info_struct *si,
+				  unsigned int nr_entries)
 {
-	long val = atomic_long_add_return_relaxed(nr_entries, &si->inuse_pages);
+	long inuse_pages;
 
 	/*
 	 * 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 swap_usage_sub(struct swap_info_struct *si, unsigned int nr_entries)
-{
-	long val = atomic_long_sub_return_relaxed(nr_entries, &si->inuse_pages);
-
-	/*
-	 * If device is not full, and SWAP_USAGE_OFFLIST_BIT is set,
-	 * add it to the plist.
-	 */
-	if (unlikely(val & SWAP_USAGE_OFFLIST_BIT))
-		add_to_avail_list(si);
-}
-
-static void swap_range_alloc(struct swap_info_struct *si,
-			     unsigned int nr_entries)
-{
-	if (swap_usage_add(si, nr_entries)) {
+	inuse_pages = atomic_long_add_return_relaxed(nr_entries, &si->inuse_pages);
+	if (unlikely(inuse_pages == si->pages)) {
 		if (vm_swap_full())
 			schedule_work(&si->reclaim_work);
+		del_from_avail_list(si, false);
 	}
+
 	atomic_long_sub(nr_entries, &nr_swap_pages);
 }
 
-static void swap_range_free(struct swap_info_struct *si, unsigned long offset,
-			    unsigned int nr_entries)
+static void swap_device_inuse_sub(struct swap_info_struct *si, unsigned long offset,
+				  unsigned int nr_entries)
 {
 	unsigned long end = offset + nr_entries - 1;
 	void (*swap_slot_free_notify)(struct block_device *, unsigned long);
+	long inuse_pages;
 	unsigned int i;
 
 	for (i = 0; i < nr_entries; i++)
@@ -1362,7 +1345,14 @@ static void swap_range_free(struct swap_info_struct *si, unsigned long offset,
 	 */
 	smp_wmb();
 	atomic_long_add(nr_entries, &nr_swap_pages);
-	swap_usage_sub(si, nr_entries);
+
+	/*
+	 * If device is not full, and SWAP_USAGE_OFFLIST_BIT is set,
+	 * add it back to the plist.
+	 */
+	inuse_pages = atomic_long_sub_return_relaxed(nr_entries, &si->inuse_pages);
+	if (unlikely(inuse_pages & SWAP_USAGE_OFFLIST_BIT))
+		add_to_avail_list(si);
 }
 
 static bool get_swap_device_info(struct swap_info_struct *si)
@@ -1975,7 +1965,7 @@ void __swap_cluster_free_entries(struct swap_info_struct *si,
 	if (batch_id)
 		mem_cgroup_uncharge_swap(batch_id, ci_off - batch_off);
 
-	swap_range_free(si, ci_head + ci_start, nr_pages);
+	swap_device_inuse_sub(si, ci_head + ci_start, nr_pages);
 	swap_cluster_assert_empty(ci, ci_start, nr_pages, false);
 
 	if (!ci->count)
@@ -2834,7 +2824,7 @@ static int try_to_unuse(unsigned int type)
 success:
 	/*
 	 * Make sure that further cleanups after try_to_unuse() returns happen
-	 * after swap_range_free() reduces si->inuse_pages to 0.
+	 * after swap_device_inuse_sub() reduces si->inuse_pages to 0.
 	 */
 	smp_mb();
 	return 0;
-- 
2.55.0

[RESEND RFC PATCH v2 08/13] mm/swap: change back to use each swap device's percpu cluster

[Lian Wang (ProcessMission)]

From: Youngjun Park <youngjun.park@lge.com>

This reverts commit 1b7e90020eb7 ("mm, swap: use percpu cluster as
allocation fast path").

The global percpu cluster causes several issues:

1) It prevents efficient swap allocation for users that do not want
   the default swap device policy.  Because the cluster cache sits
   above device selection, all allocation users must stick with the
   default allocation policy.  This fundamentally conflicts with
   incoming ideas like swap tiering.

2) It can cause priority inversion.  Consider two memcgs: memcg1 can
   access devices A and B, where A has higher priority; memcg2 can
   only access B.  Memcg2 can write the global percpu cluster with
   device B, then memcg1 takes B in the fast path even though
   higher-priority device A is not exhausted.

3) The fast-path / slow-path design bundled with the global per-CPU
   cache is problematic: only the fast path uses the global cache,
   and the slow path is forced to rotate the allocation list.  This
   complicates consumers like discard.

Revert to per-device percpu clusters first.  A later patch will
introduce new infrastructure for a high-performance global
allocation path.

Suggested-by: Kairui Song <kasong@tencent.com>
Co-developed-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Youngjun Park <youngjun.park@lge.com>
Signed-off-by: Kairui Song <kasong@tencent.com>
Signed-off-by: Lian Wang (ProcessMission) <lianux.mm@gmail.com>
Tested-by: Kunwu Chan <kunwu.chan@linux.dev>
---
 include/linux/swap.h |  13 ++-
 mm/swapfile.c        | 184 +++++++++++++++----------------------------
 2 files changed, 72 insertions(+), 125 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 7b8ba0d1903f..aa66d8454186 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -245,10 +245,17 @@ enum {
 #endif
 
 /*
- * We keep using same cluster for rotational device so IO will be sequential.
- * The purpose is to optimize SWAP throughput on these device.
+ * We assign a cluster to each CPU, so each CPU can allocate swap entry from
+ * its own cluster and swapout sequentially. The purpose is to optimize swapout
+ * throughput.
  */
+struct percpu_cluster {
+	local_lock_t lock; /* Protect the percpu_cluster above */
+	unsigned int next[SWAP_NR_ORDERS]; /* Likely next allocation offset */
+};
+
 struct swap_sequential_cluster {
+	spinlock_t lock; /* Serialize usage of global cluster */
 	unsigned int next[SWAP_NR_ORDERS]; /* Likely next allocation offset */
 };
 
@@ -271,8 +278,8 @@ struct swap_info_struct {
 					/* list of cluster that are fragmented or contented */
 	unsigned int pages;		/* total of usable pages of swap */
 	atomic_long_t inuse_pages;	/* number of those currently in use */
+	struct percpu_cluster	__percpu *percpu_cluster; /* per cpu's swap location */
 	struct swap_sequential_cluster *global_cluster; /* Use one global cluster for rotating device */
-	spinlock_t global_cluster_lock;	/* Serialize usage of global cluster */
 	struct rb_root swap_extent_root;/* root of the swap extent rbtree */
 	struct block_device *bdev;	/* swap device or bdev of swap file */
 	struct file *swap_file;		/* seldom referenced */
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 1fa6945d6415..79ecff2d0bd3 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -146,18 +146,6 @@ static atomic_t proc_poll_event = ATOMIC_INIT(0);
 
 atomic_t nr_rotate_swap = ATOMIC_INIT(0);
 
-struct percpu_swap_cluster {
-	struct swap_info_struct *si[SWAP_NR_ORDERS];
-	unsigned long offset[SWAP_NR_ORDERS];
-	local_lock_t lock;
-};
-
-static DEFINE_PER_CPU(struct percpu_swap_cluster, percpu_swap_cluster) = {
-	.si = { NULL },
-	.offset = { SWAP_ENTRY_INVALID },
-	.lock = INIT_LOCAL_LOCK(),
-};
-
 /* May return NULL on invalid type, caller must check for NULL return */
 static struct swap_info_struct *swap_type_to_info(int type)
 {
@@ -562,9 +550,10 @@ swap_cluster_populate(struct swap_info_struct *si,
 	 * Only cluster isolation from the allocator does table allocation.
 	 * Swap allocator uses percpu clusters and holds the local lock.
 	 */
-	lockdep_assert_held(&this_cpu_ptr(&percpu_swap_cluster)->lock);
-	if (!(si->flags & SWP_SOLIDSTATE))
-		lockdep_assert_held(&si->global_cluster_lock);
+	if (si->flags & SWP_SOLIDSTATE)
+		lockdep_assert_held(this_cpu_ptr(&si->percpu_cluster->lock));
+	else
+		lockdep_assert_held(&si->global_cluster->lock);
 	lockdep_assert_held(&ci->lock);
 
 	if (!swap_cluster_alloc_table(ci, __GFP_HIGH | __GFP_NOMEMALLOC |
@@ -577,9 +566,10 @@ swap_cluster_populate(struct swap_info_struct *si,
 	 * the potential recursive allocation is limited.
 	 */
 	spin_unlock(&ci->lock);
-	if (!(si->flags & SWP_SOLIDSTATE))
-		spin_unlock(&si->global_cluster_lock);
-	local_unlock(&percpu_swap_cluster.lock);
+	if (si->flags & SWP_SOLIDSTATE)
+		local_unlock(&si->percpu_cluster->lock);
+	else
+		spin_unlock(&si->global_cluster->lock);
 
 	ret = swap_cluster_alloc_table(ci, __GFP_HIGH | __GFP_NOMEMALLOC |
 					   GFP_KERNEL);
@@ -592,9 +582,10 @@ swap_cluster_populate(struct swap_info_struct *si,
 	 * could happen with ignoring the percpu cluster is fragmentation,
 	 * which is acceptable since this fallback and race is rare.
 	 */
-	local_lock(&percpu_swap_cluster.lock);
-	if (!(si->flags & SWP_SOLIDSTATE))
-		spin_lock(&si->global_cluster_lock);
+	if (si->flags & SWP_SOLIDSTATE)
+		local_lock(&si->percpu_cluster->lock);
+	else
+		spin_lock(&si->global_cluster->lock);
 	spin_lock(&ci->lock);
 
 	if (ret) {
@@ -700,7 +691,7 @@ static bool swap_do_scheduled_discard(struct swap_info_struct *si)
 		ci = list_first_entry(&si->discard_clusters, struct swap_cluster_info, list);
 		/*
 		 * Delete the cluster from list to prepare for discard, but keep
-		 * the CLUSTER_FLAG_DISCARD flag, percpu_swap_cluster could be
+		 * the CLUSTER_FLAG_DISCARD flag, there could be percpu_cluster
 		 * pointing to it, or ran into by relocate_cluster.
 		 */
 		list_del(&ci->list);
@@ -1032,12 +1023,10 @@ static unsigned int alloc_swap_scan_cluster(struct swap_info_struct *si,
 out:
 	relocate_cluster(si, ci);
 	swap_cluster_unlock(ci);
-	if (si->flags & SWP_SOLIDSTATE) {
-		this_cpu_write(percpu_swap_cluster.offset[order], next);
-		this_cpu_write(percpu_swap_cluster.si[order], si);
-	} else {
+	if (si->flags & SWP_SOLIDSTATE)
+		this_cpu_write(si->percpu_cluster->next[order], next);
+	else
 		si->global_cluster->next[order] = next;
-	}
 	return found;
 }
 
@@ -1138,13 +1127,17 @@ static unsigned long cluster_alloc_swap_entry(struct swap_info_struct *si,
 	if (order && !(si->flags & SWP_BLKDEV))
 		return 0;
 
-	if (!(si->flags & SWP_SOLIDSTATE)) {
+	if (si->flags & SWP_SOLIDSTATE) {
+		/* Fast path using per CPU cluster */
+		local_lock(&si->percpu_cluster->lock);
+		offset = __this_cpu_read(si->percpu_cluster->next[order]);
+	} else {
 		/* Serialize HDD SWAP allocation for each device. */
-		spin_lock(&si->global_cluster_lock);
+		spin_lock(&si->global_cluster->lock);
 		offset = si->global_cluster->next[order];
-		if (offset == SWAP_ENTRY_INVALID)
-			goto new_cluster;
+	}
 
+	if (offset != SWAP_ENTRY_INVALID) {
 		ci = swap_cluster_lock(si, offset);
 		/* Cluster could have been used by another order */
 		if (cluster_is_usable(ci, order)) {
@@ -1158,7 +1151,6 @@ static unsigned long cluster_alloc_swap_entry(struct swap_info_struct *si,
 			goto done;
 	}
 
-new_cluster:
 	/*
 	 * If the device need discard, prefer new cluster over nonfull
 	 * to spread out the writes.
@@ -1215,8 +1207,10 @@ static unsigned long cluster_alloc_swap_entry(struct swap_info_struct *si,
 			goto done;
 	}
 done:
-	if (!(si->flags & SWP_SOLIDSTATE))
-		spin_unlock(&si->global_cluster_lock);
+	if (si->flags & SWP_SOLIDSTATE)
+		local_unlock(&si->percpu_cluster->lock);
+	else
+		spin_unlock(&si->global_cluster->lock);
 
 	return found;
 }
@@ -1371,41 +1365,8 @@ static bool get_swap_device_info(struct swap_info_struct *si)
 	return true;
 }
 
-/*
- * Fast path try to get swap entries with specified order from current
- * CPU's swap entry pool (a cluster).
- */
-static bool swap_alloc_fast(struct folio *folio)
-{
-	unsigned int order = folio_order(folio);
-	struct swap_cluster_info *ci;
-	struct swap_info_struct *si;
-	unsigned int offset;
-
-	/*
-	 * Once allocated, swap_info_struct will never be completely freed,
-	 * so checking it's liveness by get_swap_device_info is enough.
-	 */
-	si = this_cpu_read(percpu_swap_cluster.si[order]);
-	offset = this_cpu_read(percpu_swap_cluster.offset[order]);
-	if (!si || !offset || !get_swap_device_info(si))
-		return false;
-
-	ci = swap_cluster_lock(si, offset);
-	if (cluster_is_usable(ci, order)) {
-		if (cluster_is_empty(ci))
-			offset = cluster_offset(si, ci);
-		alloc_swap_scan_cluster(si, ci, folio, offset);
-	} else {
-		swap_cluster_unlock(ci);
-	}
-
-	put_swap_device(si);
-	return folio_test_swapcache(folio);
-}
-
 /* Rotate the device and switch to a new cluster */
-static void swap_alloc_slow(struct folio *folio)
+static void swap_alloc_entry(struct folio *folio)
 {
 	struct swap_info_struct *si, *next;
 
@@ -1775,10 +1736,7 @@ int folio_alloc_swap(struct folio *folio)
 	}
 
 again:
-	local_lock(&percpu_swap_cluster.lock);
-	if (!swap_alloc_fast(folio))
-		swap_alloc_slow(folio);
-	local_unlock(&percpu_swap_cluster.lock);
+	swap_alloc_entry(folio);
 
 	if (!order && unlikely(!folio_test_swapcache(folio))) {
 		if (swap_sync_discard())
@@ -2167,31 +2125,14 @@ void swap_put_entries_direct(swp_entry_t entry, int nr)
  */
 swp_entry_t swap_alloc_hibernation_slot(int type)
 {
-	struct swap_info_struct *pcp_si, *si = swap_type_to_info(type);
-	unsigned long pcp_offset, offset = SWAP_ENTRY_INVALID;
-	struct swap_cluster_info *ci;
+	struct swap_info_struct *si = swap_type_to_info(type);
+	unsigned long offset = SWAP_ENTRY_INVALID;
 	swp_entry_t entry = {0};
 
 	if (!si)
 		goto fail;
 
-	/*
-	 * Try the local cluster first if it matches the device. If
-	 * not, try grab a new cluster and override local cluster.
-	 */
-	local_lock(&percpu_swap_cluster.lock);
-	pcp_si = this_cpu_read(percpu_swap_cluster.si[0]);
-	pcp_offset = this_cpu_read(percpu_swap_cluster.offset[0]);
-	if (pcp_si == si && pcp_offset) {
-		ci = swap_cluster_lock(si, pcp_offset);
-		if (cluster_is_usable(ci, 0))
-			offset = alloc_swap_scan_cluster(si, ci, NULL, pcp_offset);
-		else
-			swap_cluster_unlock(ci);
-	}
-	if (!offset)
-		offset = cluster_alloc_swap_entry(si, NULL);
-	local_unlock(&percpu_swap_cluster.lock);
+	offset = cluster_alloc_swap_entry(si, NULL);
 	if (offset)
 		entry = swp_entry(si->type, offset);
 
@@ -3082,27 +3023,6 @@ static void free_swap_cluster_info(struct swap_cluster_info *cluster_info,
 	kvfree(cluster_info);
 }
 
-/*
- * Called after swap device's reference count is dead, so
- * neither scan nor allocation will use it.
- */
-static void flush_percpu_swap_cluster(struct swap_info_struct *si)
-{
-	int cpu, i;
-	struct swap_info_struct **pcp_si;
-
-	for_each_possible_cpu(cpu) {
-		pcp_si = per_cpu_ptr(percpu_swap_cluster.si, cpu);
-		/*
-		 * Invalidate the percpu swap cluster cache, si->users
-		 * is dead, so no new user will point to it, just flush
-		 * any existing user.
-		 */
-		for (i = 0; i < SWAP_NR_ORDERS; i++)
-			cmpxchg(&pcp_si[i], si, NULL);
-	}
-}
-
 SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 {
 	struct swap_info_struct *p = NULL;
@@ -3154,7 +3074,6 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 
 	flush_work(&p->discard_work);
 	flush_work(&p->reclaim_work);
-	flush_percpu_swap_cluster(p);
 
 	destroy_swap_extents(p, p->swap_file);
 
@@ -3175,6 +3094,8 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 	percpu_up_write(&swapon_rwsem);
 	arch_swap_invalidate_area(p->type);
 	zswap_swapoff(p->type);
+	free_percpu(p->percpu_cluster);
+	p->percpu_cluster = NULL;
 	kfree(p->global_cluster);
 	p->global_cluster = NULL;
 	free_swap_cluster_info(cluster_info, maxpages);
@@ -3523,7 +3444,7 @@ static int setup_swap_clusters_info(struct swap_info_struct *si,
 {
 	unsigned long nr_clusters = DIV_ROUND_UP(maxpages, SWAPFILE_CLUSTER);
 	struct swap_cluster_info *cluster_info;
-	int err = -ENOMEM;
+	int cpu, err = -ENOMEM;
 	unsigned long i;
 
 	cluster_info = kvzalloc_objs(*cluster_info, nr_clusters);
@@ -3533,13 +3454,26 @@ static int setup_swap_clusters_info(struct swap_info_struct *si,
 	for (i = 0; i < nr_clusters; i++)
 		spin_lock_init(&cluster_info[i].lock);
 
-	if (!(si->flags & SWP_SOLIDSTATE)) {
+	if (si->flags & SWP_SOLIDSTATE) {
+		si->percpu_cluster = alloc_percpu(struct percpu_cluster);
+		if (!si->percpu_cluster)
+			goto err;
+
+		for_each_possible_cpu(cpu) {
+			struct percpu_cluster *cluster;
+
+			cluster = per_cpu_ptr(si->percpu_cluster, cpu);
+			for (i = 0; i < SWAP_NR_ORDERS; i++)
+				cluster->next[i] = SWAP_ENTRY_INVALID;
+			local_lock_init(&cluster->lock);
+		}
+	} else {
 		si->global_cluster = kmalloc_obj(*si->global_cluster);
 		if (!si->global_cluster)
 			goto err;
 		for (i = 0; i < SWAP_NR_ORDERS; i++)
 			si->global_cluster->next[i] = SWAP_ENTRY_INVALID;
-		spin_lock_init(&si->global_cluster_lock);
+		spin_lock_init(&si->global_cluster->lock);
 	}
 
 	/*
@@ -3708,11 +3642,6 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 
 	maxpages = si->max;
 
-	/* Set up the swap cluster info */
-	error = setup_swap_clusters_info(si, swap_header, maxpages);
-	if (error)
-		goto bad_swap_unlock_inode;
-
 	if (si->bdev && bdev_stable_writes(si->bdev))
 		si->flags |= SWP_STABLE_WRITES;
 
@@ -3726,6 +3655,15 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 		inced_nr_rotate_swap = true;
 	}
 
+	/*
+	 * Set up the swap cluster info. This must run after SWP_SOLIDSTATE
+	 * is determined above, as it decides whether to allocate the per-CPU
+	 * cluster (solid state) or the global cluster (rotational).
+	 */
+	error = setup_swap_clusters_info(si, swap_header, maxpages);
+	if (error)
+		goto bad_swap_unlock_inode;
+
 	if ((swap_flags & SWAP_FLAG_DISCARD) &&
 	    si->bdev && bdev_max_discard_sectors(si->bdev)) {
 		/*
@@ -3807,6 +3745,8 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 bad_swap_unlock_inode:
 	inode_unlock(inode);
 bad_swap:
+	free_percpu(si->percpu_cluster);
+	si->percpu_cluster = NULL;
 	kfree(si->global_cluster);
 	si->global_cluster = NULL;
 	inode = NULL;
-- 
2.55.0

[RESEND RFC PATCH v2 09/13] mm/swap: add priority queue for swap device allocation

[Lian Wang (ProcessMission)]

From: Kairui Song <kasong@tencent.com>

The swap allocator uses swap_avail_head, a plist ordered by priority,
to select devices. Devices at the same priority are rotated with
plist_requeue(), so every cluster transition serializes allocators on
swap_avail_lock and repeatedly drops and reacquires that global lock.

Replace device selection with a priority queue made of immutable rings.
Each ring contains devices at one priority, and each CPU has a local
reader that rotates through a ring after a fixed allocation quota. A
task-local cursor keeps a retry walk stable even if another task updates
the shared reader between attempts, so each peer is visited exactly once
before the allocator considers a lower priority.

Disable task migration across the retry walk so queue selection, quota
accounting and per-CPU cluster allocation stay on the same CPU. This
preserves per-CPU pacing without making the sleepable allocation loop an
atomic context.

Keep the queue structure stable under swapon_rwsem. Full or disabled
devices remain in their ring with a tag in the low bit of the stored
pointer. Serialize tag writers with swap_queue_update_lock and pair the
lockless full-pointer reads and writes with READ_ONCE() and WRITE_ONCE().
The in-use counter carries a separate off-list bit so full-to-available
transitions update the counter and pointer tag consistently.

Publish swap_file, the live percpu reference, queue membership and
SWP_WRITEOK in one swapon writer section. Preserve the writer-serialized
swapoff lookup and disable invariant established earlier in the series.

For large folios, try every device in the selected priority ring before
returning -E2BIG to request a split. Do not fall back to a lower priority
device merely because the first same-priority device is fragmented.

The old available plist is still maintained in parallel in this commit
so the transition remains bisectable. It is removed by the next patch.

Link: https://lore.kernel.org/20260714-swap-pcp-priq-v1-9-de9b164ed419@tencent.com
Link: https://lore.kernel.org/alZ7UBXweuuOX4qz@yjaykim-PowerEdge-T330
Link: https://lore.kernel.org/77d6da3d-10af-49a1-a356-72aa8b462e85@gmail.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Co-developed-by: Lian Wang (ProcessMission) <lianux.mm@gmail.com>
Signed-off-by: Lian Wang (ProcessMission) <lianux.mm@gmail.com>
Tested-by: Kunwu Chan <kunwu.chan@linux.dev>
---
 include/linux/swap.h |   5 +-
 mm/swapfile.c        | 526 +++++++++++++++++++++++++++++++++++++------
 2 files changed, 464 insertions(+), 67 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index aa66d8454186..37fe2e4d2774 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -201,8 +201,9 @@ struct swap_extent {
  * - SWP_USED: Protected by swapon_rwsem. Indicates the device is inuse. Once
  *   set, won't be cleared unless all reference to this device is freed and
  *   swapoff finished.
- * - SWP_WRITEOK: Protected by both swapon_rwsem and swap_avail_lock, clearing
- *   this flag also waits for all current cluster lock users to exit so
+ * - SWP_WRITEOK: Protected by both swapon_rwsem and swap_queue_update_lock.
+ *   Clearing this flag is followed by waiting for all current cluster lock
+ *   users to exit, so
  *   checking this flag while holding any of these locks ensures the device
  *   is safe to use at the moment. Note: clearing this flag doesn't affect
  *   pending IO or async requests, it only prevents further entry allocation
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 79ecff2d0bd3..1b7bc968b5f7 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -55,6 +55,7 @@ static bool folio_swapcache_freeable(struct folio *folio);
 static void move_cluster(struct swap_info_struct *si,
 			 struct swap_cluster_info *ci, struct list_head *list,
 			 enum swap_cluster_flags new_flags);
+static bool get_swap_device_info(struct swap_info_struct *si);
 
 /*
  * Serializes swapon/swapoff (writers) and protects the swap_info
@@ -160,20 +161,382 @@ static struct swap_info_struct *swap_entry_to_info(swp_entry_t entry)
 	return swap_type_to_info(swp_type(entry));
 }
 
+/*
+ * All available swap_info_structs are grouped by priority rings, the rings
+ * are ordered in a queue by priority (higher prio value = higher priority).
+ * The allocator iterates and rotates devices within each priority ring.
+ * When all devices in a ring are iterated, it goes to the next lower
+ * priority ring.
+ */
+struct swap_prio_ring {
+	int prio;
+	unsigned int size;
+	struct swap_info_struct *dev[] __counted_by(size);
+};
+
+/*
+ * The ring is protected by swapon_rwsem so updating it is costly. To make
+ * the allocator and other users skip full devices faster, the lowest bit of
+ * a device pointer is used to mark it disabled (temporarily unavailable).
+ * This relies on the natural alignment of struct swap_info_struct.
+ */
+#define SWAP_DEVICE_MASKED_SHIFT	0
+#define SWAP_DEVICE_MASKED_BIT		BIT(SWAP_DEVICE_MASKED_SHIFT)
+static_assert(__alignof__(struct swap_info_struct) >= 2);
+
+/*
+ * Serializes queue content mutations and keeps SWAP_USAGE_OFFLIST_BIT
+ * consistent with the masked state of each device pointer.
+ */
+static DEFINE_SPINLOCK(swap_queue_update_lock);
+
+/*
+ * Swap queue is protected by both swap_queue_update_lock and swapon_rwsem.
+ * Only swapon/swapoff will take the write lock, and modify the queue length
+ * or any ring's length. swap_queue_update_lock protects the content so
+ * devices can be masked easily without taking the writelock, which is heavy.
+ */
+static struct swap_prio_ring **swap_queue;
+static unsigned int swap_queue_len;
+
+/*
+ * Each CPU has its read iterator, so the queue itself will remain read
+ * only and the CPU side reader rotates by iterating the devices
+ * periodically using the counter.
+ */
+#define SWAP_ROUND_ROBIN_QUOTA SWAPFILE_CLUSTER
+struct swap_ring_iterator {
+	int offset;
+	long rr_counter;
+};
+
+struct swap_queue_reader {
+	local_lock_t lock;
+	struct swap_ring_iterator ri[];
+};
+
+struct swap_queue_cursor {
+	bool valid;
+	bool ring_only;
+	unsigned int ring_idx;
+	unsigned int offset;
+};
+
+static __percpu struct swap_queue_reader *swap_queue_readers;
+
+static inline bool swap_device_masked(struct swap_info_struct *si)
+{
+	return (unsigned long)si & SWAP_DEVICE_MASKED_BIT;
+}
+
+static inline struct swap_info_struct *swap_device_mask_ptr(struct swap_info_struct *si)
+{
+	return (struct swap_info_struct *)((unsigned long)si |
+					   SWAP_DEVICE_MASKED_BIT);
+}
+
+static inline struct swap_info_struct *swap_device_unmask_ptr(struct swap_info_struct *si)
+{
+	return (struct swap_info_struct *)((unsigned long)si & ~SWAP_DEVICE_MASKED_BIT);
+}
+
+static struct swap_queue_reader __percpu *swap_queue_prealloc_readers(int nr_rings, gfp_t gfp)
+{
+	struct swap_queue_reader __percpu *readers;
+
+	if (!nr_rings)
+		return NULL;
+
+	readers = __alloc_percpu_gfp(struct_size(readers, ri, nr_rings),
+				     __alignof__(*readers), gfp);
+	return readers;
+}
+
+static void swap_queue_install_readers(struct swap_queue_reader __percpu *readers)
+{
+	int ring_idx, cpu;
+	struct swap_prio_ring *ring;
+
+	free_percpu(swap_queue_readers);
+	swap_queue_readers = readers;
+	if (!readers)
+		return;
+
+	/* Distribute each CPU's swap IO fairly across devices. */
+	for_each_possible_cpu(cpu) {
+		local_lock_init(&per_cpu_ptr(readers, cpu)->lock);
+
+		for (ring_idx = 0; ring_idx < swap_queue_len; ring_idx++) {
+			ring = swap_queue[ring_idx];
+			per_cpu_ptr(readers, cpu)->ri[ring_idx].offset =
+				    cpu % ring->size;
+			per_cpu_ptr(readers, cpu)->ri[ring_idx].rr_counter =
+				    SWAP_ROUND_ROBIN_QUOTA;
+		}
+	}
+}
+
+static struct swap_info_struct *swap_queue_get_device(long nr_alloc, int nr_iter,
+						      struct swap_queue_cursor *cursor)
+{
+	bool rotate = false;
+	struct swap_info_struct *si;
+	struct swap_ring_iterator *ri;
+	struct swap_prio_ring *ring;
+	unsigned int dev_idx, queue_idx;
+
+	if (!swap_queue_len)
+		return ERR_PTR(-ENOENT);
+
+	queue_idx = 0;
+	while (nr_iter >= swap_queue[queue_idx]->size) {
+		nr_iter -= swap_queue[queue_idx]->size;
+		if (++queue_idx >= swap_queue_len)
+			return ERR_PTR(cursor->ring_only ? -E2BIG : -ENOENT);
+	}
+	if (cursor->ring_only && cursor->ring_idx != queue_idx)
+		return ERR_PTR(-E2BIG);
+
+	ring = swap_queue[queue_idx];
+	local_lock(&swap_queue_readers->lock);
+	ri = this_cpu_ptr(&swap_queue_readers->ri[queue_idx]);
+	/*
+	 * Snapshot the starting offset for this allocation's walk. The shared
+	 * iterator can move between retries, but the cursor must visit every
+	 * device in the ring exactly once before falling through.
+	 */
+	if (!cursor->valid || cursor->ring_idx != queue_idx) {
+		cursor->valid = true;
+		cursor->ring_idx = queue_idx;
+		if (ri->rr_counter < nr_alloc)
+			rotate = true;
+		else if (ri->offset >= ring->size)
+			rotate = true;
+		if (rotate) {
+			ri->offset++;
+			ri->offset %= ring->size;
+			ri->rr_counter = SWAP_ROUND_ROBIN_QUOTA;
+		}
+		cursor->offset = ri->offset;
+	}
+
+	dev_idx = (cursor->offset + nr_iter) % ring->size;
+	if (nr_iter) {
+		ri->offset = dev_idx;
+		ri->rr_counter = SWAP_ROUND_ROBIN_QUOTA;
+	}
+	ri->rr_counter -= nr_alloc;
+	si = READ_ONCE(ring->dev[dev_idx]);
+	local_unlock(&swap_queue_readers->lock);
+
+	if (swap_device_masked(si))
+		return ERR_PTR(-EBUSY);
+
+	si = swap_device_unmask_ptr(si);
+	return si;
+}
+
+static bool swap_queue_find(struct swap_info_struct *si,
+			    unsigned int *ring_idx, unsigned int *dev_idx)
+{
+	unsigned int i, j;
+	struct swap_prio_ring *ring;
+
+	lockdep_assert(lockdep_is_held(&swapon_rwsem) ||
+		       lockdep_is_held(&swap_queue_update_lock));
+
+	for (i = 0; i < swap_queue_len; i++) {
+		ring = swap_queue[i];
+		if (ring->prio != si->prio)
+			continue;
+		for (j = 0; j < ring->size; j++) {
+			if (swap_device_unmask_ptr(READ_ONCE(ring->dev[j])) != si)
+				continue;
+			*ring_idx = i;
+			*dev_idx = j;
+			return true;
+		}
+	}
+	return false;
+}
+
+static void swap_queue_mask(struct swap_info_struct *si)
+{
+	unsigned int ring_idx, dev_idx;
+
+	lockdep_assert_held(&swap_queue_update_lock);
+	if (swap_queue_find(si, &ring_idx, &dev_idx))
+		WRITE_ONCE(swap_queue[ring_idx]->dev[dev_idx],
+			   swap_device_mask_ptr(si));
+}
+
+static void swap_queue_unmask(struct swap_info_struct *si)
+{
+	unsigned int ring_idx, dev_idx;
+
+	lockdep_assert_held(&swap_queue_update_lock);
+	if (swap_queue_find(si, &ring_idx, &dev_idx))
+		WRITE_ONCE(swap_queue[ring_idx]->dev[dev_idx], si);
+}
+
+static int swap_queue_add(struct swap_info_struct *si)
+{
+	struct swap_prio_ring **new_queue = NULL, **old_queue = NULL;
+	struct swap_queue_reader __percpu *new_readers = NULL;
+	struct swap_prio_ring *ring, *new_ring = NULL, *old_ring = NULL;
+	int prio = si->prio;
+	int i, pos, err = -ENOMEM;
+	gfp_t gfp;
+
+	/* Swap not usable here because this is swap, just reclaim cache. */
+	gfp = GFP_NOIO | __GFP_HIGH;
+	lockdep_assert_held_write(&swapon_rwsem);
+
+	for (pos = 0; pos < swap_queue_len; pos++) {
+		if (swap_queue[pos]->prio == prio)
+			goto add_to_ring;
+		if (swap_queue[pos]->prio < prio)
+			break;
+	}
+
+	/* No ring at this priority: insert a new one at pos. */
+	new_readers = swap_queue_prealloc_readers(swap_queue_len + 1, gfp);
+	if (!new_readers)
+		goto failed;
+	new_queue = kmalloc_array(swap_queue_len + 1, sizeof(*swap_queue), gfp);
+	if (!new_queue)
+		goto failed;
+	new_ring = kmalloc(struct_size(new_ring, dev, 1), gfp);
+	if (!new_ring)
+		goto failed;
+	if (!get_swap_device_info(si))
+		goto failed;
+
+	new_ring->prio = prio;
+	new_ring->size = 1;
+	new_ring->dev[0] = si;
+	for (i = 0; i < pos; i++)
+		new_queue[i] = swap_queue[i];
+	new_queue[pos] = new_ring;
+	for (i = pos; i < swap_queue_len; i++)
+		new_queue[i + 1] = swap_queue[i];
+
+	spin_lock(&swap_queue_update_lock);
+	old_queue = swap_queue;
+	swap_queue = new_queue;
+	swap_queue_len++;
+	spin_unlock(&swap_queue_update_lock);
+	kfree(old_queue);
+
+	swap_queue_install_readers(new_readers);
+	return 0;
+
+add_to_ring:
+	ring = swap_queue[pos];
+	new_ring = kmalloc(struct_size(ring, dev, ring->size + 1), gfp);
+	if (!new_ring)
+		goto failed;
+	if (!get_swap_device_info(si))
+		goto failed;
+	spin_lock(&swap_queue_update_lock);
+	memcpy(new_ring, ring, struct_size(ring, dev, ring->size));
+	new_ring->size++;
+	new_ring->dev[new_ring->size - 1] = si;
+	old_ring = swap_queue[pos];
+	swap_queue[pos] = new_ring;
+	spin_unlock(&swap_queue_update_lock);
+	kfree(old_ring);
+	return 0;
+
+failed:
+	free_percpu(new_readers);
+	kfree(new_queue);
+	kfree(new_ring);
+	return err;
+}
+
+static void swap_queue_del(struct swap_info_struct *si)
+{
+	gfp_t gfp;
+	unsigned int ring_idx, dev_idx;
+	struct swap_queue_reader __percpu *new_readers = NULL;
+	struct swap_prio_ring *ring, *new_ring = NULL, *old_ring = NULL;
+	struct swap_prio_ring **new_queue = NULL, **old_queue = NULL;
+
+	lockdep_assert_held_write(&swapon_rwsem);
+	if (!swap_queue_find(si, &ring_idx, &dev_idx)) {
+		WARN_ON(1);
+		return;
+	}
+
+	/*
+	 * To shrink memory usage, pre-allocate new smaller data before
+	 * locking. Failure is fine, swapoff will release them anyway.
+	 */
+	gfp = GFP_NOIO | __GFP_HIGH;
+	ring = swap_queue[ring_idx];
+	if (ring->size > 1)
+		new_ring = kmalloc(struct_size(ring, dev, ring->size - 1), gfp);
+	if (ring->size == 1 && swap_queue_len > 1) {
+		new_readers = swap_queue_prealloc_readers(swap_queue_len - 1,
+							  gfp);
+		new_queue = kmalloc(sizeof(*swap_queue) *
+				    (swap_queue_len - 1), gfp);
+	}
+
+	spin_lock(&swap_queue_update_lock);
+	if (ring->size > 1) {
+		/* Shift trailing devices left to fill the gap. */
+		while (++dev_idx < ring->size)
+			ring->dev[dev_idx - 1] =
+				ring->dev[dev_idx];
+		ring->size--;
+		if (new_ring) {
+			memcpy(new_ring, ring,
+			       struct_size(ring, dev, ring->size));
+			old_ring = ring;
+			swap_queue[ring_idx] = new_ring;
+		}
+	} else {
+		/* Last device in this ring: remove the ring. */
+		old_ring = ring;
+		swap_queue_len--;
+		while (++ring_idx <= swap_queue_len)
+			swap_queue[ring_idx - 1] =
+				swap_queue[ring_idx];
+		if (new_queue) {
+			memcpy(new_queue, swap_queue,
+			       sizeof(*swap_queue) * swap_queue_len);
+			old_queue = swap_queue;
+			swap_queue = new_queue;
+		} else if (!swap_queue_len) {
+			old_queue = swap_queue;
+			swap_queue = NULL;
+		}
+		if (new_readers || !swap_queue_len)
+			swap_queue_install_readers(new_readers);
+	}
+	spin_unlock(&swap_queue_update_lock);
+
+	kfree(old_ring);
+	kfree(old_queue);
+	put_swap_device(si);
+}
+
 /*
  * 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
+ * if one swap device is unavailable for allocation, 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.
+ * be in the available queue 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.
+ * This bit will be set if the device is not in the available queue
+ * and not usable, will be cleared if the device is in the queue.
  */
-#define SWAP_USAGE_OFFLIST_BIT (1UL << (BITS_PER_TYPE(atomic_t) - 2))
+#define SWAP_USAGE_OFFLIST_BIT BIT(BITS_PER_LONG - 2)
 #define SWAP_USAGE_COUNTER_MASK (~SWAP_USAGE_OFFLIST_BIT)
 static long swap_usage_in_pages(struct swap_info_struct *si)
 {
@@ -1221,6 +1584,7 @@ static void del_from_avail_list(struct swap_info_struct *si, bool swapoff)
 	unsigned long pages;
 
 	spin_lock(&swap_avail_lock);
+	spin_lock(&swap_queue_update_lock);
 
 	/*
 	 * Force remove it only for swapoff. Else, take it off-list only if
@@ -1238,9 +1602,10 @@ static void del_from_avail_list(struct swap_info_struct *si, bool swapoff)
 		atomic_long_or(SWAP_USAGE_OFFLIST_BIT, &si->inuse_pages);
 	}
 
+	swap_queue_mask(si);
 	plist_del(&si->avail_list, &swap_avail_head);
-
 skip:
+	spin_unlock(&swap_queue_update_lock);
 	spin_unlock(&swap_avail_lock);
 }
 
@@ -1251,12 +1616,12 @@ static void add_to_avail_list(struct swap_info_struct *si)
 	unsigned long pages;
 
 	spin_lock(&swap_avail_lock);
+	spin_lock(&swap_queue_update_lock);
 
 	/*
-	 * Add the device to the avail list if SWP_WRITEOK is set and
-	 * SWAP_USAGE_OFFLIST_BIT is still set. Swapoff clears
-	 * SWP_WRITEOK first, so the device won't be re-added after
-	 * swapoff starts unless swap_device_enable resurrects it.
+	 * Mark the device as avail if SWP_WRITEOK is set. Swapoff clears
+	 * SWP_WRITEOK first, so check that first so the device won't be
+	 * re-added after swapoff started.
 	 */
 	if (!(si->flags & SWP_WRITEOK))
 		goto skip;
@@ -1267,21 +1632,23 @@ static void add_to_avail_list(struct swap_info_struct *si)
 	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.
+	 * When device is full and marked as available, one reader will see
+	 * (inuse_pages == si->pages) and should mark it as unavailable and
+	 * set SWAP_USAGE_OFFLIST_BIT. If that updater happens to be here, just
+	 * skip the rest.
 	 */
 	pages = si->pages;
-	if (val == pages) {
+	if ((val & SWAP_USAGE_COUNTER_MASK) == 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;
 	}
 
+	swap_queue_unmask(si);
 	plist_add(&si->avail_list, &swap_avail_head);
-
 skip:
+	spin_unlock(&swap_queue_update_lock);
 	spin_unlock(&swap_avail_lock);
 }
 
@@ -1298,7 +1665,7 @@ static void swap_device_inuse_add(struct swap_info_struct *si,
 
 	/*
 	 * If device is full, and SWAP_USAGE_OFFLIST_BIT is not set,
-	 * remove it from the plist.
+	 * mark it unavailable.
 	 */
 	inuse_pages = atomic_long_add_return_relaxed(nr_entries, &si->inuse_pages);
 	if (unlikely(inuse_pages == si->pages)) {
@@ -1342,7 +1709,7 @@ static void swap_device_inuse_sub(struct swap_info_struct *si, unsigned long off
 
 	/*
 	 * If device is not full, and SWAP_USAGE_OFFLIST_BIT is set,
-	 * add it back to the plist.
+	 * add it back to the available queue.
 	 */
 	inuse_pages = atomic_long_sub_return_relaxed(nr_entries, &si->inuse_pages);
 	if (unlikely(inuse_pages & SWAP_USAGE_OFFLIST_BIT))
@@ -1365,41 +1732,44 @@ static bool get_swap_device_info(struct swap_info_struct *si)
 	return true;
 }
 
-/* Rotate the device and switch to a new cluster */
-static void swap_alloc_entry(struct folio *folio)
+static int swap_alloc_entry(struct folio *folio)
 {
-	struct swap_info_struct *si, *next;
+	struct swap_queue_cursor cursor = {};
+	long nr_pages = folio_nr_pages(folio);
+	struct swap_info_struct *si;
+	int nr_iter, ret;
 
-	spin_lock(&swap_avail_lock);
-start_over:
-	plist_for_each_entry_safe(si, next, &swap_avail_head, avail_list) {
-		/* Rotate the device and switch to a new cluster */
-		plist_requeue(&si->avail_list, &swap_avail_head);
-		spin_unlock(&swap_avail_lock);
-		if (get_swap_device_info(si)) {
-			cluster_alloc_swap_entry(si, folio);
-			put_swap_device(si);
-			if (folio_test_swapcache(folio))
-				return;
-			if (folio_test_large(folio))
-				return;
+	percpu_down_read(&swapon_rwsem);
+	migrate_disable();
+	for (nr_iter = 0;; nr_iter++) {
+		si = swap_queue_get_device(nr_pages, nr_iter, &cursor);
+		if (IS_ERR(si)) {
+			ret = PTR_ERR(si);
+			if (ret == -EBUSY)
+				continue;
+			break;
+		}
+		cluster_alloc_swap_entry(si, folio);
+
+		if (folio_test_swapcache(folio)) {
+			ret = 0;
+			break;
 		}
 
-		spin_lock(&swap_avail_lock);
 		/*
-		 * if we got here, it's likely that si was almost full before,
-		 * multiple callers probably all tried to get a page from the
-		 * same si and it filled up before we could get one; or, the si
-		 * filled up between us dropping swap_avail_lock.
-		 * Since we dropped the swap_avail_lock, the swap_avail_list
-		 * may have been modified; so if next is still in the
-		 * swap_avail_head list then try it, otherwise start over if we
-		 * have not gotten any slots.
+		 * For large allocations, try every device at the same priority,
+		 * but ask the caller to split instead of falling back to a lower
+		 * priority ring.
 		 */
-		if (plist_node_empty(&next->avail_list))
-			goto start_over;
+		if (folio_test_large(folio)) {
+			cursor.ring_only = true;
+			continue;
+		}
 	}
-	spin_unlock(&swap_avail_lock);
+
+	migrate_enable();
+	percpu_up_read(&swapon_rwsem);
+	return ret;
 }
 
 /*
@@ -1713,6 +2083,7 @@ int folio_alloc_swap(struct folio *folio)
 {
 	unsigned int order = folio_order(folio);
 	unsigned int size = 1 << order;
+	int ret;
 
 	VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
 	VM_BUG_ON_FOLIO(!folio_test_uptodate(folio), folio);
@@ -1736,7 +2107,7 @@ int folio_alloc_swap(struct folio *folio)
 	}
 
 again:
-	swap_alloc_entry(folio);
+	ret = swap_alloc_entry(folio);
 
 	if (!order && unlikely(!folio_test_swapcache(folio))) {
 		if (swap_sync_discard())
@@ -1748,7 +2119,7 @@ int folio_alloc_swap(struct folio *folio)
 		swap_cache_del_folio(folio);
 
 	if (unlikely(!folio_test_swapcache(folio)))
-		return -ENOMEM;
+		return ret ? ret : -ENOMEM;
 
 	return 0;
 }
@@ -2921,24 +3292,29 @@ static int setup_swap_extents(struct swap_info_struct *sis,
 }
 
 /*
- * Mark a fully initialized swap device writable and expose it to the
- * allocator. The caller must have resurrected its percpu ref first.
+ * Mark a fully initialized swap device writable and expose it to the allocator.
+ * The caller must have resurrected its percpu ref before entering this helper.
  */
-static void swap_device_enable(struct swap_info_struct *si)
+static void __swap_device_enable(struct swap_info_struct *si)
 {
-	percpu_down_write(&swapon_rwsem);
-	spin_lock(&swap_avail_lock);
-	si->flags |= SWP_WRITEOK;
-	spin_unlock(&swap_avail_lock);
+	lockdep_assert_held_write(&swapon_rwsem);
 
+	spin_lock(&swap_queue_update_lock);
+	si->flags |= SWP_WRITEOK;
+	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);
-	percpu_up_write(&swapon_rwsem);
-
 	add_to_avail_list(si);
 }
 
+static void swap_device_enable(struct swap_info_struct *si)
+{
+	percpu_down_write(&swapon_rwsem);
+	__swap_device_enable(si);
+	percpu_up_write(&swapon_rwsem);
+}
+
 static int swap_device_disable(struct address_space *mapping,
 			       struct swap_info_struct **swap_info)
 {
@@ -2972,10 +3348,9 @@ static int swap_device_disable(struct address_space *mapping,
 	}
 	vm_unacct_memory(si->pages);
 
-	spin_lock(&swap_avail_lock);
+	spin_lock(&swap_queue_update_lock);
 	si->flags &= ~SWP_WRITEOK;
-	spin_unlock(&swap_avail_lock);
-
+	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);
@@ -3060,6 +3435,11 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 		return err;
 	}
 
+	percpu_down_write(&swapon_rwsem);
+	swap_queue_del(p);
+	percpu_ref_kill(&p->users);
+	percpu_up_write(&swapon_rwsem);
+
 	/*
 	 * Wait for swap operations protected by get/put_swap_device()
 	 * to complete.  Because of synchronize_rcu() here, all swap
@@ -3068,7 +3448,6 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 	 * prevent folio_test_swapcache() and the following swap cache
 	 * operations from racing with swapoff.
 	 */
-	percpu_ref_kill(&p->users);
 	synchronize_rcu();
 	wait_for_completion(&p->comp);
 
@@ -3721,11 +4100,28 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 	si->prio = prio;
 	si->list.prio = -si->prio;
 	si->avail_list.prio = -si->prio;
-	si->swap_file = swap_file;
 
-	/* Sets SWP_WRITEOK, resurrect the percpu ref, expose the swap device */
+	/*
+	 * Publish swap_file before making the percpu ref live, then add the device
+	 * to the queue and make it writable under the same write-side lock. This
+	 * keeps lockless ref users and /proc/swaps from observing partial state.
+	 */
+	percpu_down_write(&swapon_rwsem);
+	si->swap_file = swap_file;
 	percpu_ref_resurrect(&si->users);
-	swap_device_enable(si);
+	error = swap_queue_add(si);
+	if (error) {
+		si->swap_file = NULL;
+		percpu_ref_kill(&si->users);
+	} else {
+		__swap_device_enable(si);
+	}
+	percpu_up_write(&swapon_rwsem);
+	if (error) {
+		wait_for_completion(&si->comp);
+		inode->i_flags &= ~S_SWAPFILE;
+		goto free_swap_zswap;
+	}
 
 	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,
-- 
2.55.0

[RESEND RFC PATCH v2 10/13] mm/swap: remove available list

[Lian Wang (ProcessMission)]

From: Kairui Song <kasong@tencent.com>

Pure cleanup, no functional change.

After the priority queue replaced the allocator's device selection,
swap_avail_head and swap_avail_lock are no longer used. Remove them.

The throttle path now walks swap_active_head under swapon_rwsem.
Check SWP_WRITEOK and the existing off-list state. This preserves the old
priority ordering and does not select a full swap device.

Signed-off-by: Kairui Song <kasong@tencent.com>
Signed-off-by: Lian Wang (ProcessMission) <lianux.mm@gmail.com>
Tested-by: Kunwu Chan <kunwu.chan@linux.dev>
---
 include/linux/swap.h |  1 -
 mm/swapfile.c        | 33 ++++++---------------------------
 2 files changed, 6 insertions(+), 28 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 37fe2e4d2774..0c8c1ebcab9c 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -293,7 +293,6 @@ struct swap_info_struct {
 	struct work_struct discard_work; /* discard worker */
 	struct work_struct reclaim_work; /* reclaim worker */
 	struct list_head discard_clusters; /* discard clusters list */
-	struct plist_node avail_list;   /* entry in swap_avail_head */
 	const struct swap_ops *ops;
 };
 
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 1b7bc968b5f7..e183dfad264e 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -96,21 +96,6 @@ static const char Bad_offset[] = "Bad swap offset entry ";
  */
 static PLIST_HEAD(swap_active_head);
 
-/*
- * all available (active, not full) swap_info_structs
- * protected with swap_avail_lock, ordered by priority.
- * This is used by folio_alloc_swap() instead of swap_active_head
- * because swap_active_head includes all swap_info_structs,
- * but folio_alloc_swap() doesn't need to look at full ones.
- * This uses its own lock instead of swapon_rwsem because when a
- * swap_info_struct changes between not-full/full, it needs to
- * add/remove itself to/from this list, but the swap_info_struct->lock
- * is held and the locking order requires swapon_rwsem to be taken
- * before any swap_info_struct->lock.
- */
-static PLIST_HEAD(swap_avail_head);
-static DEFINE_SPINLOCK(swap_avail_lock);
-
 static inline struct swap_info_struct *__swap_iter(int *i, unsigned long flag)
 {
 	lockdep_assert_held(&swapon_rwsem);
@@ -1583,7 +1568,6 @@ static void del_from_avail_list(struct swap_info_struct *si, bool swapoff)
 {
 	unsigned long pages;
 
-	spin_lock(&swap_avail_lock);
 	spin_lock(&swap_queue_update_lock);
 
 	/*
@@ -1603,10 +1587,8 @@ static void del_from_avail_list(struct swap_info_struct *si, bool swapoff)
 	}
 
 	swap_queue_mask(si);
-	plist_del(&si->avail_list, &swap_avail_head);
 skip:
 	spin_unlock(&swap_queue_update_lock);
-	spin_unlock(&swap_avail_lock);
 }
 
 /* SWAP_USAGE_OFFLIST_BIT can only be cleared by this helper. */
@@ -1615,7 +1597,6 @@ static void add_to_avail_list(struct swap_info_struct *si)
 	long val;
 	unsigned long pages;
 
-	spin_lock(&swap_avail_lock);
 	spin_lock(&swap_queue_update_lock);
 
 	/*
@@ -1646,10 +1627,8 @@ static void add_to_avail_list(struct swap_info_struct *si)
 	}
 
 	swap_queue_unmask(si);
-	plist_add(&si->avail_list, &swap_avail_head);
 skip:
 	spin_unlock(&swap_queue_update_lock);
-	spin_unlock(&swap_avail_lock);
 }
 
 /*
@@ -3684,7 +3663,6 @@ static struct swap_info_struct *alloc_swap_info(void)
 	}
 	p->swap_extent_root = RB_ROOT;
 	plist_node_init(&p->list, 0);
-	plist_node_init(&p->avail_list, 0);
 	p->flags = SWP_USED;
 	percpu_up_write(&swapon_rwsem);
 	if (defer) {
@@ -4099,7 +4077,6 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 	 */
 	si->prio = prio;
 	si->list.prio = -si->prio;
-	si->avail_list.prio = -si->prio;
 
 	/*
 	 * Publish swap_file before making the percpu ref live, then add the device
@@ -4243,14 +4220,16 @@ void __folio_throttle_swaprate(struct folio *folio, gfp_t gfp)
 	if (current->throttle_disk)
 		return;
 
-	spin_lock(&swap_avail_lock);
-	plist_for_each_entry(si, &swap_avail_head, avail_list) {
-		if (si->bdev) {
+	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;
 		}
 	}
-	spin_unlock(&swap_avail_lock);
+	percpu_up_read(&swapon_rwsem);
 }
 #endif
 
-- 
2.55.0

[RESEND RFC PATCH v2 11/13] mm/swap: bound synchronous discard during allocation

[Lian Wang (ProcessMission)]

From: Kairui Song <kasong@tencent.com>

The old global per-CPU cluster cache made it difficult to select a
specific device before handling its pending discards.  Now that cluster
state is per-device, let an allocator issue discard for that device when
its free-cluster list is drained instead of waiting for a global fallback
scan.

Do not drain the entire discard list from one allocation.  The list is
also updated by freeing paths, so an unlocked non-empty check races with
those writers, and a full synchronous drain has no fixed latency bound
when other CPUs continue to enqueue clusters.

Factor out swap_discard_one_cluster().  Isolate one cluster under
si->lock, retain CLUSTER_FLAG_DISCARD while I/O is in flight, and return
it to the free list after the discard completes.  The background worker
continues looping until the list is empty, while an allocator handles at
most one cluster before retrying allocation.

This keeps the proactive per-device policy while giving each allocation
a fixed synchronous discard budget.

Link: https://lore.kernel.org/all/CAMgjq7CsYhEjvtN85XGkrONYAJxve7gG593TFeOGV-oax++kWA@mail.gmail.com/
Signed-off-by: Kairui Song <kasong@tencent.com>
Co-developed-by: Lian Wang (ProcessMission) <lianux.mm@gmail.com>
Signed-off-by: Lian Wang (ProcessMission) <lianux.mm@gmail.com>
Tested-by: Kunwu Chan <kunwu.chan@linux.dev>
---
 mm/swapfile.c | 149 ++++++++++++++++++++++----------------------------
 1 file changed, 64 insertions(+), 85 deletions(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index e183dfad264e..cb27f2e246f0 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -528,6 +528,26 @@ static long swap_usage_in_pages(struct swap_info_struct *si)
 	return atomic_long_read(&si->inuse_pages) & SWAP_USAGE_COUNTER_MASK;
 }
 
+/*
+ * Serialize the allocation on single CPU or globally to avoid
+ * fragmentation and make the workflow easier to follow.
+ */
+static void swap_alloc_lock_device(struct swap_info_struct *si)
+{
+	if (si->flags & SWP_SOLIDSTATE)
+		local_lock(&si->percpu_cluster->lock);
+	else
+		spin_lock(&si->global_cluster->lock);
+}
+
+static void swap_alloc_unlock_device(struct swap_info_struct *si)
+{
+	if (si->flags & SWP_SOLIDSTATE)
+		local_unlock(&si->percpu_cluster->lock);
+	else
+		spin_unlock(&si->global_cluster->lock);
+}
+
 /* Reclaim the swap entry anyway if possible */
 #define TTRS_ANYWAY		0x1
 /*
@@ -914,10 +934,7 @@ swap_cluster_populate(struct swap_info_struct *si,
 	 * the potential recursive allocation is limited.
 	 */
 	spin_unlock(&ci->lock);
-	if (si->flags & SWP_SOLIDSTATE)
-		local_unlock(&si->percpu_cluster->lock);
-	else
-		spin_unlock(&si->global_cluster->lock);
+	swap_alloc_unlock_device(si);
 
 	ret = swap_cluster_alloc_table(ci, __GFP_HIGH | __GFP_NOMEMALLOC |
 					   GFP_KERNEL);
@@ -930,10 +947,7 @@ swap_cluster_populate(struct swap_info_struct *si,
 	 * could happen with ignoring the percpu cluster is fragmentation,
 	 * which is acceptable since this fallback and race is rare.
 	 */
-	if (si->flags & SWP_SOLIDSTATE)
-		local_lock(&si->percpu_cluster->lock);
-	else
-		spin_lock(&si->global_cluster->lock);
+	swap_alloc_lock_device(si);
 	spin_lock(&ci->lock);
 
 	if (ret) {
@@ -1023,44 +1037,40 @@ static struct swap_cluster_info *isolate_lock_cluster(
 }
 
 /*
- * Doing discard actually. After a cluster discard is finished, the cluster
- * will be added to free cluster list. Discard cluster is a bit special as
- * they don't participate in allocation or reclaim, so clusters marked as
- * CLUSTER_FLAG_DISCARD must remain off-list or on discard list.
+ * Discard one cluster. After the discard is finished, the cluster will be
+ * added to the free cluster list. Discard clusters are special because they
+ * don't participate in allocation or reclaim, so CLUSTER_FLAG_DISCARD must
+ * remain set while a cluster is either queued or being discarded.
  */
-static bool swap_do_scheduled_discard(struct swap_info_struct *si)
+static bool swap_discard_one_cluster(struct swap_info_struct *si)
 {
 	struct swap_cluster_info *ci;
-	bool ret = false;
 	unsigned int idx;
 
 	spin_lock(&si->lock);
-	while (!list_empty(&si->discard_clusters)) {
-		ci = list_first_entry(&si->discard_clusters, struct swap_cluster_info, list);
-		/*
-		 * Delete the cluster from list to prepare for discard, but keep
-		 * the CLUSTER_FLAG_DISCARD flag, there could be percpu_cluster
-		 * pointing to it, or ran into by relocate_cluster.
-		 */
-		list_del(&ci->list);
-		idx = cluster_index(si, ci);
+	if (list_empty(&si->discard_clusters)) {
 		spin_unlock(&si->lock);
-		discard_swap_cluster(si, idx * SWAPFILE_CLUSTER,
-				SWAPFILE_CLUSTER);
-
-		spin_lock(&ci->lock);
-		/*
-		 * Discard is done, clear its flags as it's off-list, then
-		 * return the cluster to allocation list.
-		 */
-		ci->flags = CLUSTER_FLAG_NONE;
-		__free_cluster(si, ci);
-		spin_unlock(&ci->lock);
-		ret = true;
-		spin_lock(&si->lock);
+		return false;
 	}
+
+	ci = list_first_entry(&si->discard_clusters,
+			      struct swap_cluster_info, list);
+	/*
+	 * Delete the cluster from the list, but keep CLUSTER_FLAG_DISCARD
+	 * set while the discard is in flight. A percpu cluster may still
+	 * point to it, or relocate_cluster() may encounter it.
+	 */
+	list_del(&ci->list);
+	idx = cluster_index(si, ci);
 	spin_unlock(&si->lock);
-	return ret;
+
+	discard_swap_cluster(si, idx * SWAPFILE_CLUSTER, SWAPFILE_CLUSTER);
+
+	spin_lock(&ci->lock);
+	ci->flags = CLUSTER_FLAG_NONE;
+	__free_cluster(si, ci);
+	spin_unlock(&ci->lock);
+	return true;
 }
 
 static void swap_discard_work(struct work_struct *work)
@@ -1069,7 +1079,8 @@ static void swap_discard_work(struct work_struct *work)
 
 	si = container_of(work, struct swap_info_struct, discard_work);
 
-	swap_do_scheduled_discard(si);
+	while (swap_discard_one_cluster(si))
+		cond_resched();
 }
 
 static void swap_users_ref_free(struct percpu_ref *ref)
@@ -1467,6 +1478,7 @@ static unsigned long cluster_alloc_swap_entry(struct swap_info_struct *si,
 	struct swap_cluster_info *ci;
 	unsigned int order = likely(folio) ? folio_order(folio) : 0;
 	unsigned int offset = SWAP_ENTRY_INVALID, found = SWAP_ENTRY_INVALID;
+	bool discarded = false;
 
 	/*
 	 * Swapfile is not block device so unable
@@ -1475,15 +1487,12 @@ static unsigned long cluster_alloc_swap_entry(struct swap_info_struct *si,
 	if (order && !(si->flags & SWP_BLKDEV))
 		return 0;
 
-	if (si->flags & SWP_SOLIDSTATE) {
-		/* Fast path using per CPU cluster */
-		local_lock(&si->percpu_cluster->lock);
+restart:
+	swap_alloc_lock_device(si);
+	if (si->flags & SWP_SOLIDSTATE)
 		offset = __this_cpu_read(si->percpu_cluster->next[order]);
-	} else {
-		/* Serialize HDD SWAP allocation for each device. */
-		spin_lock(&si->global_cluster->lock);
+	else
 		offset = si->global_cluster->next[order];
-	}
 
 	if (offset != SWAP_ENTRY_INVALID) {
 		ci = swap_cluster_lock(si, offset);
@@ -1507,6 +1516,15 @@ static unsigned long cluster_alloc_swap_entry(struct swap_info_struct *si,
 		found = alloc_swap_scan_list(si, &si->free_clusters, folio, false);
 		if (found)
 			goto done;
+
+		if (!discarded) {
+			swap_alloc_unlock_device(si);
+			if (swap_discard_one_cluster(si)) {
+				discarded = true;
+				goto restart;
+			}
+			swap_alloc_lock_device(si);
+		}
 	}
 
 	if (order < PMD_ORDER) {
@@ -1555,10 +1573,7 @@ static unsigned long cluster_alloc_swap_entry(struct swap_info_struct *si,
 			goto done;
 	}
 done:
-	if (si->flags & SWP_SOLIDSTATE)
-		local_unlock(&si->percpu_cluster->lock);
-	else
-		spin_unlock(&si->global_cluster->lock);
+	swap_alloc_unlock_device(si);
 
 	return found;
 }
@@ -1751,36 +1766,6 @@ static int swap_alloc_entry(struct folio *folio)
 	return ret;
 }
 
-/*
- * Discard pending clusters in a synchronized way when under high pressure.
- * Return: true if any cluster is discarded.
- */
-static bool swap_sync_discard(void)
-{
-	bool ret = false;
-	struct swap_info_struct *si, *next;
-
-	percpu_down_read(&swapon_rwsem);
-start_over:
-	plist_for_each_entry_safe(si, next, &swap_active_head, list) {
-		percpu_up_read(&swapon_rwsem);
-		if (get_swap_device_info(si)) {
-			if (si->flags & SWP_PAGE_DISCARD)
-				ret = swap_do_scheduled_discard(si);
-			put_swap_device(si);
-		}
-		if (ret)
-			return true;
-
-		percpu_down_read(&swapon_rwsem);
-		if (plist_node_empty(&next->list))
-			goto start_over;
-	}
-	percpu_up_read(&swapon_rwsem);
-
-	return false;
-}
-
 static int swap_extend_table_alloc(struct swap_info_struct *si,
 				   struct swap_cluster_info *ci,
 				   unsigned int ci_off, gfp_t gfp)
@@ -2085,14 +2070,8 @@ int folio_alloc_swap(struct folio *folio)
 		}
 	}
 
-again:
 	ret = swap_alloc_entry(folio);
 
-	if (!order && unlikely(!folio_test_swapcache(folio))) {
-		if (swap_sync_discard())
-			goto again;
-	}
-
 	/* Need to call this even if allocation failed, for MEMCG_SWAP_FAIL. */
 	if (unlikely(mem_cgroup_try_charge_swap(folio)))
 		swap_cache_del_folio(folio);
-- 
2.55.0

[RESEND RFC PATCH v2 12/13] mm/swap: drop swap active plist

[Lian Wang (ProcessMission)]

From: Kairui Song <kasong@tencent.com>

The swap_active_head plist tracked all writable swap devices, ordered
by priority.  Now that every consumer that needed priority ordering
has moved to the priority queue, the remaining users only need to
find any writable device.

Remove swap_active_head, the associated plist_node from
swap_info_struct, and migrate the remaining plist operations.

Signed-off-by: Kairui Song <kasong@tencent.com>
Signed-off-by: Lian Wang (ProcessMission) <lianux.mm@gmail.com>
Tested-by: Kunwu Chan <kunwu.chan@linux.dev>
---
 include/linux/swap.h |  1 -
 mm/swapfile.c        | 27 +++++----------------------
 2 files changed, 5 insertions(+), 23 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 0c8c1ebcab9c..9d86db144b94 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -267,7 +267,6 @@ 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; /* cluster info. Only for SSD */
diff --git a/mm/swapfile.c b/mm/swapfile.c
index cb27f2e246f0..0acb1f31df8c 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);
@@ -3262,7 +3255,6 @@ 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);
 }
 
@@ -3282,9 +3274,8 @@ 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;
 		}
@@ -3309,7 +3300,6 @@ 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);
 
@@ -3641,7 +3631,6 @@ 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) {
@@ -4050,12 +4039,7 @@ 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
@@ -4176,7 +4160,7 @@ 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;
 }
 
 void __folio_throttle_swaprate(struct folio *folio, gfp_t gfp)
@@ -4200,9 +4184,8 @@ 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) &
+	for_each_avail_swap(si) {
+		if (!(atomic_long_read(&si->inuse_pages) &
 		      SWAP_USAGE_OFFLIST_BIT) && si->bdev) {
 			blkcg_schedule_throttle(si->bdev->bd_disk, true);
 			break;
-- 
2.55.0

[RESEND RFC PATCH v2 13/13] lib/plist.c: remove requeue function

[Lian Wang (ProcessMission)]

From: Kairui Song <kasong@tencent.com>

Now the last user of plist requeue is gone, this function can be
removed.

Signed-off-by: Kairui Song <kasong@tencent.com>
Signed-off-by: Lian Wang (ProcessMission) <lianux.mm@gmail.com>
Tested-by: Kunwu Chan <kunwu.chan@linux.dev>
---
 include/linux/plist.h |  2 --
 lib/plist.c           | 64 -------------------------------------------
 2 files changed, 66 deletions(-)

diff --git a/include/linux/plist.h b/include/linux/plist.h
index 16cf4355b5c1..73d359c1a053 100644
--- a/include/linux/plist.h
+++ b/include/linux/plist.h
@@ -132,8 +132,6 @@ static inline void plist_node_init(struct plist_node *node, int prio)
 extern void plist_add(struct plist_node *node, struct plist_head *head);
 extern void plist_del(struct plist_node *node, struct plist_head *head);
 
-extern void plist_requeue(struct plist_node *node, struct plist_head *head);
-
 /**
  * plist_for_each - iterate over the plist
  * @pos:	the type * to use as a loop counter
diff --git a/lib/plist.c b/lib/plist.c
index a5bef38add43..b05ae7ffea87 100644
--- a/lib/plist.c
+++ b/lib/plist.c
@@ -142,58 +142,6 @@ void plist_del(struct plist_node *node, struct plist_head *head)
 	plist_check_head(head);
 }
 
-/**
- * plist_requeue - Requeue @node at end of same-prio entries.
- *
- * This is essentially an optimized plist_del() followed by
- * plist_add().  It moves an entry already in the plist to
- * after any other same-priority entries.
- *
- * @node:	&struct plist_node pointer - entry to be moved
- * @head:	&struct plist_head pointer - list head
- */
-void plist_requeue(struct plist_node *node, struct plist_head *head)
-{
-	struct plist_node *iter;
-	struct list_head *node_next = &head->node_list;
-
-	plist_check_head(head);
-	BUG_ON(plist_head_empty(head));
-	BUG_ON(plist_node_empty(node));
-
-	if (node == plist_last(head))
-		return;
-
-	iter = plist_next(node);
-
-	if (node->prio != iter->prio)
-		return;
-
-	plist_del(node, head);
-
-	/*
-	 * After plist_del(), iter is the replacement of the node.  If the node
-	 * was on prio_list, take shortcut to find node_next instead of looping.
-	 */
-	if (!list_empty(&iter->prio_list)) {
-		iter = list_entry(iter->prio_list.next, struct plist_node,
-				  prio_list);
-		node_next = &iter->node_list;
-		goto queue;
-	}
-
-	plist_for_each_continue(iter, head) {
-		if (node->prio != iter->prio) {
-			node_next = &iter->node_list;
-			break;
-		}
-	}
-queue:
-	list_add_tail(&node->node_list, node_next);
-
-	plist_check_head(head);
-}
-
 #ifdef CONFIG_DEBUG_PLIST
 #include <linux/sched.h>
 #include <linux/sched/clock.h>
@@ -231,14 +179,6 @@ static void __init plist_test_check(int nr_expect)
 	BUG_ON(prio_pos->prio_list.next != &first->prio_list);
 }
 
-static void __init plist_test_requeue(struct plist_node *node)
-{
-	plist_requeue(node, &test_head);
-
-	if (node != plist_last(&test_head))
-		BUG_ON(node->prio == plist_next(node)->prio);
-}
-
 static int  __init plist_test(void)
 {
 	int nr_expect = 0, i, loop;
@@ -262,10 +202,6 @@ static int  __init plist_test(void)
 			nr_expect--;
 		}
 		plist_test_check(nr_expect);
-		if (!plist_node_empty(test_node + i)) {
-			plist_test_requeue(test_node + i);
-			plist_test_check(nr_expect);
-		}
 	}
 
 	for (i = 0; i < ARRAY_SIZE(test_node); i++) {
-- 
2.55.0

Re: [RESEND RFC PATCH v2 00/13] mm/swap: introduce per-priority allocation queues

[Kunwu Chan]

On Sat, 29 Aug 2026 15:43:50 +0800 "Lian Wang (ProcessMission)" <lianux.mm@gmail.com> wrote:

> Hi all,
> 
> RESEND: Only the first cover letter was sent, with an outdated sender
> display name.  No patches from the series were sent.  This resend uses
> the corrected sender identity and starts the complete thread.  There are
> no code or content changes from the previously reviewed v2 draft.
> 
> This is v2 of Kairui's swap priority queue RFC [1].  With Kairui's
> agreement, I am carrying this revision while preserving the original
> authorship of his patches and Youngjun's per-device percpu-cluster patch.
> 
> My substantive code changes are concentrated in patches 3, 9 and 11.
> Patches 5, 10 and 12 contain the dependent adaptations needed to keep
> the stack coherent and bisectable.  The code patch-ids of patches 1, 2,
> 4, 6, 7, 8 and 13 are unchanged from v1.  My Signed-off-by on the full
> series records the handoff and DCO chain; Co-developed-by is limited to
> the three patches with substantive changes.
> 
> The current swap allocator rotates swap_avail_head with plist_requeue()
> when it moves between devices of the same priority.  That couples device
> selection to the global cluster cache and serializes cluster transitions
> on swap_avail_lock.
> 
> This series first restores per-device percpu clusters, then replaces the
> allocation-time plist rotation with a priority-ordered queue.  Each
> priority has a mostly immutable ring of devices.  Per-CPU readers rotate
> within a ring after a fixed allocation quota, so the allocator preserves
> strict priority ordering without contending on one global rotation point.
> 
> The queue is also intended to separate device selection policy from the
> per-device cluster allocator.  Youngjun's swap-tier series [2] remains
> under separate review.  This RFC does not introduce a tier ABI or assume
> that the tier series has landed: a ring is keyed by the existing swap
> priority.  If tiers land first, the same queue can become an in-tier
> allocation policy rather than a competing tier definition.
> 


I've completed the functional, stress, and A/B/C/D/E performance testing
on my side.

The results are consistent with the expected v1 performance shape, and
I don't see a significant performance regression for the full v2 series
in this workload.

I'm also reviewing the v2 code, particularly the allocation queue,
locking, and swapoff/lifetime changes. I'll follow up on the thread with
any findings.

Tested-by: Kunwu Chan <kunwu.chan@gmail.com>

Best,
Kunwu

> Changes since v1
> ================
> 
> The core allocator and lifetime changes are:
> 
> - keep a task-local cursor stable across retries, so a retry walk visits
>   each same-priority peer exactly once even if another task rotates the
>   shared per-CPU reader;
> - disable task migration across the queue walk, keeping queue accounting
>   and per-CPU cluster allocation on the same CPU while leaving the loop
>   sleepable;
> - retry every same-priority peer for a large folio before returning
>   -E2BIG, without falling through to a lower priority merely because one
>   device is fragmented;
> - initialize the per-CPU quota consistently;
> - represent full/disabled devices with tagged ring entries, serialize tag
>   writers, and use READ_ONCE()/WRITE_ONCE() for lockless readers;
> - publish swap_file, the live percpu reference, queue membership and
>   SWP_WRITEOK in one swapon writer section, and tighten swapoff disable
>   and teardown ordering;
> - fix the transitional available-list locking and publication issues
>   reported by Jihan;
> - bound allocation-path synchronous discard to one cluster, while the
>   background worker can continue draining the list; and
> - remove the transitional available/active plists and the now-unused
>   plist_requeue() API after the queue becomes authoritative.
> 
> These changes address the public review on patch 9 from Youngjun and
> Jihan [3][4].
> 
> Open questions
> ==============
> 
> Two performance-policy questions from the v1 discussion remain open:
> 
> - A one-device ring, which is likely the most common configuration,
>   still follows the common reader path and takes the local lock.  A
>   size-one fast path could return ring->dev[0] directly while
>   swapon_rwsem keeps the ring stable.  This revision keeps the tested
>   common path unchanged; feedback on whether the special case is worth
>   adding would be welcome.
> - Full devices remain as masked entries in the static ring.  Readers skip
>   them rather than rebuilding the ring on each full/available transition.
>   This keeps writers simple, but a workload with many full devices in a
>   higher-priority ring still pays the masked-entry checks before falling
>   through.  It is not clear that extra per-ring availability state would
>   justify its synchronization complexity.
> 
> Testing
> =======
> 
> Kunwu ran an A/B/C/D/E matrix on a Kunpeng 920 server with 256 CPUs,
> 249 GiB RAM and an aarch64 kernel.  The workload built a fixed kernel
> source with "make -j96 defconfig Image modules" under 2 GiB and 3 GiB
> memory cgroups, using eight equal-priority ZRAM devices.  Every arm used
> one warm-up followed by 12 measured runs.
> 
> A/B/C share the v1 base and reproduce the progression reported in v1:
> 
>   A: before the series
>   B: after patch 8, with per-device percpu clusters but the old plist
>   C: after patch 13, with the complete priority queue
> 
> D/E share the tested v2 base and isolate the current series:
> 
>   D: before the series
>   E: after all 13 v2 patches
> 
> 1. Reproducing the v1 performance shape
> 
> Average system time over 12 measured kernel builds:
> 
>                               2 GiB       3 GiB
>   Before (A)                40899.14s    22621.86s
>   After patch 8 (B)         88712.52s    53244.70s
>   After patch 13 (C)        41140.76s    22925.48s
> 
> Patch 8 alone increased system time by 116.9% in the 2 GiB workload
> and 135.4% in the 3 GiB workload.  Adding the priority queue returned
> system time to within 0.59% and 1.34% of A, respectively.  The absolute
> times differ from the v1 machine, but the expected performance shape is
> reproduced: moving the cluster cache back to device scope is expensive
> while allocation still rotates the plist, and the queue removes that
> contention.
> 
> 2. Isolating the v2 delta
> 
> Average system time over 12 measured runs:
> 
>                               2 GiB       3 GiB
>   Base (D)                  39643.77s    22139.66s
>   Full v2 (E)              40336.91s    22984.62s
>   Delta                       +1.75%       +3.82%
> 
> Average elapsed time, with sample standard deviation:
> 
>                               2 GiB       3 GiB
>   Base (D)              518.97 +/- 5.87s  334.07 +/- 4.01s
>   Full v2 (E)           527.24 +/- 5.21s  342.98 +/- 2.78s
>   Delta                       +1.59%       +2.67%
> 
> The arms were run sequentially rather than interleaved, so these small
> D/E deltas may include temporal drift.  They are reported as observed
> instead of being described as zero regression.
> 
> 3. Correctness and stress
> 
> All 120 measured builds completed with build_exit=0, oom_kill=0,
> pswpout>0 and a clean dmesg delta.  Arm E also passed tests covering:
> 
> - same-priority distribution and lower-priority isolation;
> - concurrent swapon/swapoff and /proc/swaps readers;
> - full-device mask, unmask and peer refill;
> - same-ring large-folio peer retry without fallback; and
> - dmesg cleanliness and complete cleanup.
> 
> A 600-second stress run concurrently migrated the memory-pressure tasks
> across CPUs and inserted and removed a device from the priority ring.  It
> completed with live workers, swapout progress and a clean dmesg delta.
> 
> For the 48 D/E measured runs, all eight ZRAM devices had nonzero recorded
> peak usage.  On E, the per-sample largest/smallest peak ratio averaged
> 1.021 for 2 GiB and 1.031 for 3 GiB; the worst observed ratio was 1.049.
> 
> 4. Scope and posting base
> 
> The full server matrix was run from commit 94f9b3980dd4
> ("mm/page_reporting: Add page_reporting_delay_ms module parameter") to
> commit d5c8964cf19f ("lib/plist.c: remove requeue function").  The series
> below is rebased onto current mm-unstable; range-diff shows all 13 patches
> are patch-identical.
> 
> The original 48 GiB BRD workload and a real multi-SSD workload were not
> rerun, so this revision makes no BRD or hardware-scaling claim.
> 
> Link: https://lore.kernel.org/20260714-swap-pcp-priq-v1-0-de9b164ed419@tencent.com [1]
> Link: https://lore.kernel.org/20260713025644.170839-1-youngjun.park@lge.com [2]
> Link: https://lore.kernel.org/alZ7UBXweuuOX4qz@yjaykim-PowerEdge-T330 [3]
> Link: https://lore.kernel.org/77d6da3d-10af-49a1-a356-72aa8b462e85@gmail.com [4]
> 
> Thanks to Kairui for the original design and handoff, Youngjun and Jihan
> for the v1 review, and Kunwu for the server validation.
> 
> Signed-off-by: Lian Wang (ProcessMission) <lianux.mm@gmail.com>
> ---
> 
> Kairui Song (12):
>   mm/swap: remove unused parameter for reading swap header
>   mm/swap: slightly cleanup the code for hibernation error handling
>   mm/swap: cleanup and document swap device availability flag usage
>   mm/swap: introduce swap device iteration helper
>   mm/swap: change the swapon lock into a percpu rwsem
>   mm/swap: remove swapon mutex and update proc reader
>   mm/swap: consolidate swap inuse accounting helpers
>   mm/swap: add priority queue for swap device allocation
>   mm/swap: remove available list
>   mm/swap: bound synchronous discard during allocation
>   mm/swap: drop swap active plist
>   lib/plist.c: remove requeue function
> 
> Youngjun Park (1):
>   mm/swap: change back to use each swap device's percpu cluster
> 
>  include/linux/plist.h |    2 -
>  include/linux/swap.h  |   43 +-
>  lib/plist.c           |   64 --
>  mm/swap.h             |   12 +-
>  mm/swapfile.c         | 1301 +++++++++++++++++++++++++----------------
>  5 files changed, 817 insertions(+), 605 deletions(-)
> 
> 
> base-commit: aeddb4d52acfcc5ce5e988acd48f2906fe966ca3
> -- 
> 2.55.0
> 

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