Merge tag 'mm-stable-2025-12-03-21-26' of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Pull MM updates from Andrew Morton:

  "__vmalloc()/kvmalloc() and no-block support" (Uladzislau Rezki)
     Rework the vmalloc() code to support non-blocking allocations
     (GFP_ATOIC, GFP_NOWAIT)

  "ksm: fix exec/fork inheritance" (xu xin)
     Fix a rare case where the KSM MMF_VM_MERGE_ANY prctl state is not
     inherited across fork/exec

  "mm/zswap: misc cleanup of code and documentations" (SeongJae Park)
     Some light maintenance work on the zswap code

  "mm/page_owner: add debugfs files 'show_handles' and 'show_stacks_handles'" (Mauricio Faria de Oliveira)
     Enhance the /sys/kernel/debug/page_owner debug feature by adding
     unique identifiers to differentiate the various stack traces so
     that userspace monitoring tools can better match stack traces over
     time

  "mm/page_alloc: pcp->batch cleanups" (Joshua Hahn)
     Minor alterations to the page allocator's per-cpu-pages feature

  "Improve UFFDIO_MOVE scalability by removing anon_vma lock" (Lokesh Gidra)
     Address a scalability issue in userfaultfd's UFFDIO_MOVE operation

  "kasan: cleanups for kasan_enabled() checks" (Sabyrzhan Tasbolatov)

  "drivers/base/node: fold node register and unregister functions" (Donet Tom)
     Clean up the NUMA node handling code a little

  "mm: some optimizations for prot numa" (Kefeng Wang)
     Cleanups and small optimizations to the NUMA allocation hinting
     code

  "mm/page_alloc: Batch callers of free_pcppages_bulk" (Joshua Hahn)
     Address long lock hold times at boot on large machines. These were
     causing (harmless) softlockup warnings

  "optimize the logic for handling dirty file folios during reclaim" (Baolin Wang)
     Remove some now-unnecessary work from page reclaim

  "mm/damon: allow DAMOS auto-tuned for per-memcg per-node memory usage" (SeongJae Park)
     Enhance the DAMOS auto-tuning feature

  "mm/damon: fixes for address alignment issues in DAMON_LRU_SORT and DAMON_RECLAIM" (Quanmin Yan)
     Fix DAMON_LRU_SORT and DAMON_RECLAIM with certain userspace
     configuration

  "expand mmap_prepare functionality, port more users" (Lorenzo Stoakes)
     Enhance the new(ish) file_operations.mmap_prepare() method and port
     additional callsites from the old ->mmap() over to ->mmap_prepare()

  "Fix stale IOTLB entries for kernel address space" (Lu Baolu)
     Fix a bug (and possible security issue on non-x86) in the IOMMU
     code. In some situations the IOMMU could be left hanging onto a
     stale kernel pagetable entry

  "mm/huge_memory: cleanup __split_unmapped_folio()" (Wei Yang)
     Clean up and optimize the folio splitting code

  "mm, swap: misc cleanup and bugfix" (Kairui Song)
     Some cleanups and a minor fix in the swap discard code

  "mm/damon: misc documentation fixups" (SeongJae Park)

  "mm/damon: support pin-point targets removal" (SeongJae Park)
     Permit userspace to remove a specific monitoring target in the
     middle of the current targets list

  "mm: MISC follow-up patches for linux/pgalloc.h" (Harry Yoo)
     A couple of cleanups related to mm header file inclusion

  "mm/swapfile.c: select swap devices of default priority round robin" (Baoquan He)
     improve the selection of swap devices for NUMA machines

  "mm: Convert memory block states (MEM_*) macros to enums" (Israel Batista)
     Change the memory block labels from macros to enums so they will
     appear in kernel debug info

  "ksm: perform a range-walk to jump over holes in break_ksm" (Pedro Demarchi Gomes)
     Address an inefficiency when KSM unmerges an address range

  "mm/damon/tests: fix memory bugs in kunit tests" (SeongJae Park)
     Fix leaks and unhandled malloc() failures in DAMON userspace unit
     tests

  "some cleanups for pageout()" (Baolin Wang)
     Clean up a couple of minor things in the page scanner's
     writeback-for-eviction code

  "mm/hugetlb: refactor sysfs/sysctl interfaces" (Hui Zhu)
     Move hugetlb's sysfs/sysctl handling code into a new file

  "introduce VM_MAYBE_GUARD and make it sticky" (Lorenzo Stoakes)
     Make the VMA guard regions available in /proc/pid/smaps and
     improves the mergeability of guarded VMAs

  "mm: perform guard region install/remove under VMA lock" (Lorenzo Stoakes)
     Reduce mmap lock contention for callers performing VMA guard region
     operations

  "vma_start_write_killable" (Matthew Wilcox)
     Start work on permitting applications to be killed when they are
     waiting on a read_lock on the VMA lock

  "mm/damon/tests: add more tests for online parameters commit" (SeongJae Park)
     Add additional userspace testing of DAMON's "commit" feature

  "mm/damon: misc cleanups" (SeongJae Park)

  "make VM_SOFTDIRTY a sticky VMA flag" (Lorenzo Stoakes)
     Address the possible loss of a VMA's VM_SOFTDIRTY flag when that
     VMA is merged with another

  "mm: support device-private THP" (Balbir Singh)
     Introduce support for Transparent Huge Page (THP) migration in zone
     device-private memory

  "Optimize folio split in memory failure" (Zi Yan)

  "mm/huge_memory: Define split_type and consolidate split support checks" (Wei Yang)
     Some more cleanups in the folio splitting code

  "mm: remove is_swap_[pte, pmd]() + non-swap entries, introduce leaf entries" (Lorenzo Stoakes)
     Clean up our handling of pagetable leaf entries by introducing the
     concept of 'software leaf entries', of type softleaf_t

  "reparent the THP split queue" (Muchun Song)
     Reparent the THP split queue to its parent memcg. This is in
     preparation for addressing the long-standing "dying memcg" problem,
     wherein dead memcg's linger for too long, consuming memory
     resources

  "unify PMD scan results and remove redundant cleanup" (Wei Yang)
     A little cleanup in the hugepage collapse code

  "zram: introduce writeback bio batching" (Sergey Senozhatsky)
     Improve zram writeback efficiency by introducing batched bio
     writeback support

  "memcg: cleanup the memcg stats interfaces" (Shakeel Butt)
     Clean up our handling of the interrupt safety of some memcg stats

  "make vmalloc gfp flags usage more apparent" (Vishal Moola)
     Clean up vmalloc's handling of incoming GFP flags

  "mm: Add soft-dirty and uffd-wp support for RISC-V" (Chunyan Zhang)
     Teach soft dirty and userfaultfd write protect tracking to use
     RISC-V's Svrsw60t59b extension

  "mm: swap: small fixes and comment cleanups" (Youngjun Park)
     Fix a small bug and clean up some of the swap code

  "initial work on making VMA flags a bitmap" (Lorenzo Stoakes)
     Start work on converting the vma struct's flags to a bitmap, so we
     stop running out of them, especially on 32-bit

  "mm/swapfile: fix and cleanup swap list iterations" (Youngjun Park)
     Address a possible bug in the swap discard code and clean things
     up a little

[ This merge also reverts commit ebb9aeb980e5 ("vfio/nvgrace-gpu:
  register device memory for poison handling") because it looks
  broken to me, I've asked for clarification   - Linus ]

* tag 'mm-stable-2025-12-03-21-26' of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm: (321 commits)
  mm: fix vma_start_write_killable() signal handling
  mm/swapfile: use plist_for_each_entry in __folio_throttle_swaprate
  mm/swapfile: fix list iteration when next node is removed during discard
  fs/proc/task_mmu.c: fix make_uffd_wp_huge_pte() huge pte handling
  mm/kfence: add reboot notifier to disable KFENCE on shutdown
  memcg: remove inc/dec_lruvec_kmem_state helpers
  selftests/mm/uffd: initialize char variable to Null
  mm: fix DEBUG_RODATA_TEST indentation in Kconfig
  mm: introduce VMA flags bitmap type
  tools/testing/vma: eliminate dependency on vma->__vm_flags
  mm: simplify and rename mm flags function for clarity
  mm: declare VMA flags by bit
  zram: fix a spelling mistake
  mm/page_alloc: optimize lowmem_reserve max lookup using its semantic monotonicity
  mm/vmscan: skip increasing kswapd_failures when reclaim was boosted
  pagemap: update BUDDY flag documentation
  mm: swap: remove scan_swap_map_slots() references from comments
  mm: swap: change swap_alloc_slow() to void
  mm, swap: remove redundant comment for read_swap_cache_async
  mm, swap: use SWP_SOLIDSTATE to determine if swap is rotational
  ...

2 files changed, 376 insertions, 109 deletions

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index a43074657531..5759823d6314 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -500,8 +500,31 @@ out:
 }
 
 #ifdef CONFIG_ZRAM_WRITEBACK
+#define INVALID_BDEV_BLOCK		(~0UL)
+
+struct zram_wb_ctl {
+	/* idle list is accessed only by the writeback task, no concurency */
+	struct list_head idle_reqs;
+	/* done list is accessed concurrently, protect by done_lock */
+	struct list_head done_reqs;
+	wait_queue_head_t done_wait;
+	spinlock_t done_lock;
+	atomic_t num_inflight;
+};
+
+struct zram_wb_req {
+	unsigned long blk_idx;
+	struct page *page;
+	struct zram_pp_slot *pps;
+	struct bio_vec bio_vec;
+	struct bio bio;
+
+	struct list_head entry;
+};
+
 static ssize_t writeback_limit_enable_store(struct device *dev,
-		struct device_attribute *attr, const char *buf, size_t len)
+					    struct device_attribute *attr,
+					    const char *buf, size_t len)
 {
 	struct zram *zram = dev_to_zram(dev);
 	u64 val;
@@ -510,33 +533,31 @@ static ssize_t writeback_limit_enable_store(struct device *dev,
 	if (kstrtoull(buf, 10, &val))
 		return ret;
 
-	down_read(&zram->init_lock);
-	spin_lock(&zram->wb_limit_lock);
+	down_write(&zram->init_lock);
 	zram->wb_limit_enable = val;
-	spin_unlock(&zram->wb_limit_lock);
-	up_read(&zram->init_lock);
+	up_write(&zram->init_lock);
 	ret = len;
 
 	return ret;
 }
 
 static ssize_t writeback_limit_enable_show(struct device *dev,
-		struct device_attribute *attr, char *buf)
+					   struct device_attribute *attr,
+					   char *buf)
 {
 	bool val;
 	struct zram *zram = dev_to_zram(dev);
 
 	down_read(&zram->init_lock);
-	spin_lock(&zram->wb_limit_lock);
 	val = zram->wb_limit_enable;
-	spin_unlock(&zram->wb_limit_lock);
 	up_read(&zram->init_lock);
 
 	return sysfs_emit(buf, "%d\n", val);
 }
 
 static ssize_t writeback_limit_store(struct device *dev,
-		struct device_attribute *attr, const char *buf, size_t len)
+				     struct device_attribute *attr,
+				     const char *buf, size_t len)
 {
 	struct zram *zram = dev_to_zram(dev);
 	u64 val;
@@ -545,31 +566,71 @@ static ssize_t writeback_limit_store(struct device *dev,
 	if (kstrtoull(buf, 10, &val))
 		return ret;
 
-	down_read(&zram->init_lock);
-	spin_lock(&zram->wb_limit_lock);
+	/*
+	 * When the page size is greater than 4KB, if bd_wb_limit is set to
+	 * a value that is not page - size aligned, it will cause value
+	 * wrapping. For example, when the page size is set to 16KB and
+	 * bd_wb_limit is set to 3, a single write - back operation will
+	 * cause bd_wb_limit to become -1. Even more terrifying is that
+	 * bd_wb_limit is an unsigned number.
+	 */
+	val = rounddown(val, PAGE_SIZE / 4096);
+
+	down_write(&zram->init_lock);
 	zram->bd_wb_limit = val;
-	spin_unlock(&zram->wb_limit_lock);
-	up_read(&zram->init_lock);
+	up_write(&zram->init_lock);
 	ret = len;
 
 	return ret;
 }
 
 static ssize_t writeback_limit_show(struct device *dev,
-		struct device_attribute *attr, char *buf)
+				    struct device_attribute *attr, char *buf)
 {
 	u64 val;
 	struct zram *zram = dev_to_zram(dev);
 
 	down_read(&zram->init_lock);
-	spin_lock(&zram->wb_limit_lock);
 	val = zram->bd_wb_limit;
-	spin_unlock(&zram->wb_limit_lock);
 	up_read(&zram->init_lock);
 
 	return sysfs_emit(buf, "%llu\n", val);
 }
 
+static ssize_t writeback_batch_size_store(struct device *dev,
+					  struct device_attribute *attr,
+					  const char *buf, size_t len)
+{
+	struct zram *zram = dev_to_zram(dev);
+	u32 val;
+
+	if (kstrtouint(buf, 10, &val))
+		return -EINVAL;
+
+	if (!val)
+		return -EINVAL;
+
+	down_write(&zram->init_lock);
+	zram->wb_batch_size = val;
+	up_write(&zram->init_lock);
+
+	return len;
+}
+
+static ssize_t writeback_batch_size_show(struct device *dev,
+					 struct device_attribute *attr,
+					 char *buf)
+{
+	u32 val;
+	struct zram *zram = dev_to_zram(dev);
+
+	down_read(&zram->init_lock);
+	val = zram->wb_batch_size;
+	up_read(&zram->init_lock);
+
+	return sysfs_emit(buf, "%u\n", val);
+}
+
 static void reset_bdev(struct zram *zram)
 {
 	if (!zram->backing_dev)
@@ -697,23 +758,20 @@ out:
 	return err;
 }
 
-static unsigned long alloc_block_bdev(struct zram *zram)
+static unsigned long zram_reserve_bdev_block(struct zram *zram)
 {
-	unsigned long blk_idx = 1;
-retry:
-	/* skip 0 bit to confuse zram.handle = 0 */
-	blk_idx = find_next_zero_bit(zram->bitmap, zram->nr_pages, blk_idx);
-	if (blk_idx == zram->nr_pages)
-		return 0;
+	unsigned long blk_idx;
 
-	if (test_and_set_bit(blk_idx, zram->bitmap))
-		goto retry;
+	blk_idx = find_next_zero_bit(zram->bitmap, zram->nr_pages, 0);
+	if (blk_idx == zram->nr_pages)
+		return INVALID_BDEV_BLOCK;
 
+	set_bit(blk_idx, zram->bitmap);
 	atomic64_inc(&zram->stats.bd_count);
 	return blk_idx;
 }
 
-static void free_block_bdev(struct zram *zram, unsigned long blk_idx)
+static void zram_release_bdev_block(struct zram *zram, unsigned long blk_idx)
 {
 	int was_set;
 
@@ -734,32 +792,249 @@ static void read_from_bdev_async(struct zram *zram, struct page *page,
 	submit_bio(bio);
 }
 
-static int zram_writeback_slots(struct zram *zram, struct zram_pp_ctl *ctl)
+static void release_wb_req(struct zram_wb_req *req)
 {
-	unsigned long blk_idx = 0;
-	struct page *page = NULL;
-	struct zram_pp_slot *pps;
-	struct bio_vec bio_vec;
-	struct bio bio;
+	__free_page(req->page);
+	kfree(req);
+}
+
+static void release_wb_ctl(struct zram_wb_ctl *wb_ctl)
+{
+	if (!wb_ctl)
+		return;
+
+	/* We should never have inflight requests at this point */
+	WARN_ON(atomic_read(&wb_ctl->num_inflight));
+	WARN_ON(!list_empty(&wb_ctl->done_reqs));
+
+	while (!list_empty(&wb_ctl->idle_reqs)) {
+		struct zram_wb_req *req;
+
+		req = list_first_entry(&wb_ctl->idle_reqs,
+				       struct zram_wb_req, entry);
+		list_del(&req->entry);
+		release_wb_req(req);
+	}
+
+	kfree(wb_ctl);
+}
+
+static struct zram_wb_ctl *init_wb_ctl(struct zram *zram)
+{
+	struct zram_wb_ctl *wb_ctl;
+	int i;
+
+	wb_ctl = kmalloc(sizeof(*wb_ctl), GFP_KERNEL);
+	if (!wb_ctl)
+		return NULL;
+
+	INIT_LIST_HEAD(&wb_ctl->idle_reqs);
+	INIT_LIST_HEAD(&wb_ctl->done_reqs);
+	atomic_set(&wb_ctl->num_inflight, 0);
+	init_waitqueue_head(&wb_ctl->done_wait);
+	spin_lock_init(&wb_ctl->done_lock);
+
+	for (i = 0; i < zram->wb_batch_size; i++) {
+		struct zram_wb_req *req;
+
+		/*
+		 * This is fatal condition only if we couldn't allocate
+		 * any requests at all.  Otherwise we just work with the
+		 * requests that we have successfully allocated, so that
+		 * writeback can still proceed, even if there is only one
+		 * request on the idle list.
+		 */
+		req = kzalloc(sizeof(*req), GFP_KERNEL | __GFP_NOWARN);
+		if (!req)
+			break;
+
+		req->page = alloc_page(GFP_KERNEL | __GFP_NOWARN);
+		if (!req->page) {
+			kfree(req);
+			break;
+		}
+
+		list_add(&req->entry, &wb_ctl->idle_reqs);
+	}
+
+	/* We couldn't allocate any requests, so writeabck is not possible */
+	if (list_empty(&wb_ctl->idle_reqs))
+		goto release_wb_ctl;
+
+	return wb_ctl;
+
+release_wb_ctl:
+	release_wb_ctl(wb_ctl);
+	return NULL;
+}
+
+static void zram_account_writeback_rollback(struct zram *zram)
+{
+	lockdep_assert_held_read(&zram->init_lock);
+
+	if (zram->wb_limit_enable)
+		zram->bd_wb_limit +=  1UL << (PAGE_SHIFT - 12);
+}
+
+static void zram_account_writeback_submit(struct zram *zram)
+{
+	lockdep_assert_held_read(&zram->init_lock);
+
+	if (zram->wb_limit_enable && zram->bd_wb_limit > 0)
+		zram->bd_wb_limit -=  1UL << (PAGE_SHIFT - 12);
+}
+
+static int zram_writeback_complete(struct zram *zram, struct zram_wb_req *req)
+{
+	u32 index = req->pps->index;
+	int err;
+
+	err = blk_status_to_errno(req->bio.bi_status);
+	if (err) {
+		/*
+		 * Failed wb requests should not be accounted in wb_limit
+		 * (if enabled).
+		 */
+		zram_account_writeback_rollback(zram);
+		zram_release_bdev_block(zram, req->blk_idx);
+		return err;
+	}
+
+	atomic64_inc(&zram->stats.bd_writes);
+	zram_slot_lock(zram, index);
+	/*
+	 * We release slot lock during writeback so slot can change under us:
+	 * slot_free() or slot_free() and zram_write_page(). In both cases
+	 * slot loses ZRAM_PP_SLOT flag. No concurrent post-processing can
+	 * set ZRAM_PP_SLOT on such slots until current post-processing
+	 * finishes.
+	 */
+	if (!zram_test_flag(zram, index, ZRAM_PP_SLOT)) {
+		zram_release_bdev_block(zram, req->blk_idx);
+		goto out;
+	}
+
+	zram_free_page(zram, index);
+	zram_set_flag(zram, index, ZRAM_WB);
+	zram_set_handle(zram, index, req->blk_idx);
+	atomic64_inc(&zram->stats.pages_stored);
+
+out:
+	zram_slot_unlock(zram, index);
+	return 0;
+}
+
+static void zram_writeback_endio(struct bio *bio)
+{
+	struct zram_wb_req *req = container_of(bio, struct zram_wb_req, bio);
+	struct zram_wb_ctl *wb_ctl = bio->bi_private;
+	unsigned long flags;
+
+	spin_lock_irqsave(&wb_ctl->done_lock, flags);
+	list_add(&req->entry, &wb_ctl->done_reqs);
+	spin_unlock_irqrestore(&wb_ctl->done_lock, flags);
+
+	wake_up(&wb_ctl->done_wait);
+}
+
+static void zram_submit_wb_request(struct zram *zram,
+				   struct zram_wb_ctl *wb_ctl,
+				   struct zram_wb_req *req)
+{
+	/*
+	 * wb_limit (if enabled) should be adjusted before submission,
+	 * so that we don't over-submit.
+	 */
+	zram_account_writeback_submit(zram);
+	atomic_inc(&wb_ctl->num_inflight);
+	req->bio.bi_private = wb_ctl;
+	submit_bio(&req->bio);
+}
+
+static int zram_complete_done_reqs(struct zram *zram,
+				   struct zram_wb_ctl *wb_ctl)
+{
+	struct zram_wb_req *req;
+	unsigned long flags;
 	int ret = 0, err;
-	u32 index;
 
-	page = alloc_page(GFP_KERNEL);
-	if (!page)
-		return -ENOMEM;
+	while (atomic_read(&wb_ctl->num_inflight) > 0) {
+		spin_lock_irqsave(&wb_ctl->done_lock, flags);
+		req = list_first_entry_or_null(&wb_ctl->done_reqs,
+					       struct zram_wb_req, entry);
+		if (req)
+			list_del(&req->entry);
+		spin_unlock_irqrestore(&wb_ctl->done_lock, flags);
+
+		/* ->num_inflight > 0 doesn't mean we have done requests */
+		if (!req)
+			break;
+
+		err = zram_writeback_complete(zram, req);
+		if (err)
+			ret = err;
+
+		atomic_dec(&wb_ctl->num_inflight);
+		release_pp_slot(zram, req->pps);
+		req->pps = NULL;
+
+		list_add(&req->entry, &wb_ctl->idle_reqs);
+	}
+
+	return ret;
+}
+
+static struct zram_wb_req *zram_select_idle_req(struct zram_wb_ctl *wb_ctl)
+{
+	struct zram_wb_req *req;
+
+	req = list_first_entry_or_null(&wb_ctl->idle_reqs,
+				       struct zram_wb_req, entry);
+	if (req)
+		list_del(&req->entry);
+	return req;
+}
+
+static int zram_writeback_slots(struct zram *zram,
+				struct zram_pp_ctl *ctl,
+				struct zram_wb_ctl *wb_ctl)
+{
+	unsigned long blk_idx = INVALID_BDEV_BLOCK;
+	struct zram_wb_req *req = NULL;
+	struct zram_pp_slot *pps;
+	int ret = 0, err = 0;
+	u32 index = 0;
 
 	while ((pps = select_pp_slot(ctl))) {
-		spin_lock(&zram->wb_limit_lock);
 		if (zram->wb_limit_enable && !zram->bd_wb_limit) {
-			spin_unlock(&zram->wb_limit_lock);
 			ret = -EIO;
 			break;
 		}
-		spin_unlock(&zram->wb_limit_lock);
 
-		if (!blk_idx) {
-			blk_idx = alloc_block_bdev(zram);
-			if (!blk_idx) {
+		while (!req) {
+			req = zram_select_idle_req(wb_ctl);
+			if (req)
+				break;
+
+			wait_event(wb_ctl->done_wait,
+				   !list_empty(&wb_ctl->done_reqs));
+
+			err = zram_complete_done_reqs(zram, wb_ctl);
+			/*
+			 * BIO errors are not fatal, we continue and simply
+			 * attempt to writeback the remaining objects (pages).
+			 * At the same time we need to signal user-space that
+			 * some writes (at least one, but also could be all of
+			 * them) were not successful and we do so by returning
+			 * the most recent BIO error.
+			 */
+			if (err)
+				ret = err;
+		}
+
+		if (blk_idx == INVALID_BDEV_BLOCK) {
+			blk_idx = zram_reserve_bdev_block(zram);
+			if (blk_idx == INVALID_BDEV_BLOCK) {
 				ret = -ENOSPC;
 				break;
 			}
@@ -768,74 +1043,54 @@ static int zram_writeback_slots(struct zram *zram, struct zram_pp_ctl *ctl)
 		index = pps->index;
 		zram_slot_lock(zram, index);
 		/*
-		 * scan_slots() sets ZRAM_PP_SLOT and relases slot lock, so
+		 * scan_slots() sets ZRAM_PP_SLOT and releases slot lock, so
 		 * slots can change in the meantime. If slots are accessed or
 		 * freed they lose ZRAM_PP_SLOT flag and hence we don't
 		 * post-process them.
 		 */
 		if (!zram_test_flag(zram, index, ZRAM_PP_SLOT))
 			goto next;
-		if (zram_read_from_zspool(zram, page, index))
+		if (zram_read_from_zspool(zram, req->page, index))
 			goto next;
 		zram_slot_unlock(zram, index);
 
-		bio_init(&bio, zram->bdev, &bio_vec, 1,
-			 REQ_OP_WRITE | REQ_SYNC);
-		bio.bi_iter.bi_sector = blk_idx * (PAGE_SIZE >> 9);
-		__bio_add_page(&bio, page, PAGE_SIZE, 0);
-
-		/*
-		 * XXX: A single page IO would be inefficient for write
-		 * but it would be not bad as starter.
-		 */
-		err = submit_bio_wait(&bio);
-		if (err) {
-			release_pp_slot(zram, pps);
-			/*
-			 * BIO errors are not fatal, we continue and simply
-			 * attempt to writeback the remaining objects (pages).
-			 * At the same time we need to signal user-space that
-			 * some writes (at least one, but also could be all of
-			 * them) were not successful and we do so by returning
-			 * the most recent BIO error.
-			 */
-			ret = err;
-			continue;
-		}
-
-		atomic64_inc(&zram->stats.bd_writes);
-		zram_slot_lock(zram, index);
 		/*
-		 * Same as above, we release slot lock during writeback so
-		 * slot can change under us: slot_free() or slot_free() and
-		 * reallocation (zram_write_page()). In both cases slot loses
-		 * ZRAM_PP_SLOT flag. No concurrent post-processing can set
-		 * ZRAM_PP_SLOT on such slots until current post-processing
-		 * finishes.
+		 * From now on pp-slot is owned by the req, remove it from
+		 * its pp bucket.
 		 */
-		if (!zram_test_flag(zram, index, ZRAM_PP_SLOT))
-			goto next;
+		list_del_init(&pps->entry);
+
+		req->blk_idx = blk_idx;
+		req->pps = pps;
+		bio_init(&req->bio, zram->bdev, &req->bio_vec, 1, REQ_OP_WRITE);
+		req->bio.bi_iter.bi_sector = req->blk_idx * (PAGE_SIZE >> 9);
+		req->bio.bi_end_io = zram_writeback_endio;
+		__bio_add_page(&req->bio, req->page, PAGE_SIZE, 0);
+
+		zram_submit_wb_request(zram, wb_ctl, req);
+		blk_idx = INVALID_BDEV_BLOCK;
+		req = NULL;
+		cond_resched();
+		continue;
 
-		zram_free_page(zram, index);
-		zram_set_flag(zram, index, ZRAM_WB);
-		zram_set_handle(zram, index, blk_idx);
-		blk_idx = 0;
-		atomic64_inc(&zram->stats.pages_stored);
-		spin_lock(&zram->wb_limit_lock);
-		if (zram->wb_limit_enable && zram->bd_wb_limit > 0)
-			zram->bd_wb_limit -=  1UL << (PAGE_SHIFT - 12);
-		spin_unlock(&zram->wb_limit_lock);
 next:
 		zram_slot_unlock(zram, index);
 		release_pp_slot(zram, pps);
-
-		cond_resched();
 	}
 
-	if (blk_idx)
-		free_block_bdev(zram, blk_idx);
-	if (page)
-		__free_page(page);
+	/*
+	 * Selected idle req, but never submitted it due to some error or
+	 * wb limit.
+	 */
+	if (req)
+		release_wb_req(req);
+
+	while (atomic_read(&wb_ctl->num_inflight) > 0) {
+		wait_event(wb_ctl->done_wait, !list_empty(&wb_ctl->done_reqs));
+		err = zram_complete_done_reqs(zram, wb_ctl);
+		if (err)
+			ret = err;
+	}
 
 	return ret;
 }
@@ -948,7 +1203,8 @@ static ssize_t writeback_store(struct device *dev,
 	struct zram *zram = dev_to_zram(dev);
 	u64 nr_pages = zram->disksize >> PAGE_SHIFT;
 	unsigned long lo = 0, hi = nr_pages;
-	struct zram_pp_ctl *ctl = NULL;
+	struct zram_pp_ctl *pp_ctl = NULL;
+	struct zram_wb_ctl *wb_ctl = NULL;
 	char *args, *param, *val;
 	ssize_t ret = len;
 	int err, mode = 0;
@@ -970,8 +1226,14 @@ static ssize_t writeback_store(struct device *dev,
 		goto release_init_lock;
 	}
 
-	ctl = init_pp_ctl();
-	if (!ctl) {
+	pp_ctl = init_pp_ctl();
+	if (!pp_ctl) {
+		ret = -ENOMEM;
+		goto release_init_lock;
+	}
+
+	wb_ctl = init_wb_ctl(zram);
+	if (!wb_ctl) {
 		ret = -ENOMEM;
 		goto release_init_lock;
 	}
@@ -1000,7 +1262,7 @@ static ssize_t writeback_store(struct device *dev,
 				goto release_init_lock;
 			}
 
-			scan_slots_for_writeback(zram, mode, lo, hi, ctl);
+			scan_slots_for_writeback(zram, mode, lo, hi, pp_ctl);
 			break;
 		}
 
@@ -1011,7 +1273,7 @@ static ssize_t writeback_store(struct device *dev,
 				goto release_init_lock;
 			}
 
-			scan_slots_for_writeback(zram, mode, lo, hi, ctl);
+			scan_slots_for_writeback(zram, mode, lo, hi, pp_ctl);
 			break;
 		}
 
@@ -1022,7 +1284,7 @@ static ssize_t writeback_store(struct device *dev,
 				goto release_init_lock;
 			}
 
-			scan_slots_for_writeback(zram, mode, lo, hi, ctl);
+			scan_slots_for_writeback(zram, mode, lo, hi, pp_ctl);
 			continue;
 		}
 
@@ -1033,17 +1295,18 @@ static ssize_t writeback_store(struct device *dev,
 				goto release_init_lock;
 			}
 
-			scan_slots_for_writeback(zram, mode, lo, hi, ctl);
+			scan_slots_for_writeback(zram, mode, lo, hi, pp_ctl);
 			continue;
 		}
 	}
 
-	err = zram_writeback_slots(zram, ctl);
+	err = zram_writeback_slots(zram, pp_ctl, wb_ctl);
 	if (err)
 		ret = err;
 
 release_init_lock:
-	release_pp_ctl(zram, ctl);
+	release_pp_ctl(zram, pp_ctl);
+	release_wb_ctl(wb_ctl);
 	atomic_set(&zram->pp_in_progress, 0);
 	up_read(&zram->init_lock);
 
@@ -1112,7 +1375,9 @@ static int read_from_bdev(struct zram *zram, struct page *page,
 	return -EIO;
 }
 
-static void free_block_bdev(struct zram *zram, unsigned long blk_idx) {};
+static void zram_release_bdev_block(struct zram *zram, unsigned long blk_idx)
+{
+}
 #endif
 
 #ifdef CONFIG_ZRAM_MEMORY_TRACKING
@@ -1634,7 +1899,7 @@ static void zram_free_page(struct zram *zram, size_t index)
 
 	if (zram_test_flag(zram, index, ZRAM_WB)) {
 		zram_clear_flag(zram, index, ZRAM_WB);
-		free_block_bdev(zram, zram_get_handle(zram, index));
+		zram_release_bdev_block(zram, zram_get_handle(zram, index));
 		goto out;
 	}
 
@@ -1740,14 +2005,14 @@ static int zram_read_page(struct zram *zram, struct page *page, u32 index,
 		ret = zram_read_from_zspool(zram, page, index);
 		zram_slot_unlock(zram, index);
 	} else {
+		unsigned long blk_idx = zram_get_handle(zram, index);
+
 		/*
 		 * The slot should be unlocked before reading from the backing
 		 * device.
 		 */
 		zram_slot_unlock(zram, index);
-
-		ret = read_from_bdev(zram, page, zram_get_handle(zram, index),
-				     parent);
+		ret = read_from_bdev(zram, page, blk_idx, parent);
 	}
 
 	/* Should NEVER happen. Return bio error if it does. */
@@ -2610,6 +2875,7 @@ static DEVICE_ATTR_RW(backing_dev);
 static DEVICE_ATTR_WO(writeback);
 static DEVICE_ATTR_RW(writeback_limit);
 static DEVICE_ATTR_RW(writeback_limit_enable);
+static DEVICE_ATTR_RW(writeback_batch_size);
 #endif
 #ifdef CONFIG_ZRAM_MULTI_COMP
 static DEVICE_ATTR_RW(recomp_algorithm);
@@ -2631,6 +2897,7 @@ static struct attribute *zram_disk_attrs[] = {
 	&dev_attr_writeback.attr,
 	&dev_attr_writeback_limit.attr,
 	&dev_attr_writeback_limit_enable.attr,
+	&dev_attr_writeback_batch_size.attr,
 #endif
 	&dev_attr_io_stat.attr,
 	&dev_attr_mm_stat.attr,
@@ -2692,7 +2959,7 @@ static int zram_add(void)
 
 	init_rwsem(&zram->init_lock);
 #ifdef CONFIG_ZRAM_WRITEBACK
-	spin_lock_init(&zram->wb_limit_lock);
+	zram->wb_batch_size = 32;
 #endif
 
 	/* gendisk structure */
diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
index 6cee93f9c0d0..c6d94501376c 100644
--- a/drivers/block/zram/zram_drv.h
+++ b/drivers/block/zram/zram_drv.h
@@ -127,8 +127,8 @@ struct zram {
 	bool claim; /* Protected by disk->open_mutex */
 #ifdef CONFIG_ZRAM_WRITEBACK
 	struct file *backing_dev;
-	spinlock_t wb_limit_lock;
 	bool wb_limit_enable;
+	u32 wb_batch_size;
 	u64 bd_wb_limit;
 	struct block_device *bdev;
 	unsigned long *bitmap;