@@ -898,6 +898,7 @@ struct btrfs_fs_info {
 	struct btrfs_workqueue *scrub_workers;
 	struct btrfs_workqueue *scrub_wr_completion_workers;
 	struct btrfs_workqueue *scrub_parity_workers;
+	struct btrfs_subpage_info *subpage_info;
 
 	struct btrfs_discard_ctl discard_ctl;
 

@@ -1644,6 +1644,7 @@ void btrfs_free_fs_info(struct btrfs_fs_info *fs_info)
 	btrfs_extent_buffer_leak_debug_check(fs_info);
 	kfree(fs_info->super_copy);
 	kfree(fs_info->super_for_commit);
+	kfree(fs_info->subpage_info);
 	kvfree(fs_info);
 }
 

@@ -3393,11 +3394,13 @@ int __cold open_ctree(struct super_block *sb, struct btrfs_fs_devices *fs_device
 	}
 
 	if (sectorsize != PAGE_SIZE) {
+		struct btrfs_subpage_info *subpage_info;
+
+		ASSERT(sectorsize < PAGE_SIZE);
+
 		btrfs_warn(fs_info,
 	"read-write for sector size %u with page size %lu is experimental",
 			   sectorsize, PAGE_SIZE);
-	}
-	if (sectorsize != PAGE_SIZE) {
 		if (btrfs_super_incompat_flags(fs_info->super_copy) &
 			BTRFS_FEATURE_INCOMPAT_RAID56) {
 			btrfs_err(fs_info,

@@ -3406,6 +3409,11 @@ int __cold open_ctree(struct super_block *sb, struct btrfs_fs_devices *fs_device
 			err = -EINVAL;
 			goto fail_alloc;
 		}
+		subpage_info = kzalloc(sizeof(*subpage_info), GFP_NOFS);
+		if (!subpage_info)
+			goto fail_alloc;
+		btrfs_init_subpage_info(subpage_info, sectorsize);
+		fs_info->subpage_info = subpage_info;
 	}
 
 	ret = btrfs_init_workqueues(fs_info, fs_devices);

@@ -63,6 +63,41 @@
  *   This means a slightly higher tree locking latency.
  */
 
+void btrfs_init_subpage_info(struct btrfs_subpage_info *subpage_info,
+			     u32 sectorsize)
+{
+	unsigned int cur = 0;
+	unsigned int nr_bits;
+
+	/*
+	 * Just in case we have super large PAGE_SIZE that unsigned int is not
+	 * enough to contain the number of sectors for the minimal sectorsize.
+	 */
+	BUILD_BUG_ON(UINT_MAX * SZ_4K < PAGE_SIZE);
+
+	ASSERT(IS_ALIGNED(PAGE_SIZE, sectorsize));
+
+	nr_bits = PAGE_SIZE / sectorsize;
+	subpage_info->bitmap_nr_bits = nr_bits;
+
+	subpage_info->uptodate_start = cur;
+	cur += nr_bits;
+
+	subpage_info->error_start = cur;
+	cur += nr_bits;
+
+	subpage_info->dirty_start = cur;
+	cur += nr_bits;
+
+	subpage_info->writeback_start = cur;
+	cur += nr_bits;
+
+	subpage_info->ordered_start = cur;
+	cur += nr_bits;
+
+	subpage_info->total_nr_bits = cur;
+}
+
 int btrfs_attach_subpage(const struct btrfs_fs_info *fs_info,
 			 struct page *page, enum btrfs_subpage_type type)
 {

@@ -11,6 +11,32 @@
  */
 #define BTRFS_SUBPAGE_BITMAP_SIZE	16
 
+/*
+ * Extra info for subpapge bitmap.
+ *
+ * For subpage we integrate all uptodate/error/dirty/writeback/ordered
+ * bitmaps into one larger bitmap.
+ * This structure records the basic info.
+ */
+struct btrfs_subpage_info {
+	/* Number of bits for each bitmap*/
+	unsigned int bitmap_nr_bits;
+
+	/* Total number of bits for the whole bitmap */
+	unsigned int total_nr_bits;
+
+	/*
+	 * *_start indicates where the bitmap starts, the length
+	 * is always @bitmap_size, which is calculated from
+	 * PAGE_SIZE / sectorsize.
+	 */
+	unsigned int uptodate_start;
+	unsigned int error_start;
+	unsigned int dirty_start;
+	unsigned int writeback_start;
+	unsigned int ordered_start;
+};
+
 /*
  * Structure to trace status of each sector inside a page, attached to
  * page::private for both data and metadata inodes.

@@ -53,6 +79,8 @@ enum btrfs_subpage_type {
 	BTRFS_SUBPAGE_DATA,
 };
 
+void btrfs_init_subpage_info(struct btrfs_subpage_info *subpage_info,
+			     u32 sectorsize);
 int btrfs_attach_subpage(const struct btrfs_fs_info *fs_info,
 			 struct page *page, enum btrfs_subpage_type type);
 void btrfs_detach_subpage(const struct btrfs_fs_info *fs_info,