Currently we use fixed size u16 bitmap for subpage bitmap.
This is fine for 4K sectorsize with 64K page size.

But for 4K sectorsize and larger page size, the bitmap is too small,
while for smaller page size like 16K, u16 bitmaps waste too much space.

Here we introduce a new helper structure, btrfs_subpage_bitmap_info, to
record the proper bitmap size, and where each bitmap should start at.

By this, we can later compact all subpage bitmaps into one u32 bitmap.

This patch is the first step towards such compact bitmap.

Signed-off-by: Qu Wenruo <redacted>
---
 fs/btrfs/ctree.h   |  1 +
 fs/btrfs/disk-io.c | 12 +++++++++---
 fs/btrfs/subpage.c | 35 +++++++++++++++++++++++++++++++++++
 fs/btrfs/subpage.h | 36 ++++++++++++++++++++++++++++++++++++
 4 files changed, 81 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index f07c82fafa04..a5297748d719 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h

@@ -899,6 +899,7 @@ 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;
 

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 2f9515dccce0..3355708919d0 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c

@@ -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);
 }
 

@@ -3392,12 +3393,12 @@ int __cold open_ctree(struct super_block *sb, struct btrfs_fs_devices *fs_device
 		goto fail_alloc;
 	}
 
-	if (sectorsize != PAGE_SIZE) {
+	if (sectorsize < PAGE_SIZE) {
+		struct btrfs_subpage_info *subpage_info;
+
 		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 +3407,11 @@ 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);

diff --git a/fs/btrfs/subpage.c b/fs/btrfs/subpage.c
index ae6c68370a95..c4fb2ce52207 100644
--- a/fs/btrfs/subpage.c
+++ b/fs/btrfs/subpage.c

@@ -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);