This patch introduce the following functions to handle btrfs subpage
uptodate status:
- btrfs_subpage_set_uptodate()
- btrfs_subpage_clear_uptodate()
- btrfs_subpage_test_uptodate()
   Those helpers can only be called when the range is ensured to be
   inside the page.

- btrfs_page_set_uptodate()
- btrfs_page_clear_uptodate()
- btrfs_page_test_uptodate()
   Those helpers can handle both regular sector size and subpage without
   problem.
   Although caller should still ensure that the range is inside the page.

Signed-off-by: Qu Wenruo <redacted>
---
  fs/btrfs/subpage.h | 115 +++++++++++++++++++++++++++++++++++++++++++++
  1 file changed, 115 insertions(+)

diff --git a/fs/btrfs/subpage.h b/fs/btrfs/subpage.h
index d8b34879368d..3373ef4ffec1 100644
--- a/fs/btrfs/subpage.h
+++ b/fs/btrfs/subpage.h

@@ -23,6 +23,7 @@
  struct btrfs_subpage {
  	/* Common members for both data and metadata pages */
  	spinlock_t lock;
+	u16 uptodate_bitmap;
  	union {
  		/* Structures only used by metadata */
  		bool under_alloc;

@@ -78,4 +79,118 @@ static inline void btrfs_page_end_meta_alloc(struct btrfs_fs_info *fs_info,
  int btrfs_attach_subpage(struct btrfs_fs_info *fs_info, struct page *page);
  void btrfs_detach_subpage(struct btrfs_fs_info *fs_info, struct page *page);
  
+/*
+ * Convert the [start, start + len) range into a u16 bitmap
+ *
+ * E.g. if start == page_offset() + 16K, len = 16K, we get 0x00f0.
+ */
+static inline u16 btrfs_subpage_calc_bitmap(struct btrfs_fs_info *fs_info,
+			struct page *page, u64 start, u32 len)
+{
+	int bit_start = offset_in_page(start) >> fs_info->sectorsize_bits;
+	int nbits = len >> fs_info->sectorsize_bits;
+
+	/* Basic checks */
+	ASSERT(PagePrivate(page) && page->private);
+	ASSERT(IS_ALIGNED(start, fs_info->sectorsize) &&
+	       IS_ALIGNED(len, fs_info->sectorsize));
+
+	/*
+	 * The range check only works for mapped page, we can
+	 * still have unampped page like dummy extent buffer pages.
+	 */
+	if (page->mapping)
+		ASSERT(page_offset(page) <= start &&
+			start + len <= page_offset(page) + PAGE_SIZE);