btrfs: subpage: introduce helpers for writeback status

Introduces the following functions to handle subpage writeback status:

- btrfs_subpage_set_writeback()
- btrfs_subpage_clear_writeback()
- btrfs_subpage_test_writeback()
These helpers can only be called when the range is ensured to be
inside the page.

- btrfs_page_set_writeback()
- btrfs_page_clear_writeback()
- btrfs_page_test_writeback()
These helpers can handle both regular sector size and subpage without
problem.

Signed-off-by: Qu Wenruo <[e-mail]>
Reviewed-by: David Sterba <[e-mail]>
Signed-off-by: David Sterba <[e-mail]>

fs/btrfs/subpage.c

···
 		clear_page_dirty_for_io(page);
 }
 
+void btrfs_subpage_set_writeback(const struct btrfs_fs_info *fs_info,
+		struct page *page, u64 start, u32 len)
+{
+	struct btrfs_subpage *subpage = (struct btrfs_subpage *)page->private;
+	u16 tmp = btrfs_subpage_calc_bitmap(fs_info, page, start, len);
+	unsigned long flags;
+
+	spin_lock_irqsave(&subpage->lock, flags);
+	subpage->writeback_bitmap |= tmp;
+	set_page_writeback(page);
+	spin_unlock_irqrestore(&subpage->lock, flags);
+}
+
+void btrfs_subpage_clear_writeback(const struct btrfs_fs_info *fs_info,
+		struct page *page, u64 start, u32 len)
+{
+	struct btrfs_subpage *subpage = (struct btrfs_subpage *)page->private;
+	u16 tmp = btrfs_subpage_calc_bitmap(fs_info, page, start, len);
+	unsigned long flags;
+
+	spin_lock_irqsave(&subpage->lock, flags);
+	subpage->writeback_bitmap &= ~tmp;
+	if (subpage->writeback_bitmap == 0)
+		end_page_writeback(page);
+	spin_unlock_irqrestore(&subpage->lock, flags);
+}
+
 /*
  * Unlike set/clear which is dependent on each page status, for test all bits
  * are tested in the same way.
···
 IMPLEMENT_BTRFS_SUBPAGE_TEST_OP(uptodate);
 IMPLEMENT_BTRFS_SUBPAGE_TEST_OP(error);
 IMPLEMENT_BTRFS_SUBPAGE_TEST_OP(dirty);
+IMPLEMENT_BTRFS_SUBPAGE_TEST_OP(writeback);
 
 /*
  * Note that, in selftests (extent-io-tests), we can have empty fs_info passed
···
 IMPLEMENT_BTRFS_PAGE_OPS(error, SetPageError, ClearPageError, PageError);
 IMPLEMENT_BTRFS_PAGE_OPS(dirty, set_page_dirty, clear_page_dirty_for_io,
 			 PageDirty);
+IMPLEMENT_BTRFS_PAGE_OPS(writeback, set_page_writeback, end_page_writeback,
+			 PageWriteback);

fs/btrfs/subpage.h

···
 	u16 uptodate_bitmap;
 	u16 error_bitmap;
 	u16 dirty_bitmap;
+	u16 writeback_bitmap;
 	union {
 		/*
 		 * Structures only used by metadata
···
 DECLARE_BTRFS_SUBPAGE_OPS(uptodate);
 DECLARE_BTRFS_SUBPAGE_OPS(error);
 DECLARE_BTRFS_SUBPAGE_OPS(dirty);
+DECLARE_BTRFS_SUBPAGE_OPS(writeback);
 
 bool btrfs_subpage_clear_and_test_dirty(const struct btrfs_fs_info *fs_info,
 		struct page *page, u64 start, u32 len);