btrfs: simplify the page uptodate preparation for prepare_pages()

Currently inside prepare_pages(), we handle the leading and tailing page
differently, and skip the middle pages (if any).  This is to avoid
reading pages which are fully covered by the dirty range.

Refactor the code by moving all checks (alignment check, range check,
force read check) into prepare_uptodate_page().

So that prepare_pages() only needs to iterate all the pages
unconditionally.

And since we're here, also update prepare_uptodate_page() to use
folio API other than the old page API.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/file.c

@@ -858,36 +858,42 @@ int btrfs_mark_extent_written(struct btrfs_trans_handle *trans,
  */
 static int prepare_uptodate_page(struct inode *inode,
 				 struct page *page, u64 pos,
-				 bool force_uptodate)
+				 u64 len, bool force_uptodate)
 {
 	struct folio *folio = page_folio(page);
+	u64 clamp_start = max_t(u64, pos, folio_pos(folio));
+	u64 clamp_end = min_t(u64, pos + len, folio_pos(folio) + folio_size(folio));
 	int ret = 0;
 
-	if (((pos & (PAGE_SIZE - 1)) || force_uptodate) &&
-	    !PageUptodate(page)) {
-		ret = btrfs_read_folio(NULL, folio);
-		if (ret)
-			return ret;
-		lock_page(page);
-		if (!PageUptodate(page)) {
-			unlock_page(page);
-			return -EIO;
-		}
+	if (folio_test_uptodate(folio))
+		return 0;
 
-		/*
-		 * Since btrfs_read_folio() will unlock the folio before it
-		 * returns, there is a window where btrfs_release_folio() can be
-		 * called to release the page.  Here we check both inode
-		 * mapping and PagePrivate() to make sure the page was not
-		 * released.
-		 *
-		 * The private flag check is essential for subpage as we need
-		 * to store extra bitmap using folio private.
-		 */
-		if (page->mapping != inode->i_mapping || !folio_test_private(folio)) {
-			unlock_page(page);
-			return -EAGAIN;
-		}
+	if (!force_uptodate &&
+	    IS_ALIGNED(clamp_start, PAGE_SIZE) &&
+	    IS_ALIGNED(clamp_end, PAGE_SIZE))
+		return 0;
+
+	ret = btrfs_read_folio(NULL, folio);
+	if (ret)
+		return ret;
+	folio_lock(folio);
+	if (!folio_test_uptodate(folio)) {
+		folio_unlock(folio);
+		return -EIO;
+	}
+
+	/*
+	 * Since btrfs_read_folio() will unlock the folio before it returns,
+	 * there is a window where btrfs_release_folio() can be called to
+	 * release the page.  Here we check both inode mapping and page
+	 * private to make sure the page was not released.
+	 *
+	 * The private flag check is essential for subpage as we need to store
+	 * extra bitmap using folio private.
+	 */
+	if (page->mapping != inode->i_mapping || !folio_test_private(folio)) {
+		folio_unlock(folio);
+		return -EAGAIN;
 	}
 	return 0;
 }
@@ -949,12 +955,8 @@ static noinline int prepare_pages(struct inode *inode, struct page **pages,
 			goto fail;
 		}
 
-		if (i == 0)
-			ret = prepare_uptodate_page(inode, pages[i], pos,
-						    force_uptodate);
-		if (!ret && i == num_pages - 1)
-			ret = prepare_uptodate_page(inode, pages[i],
-						    pos + write_bytes, false);
+		ret = prepare_uptodate_page(inode, pages[i], pos, write_bytes,
+					    force_uptodate);
 		if (ret) {
 			put_page(pages[i]);
 			if (!nowait && ret == -EAGAIN) {