btrfs: fix hang when run_delalloc_range() failed

[BUG]
When running subpage preparation patches on x86, btrfs/125 will hang
forever with one ordered extent never finished.

[CAUSE]
The test case btrfs/125 itself will always fail as the fix is never merged.

When the test fails at balance, btrfs needs to cleanup the ordered
extent in btrfs_cleanup_ordered_extents() for data reloc inode.

The problem is in the sequence how we cleanup the page Order bit.

Currently it works like:

  btrfs_cleanup_ordered_extents()
  |- find_get_page();
  |- btrfs_page_clear_ordered(page);
  |  Now the page doesn't have Ordered bit anymore.
  |  !!! This also includes the first (locked) page !!!
  |
  |- offset += PAGE_SIZE
  |  This is to skip the first page
  |- __endio_write_update_ordered()
     |- btrfs_mark_ordered_io_finished(NULL)
        Except the first page, all ordered extents are finished.

Then the locked page is cleaned up in __extent_writepage():

  __extent_writepage()
  |- If (PageError(page))
  |- end_extent_writepage()
     |- btrfs_mark_ordered_io_finished(page)
        |- if (btrfs_test_page_ordered(page))
        |-  !!! The page gets skipped !!!
            The ordered extent is not decreased as the page doesn't
            have ordered bit anymore.

This leaves the ordered extent with bytes_left == sectorsize, thus never
finish.

[FIX]
The fix is to ensure we never clear page Ordered bit without running the
ordered extent accounting.

Here we choose to skip the locked page in
btrfs_cleanup_ordered_extents() so that later end_extent_writepage() can
properly finish the ordered extent.
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -166,10 +166,31 @@ static inline void btrfs_cleanup_ordered_extents(struct btrfs_inode *inode,
 	struct page *page;
 
 	while (index <= end_index) {
+		/*
+		 * For locked page, we will call end_extent_writepage() on it
+		 * in run_delalloc_range() for the error handling.  That
+		 * end_extent_writepage() function will call
+		 * btrfs_mark_ordered_io_finished() to clear page Ordered and
+		 * run the ordered extent accounting.
+		 *
+		 * Here we can't just clear the Ordered bit, or
+		 * btrfs_mark_ordered_io_finished() would skip the accounting
+		 * for the page range, and the ordered extent will never finish.
+		 */
+		if (index == (page_offset(locked_page) >> PAGE_SHIFT)) {
+			index++;
+			continue;
+		}
 		page = find_get_page(inode->vfs_inode.i_mapping, index);
 		index++;
 		if (!page)
 			continue;
+
+		/*
+		 * Here we just clear all Ordered bits for every page in the
+		 * range, then __endio_write_update_ordered() will handle
+		 * the ordered extent accounting for the range.
+		 */
 		ClearPageOrdered(page);
 		put_page(page);
 	}