btrfs: relocation: Check cancel request after each data page read

When relocating a data extents with large large data extents, we spend
most of our time in relocate_file_extent_cluster() at stage "moving data
extents":

 1)               |  btrfs_relocate_block_group [btrfs]() {
 1)               |    relocate_file_extent_cluster [btrfs]() {
 1) $ 6586769 us  |    }
 1) + 18.260 us   |    relocate_file_extent_cluster [btrfs]();
 1) + 15.770 us   |    relocate_file_extent_cluster [btrfs]();
 1) $ 8916340 us  |  }
 1)               |  btrfs_relocate_block_group [btrfs]() {
 1)               |    relocate_file_extent_cluster [btrfs]() {
 1) $ 11611586 us |    }
 1) + 16.930 us   |    relocate_file_extent_cluster [btrfs]();
 1) + 15.870 us   |    relocate_file_extent_cluster [btrfs]();
 1) $ 14986130 us |  }

To make data relocation cancelling quicker, add extra balance cancelling
check after each page read in relocate_file_extent_cluster().

Cleanup and error handling uses the same mechanism as if the whole
process finished
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/relocation.c

@@ -3445,6 +3445,10 @@ static int relocate_file_extent_cluster(struct inode *inode,
 		btrfs_delalloc_release_extents(BTRFS_I(inode), PAGE_SIZE);
 		balance_dirty_pages_ratelimited(inode->i_mapping);
 		btrfs_throttle(fs_info);
+		if (btrfs_should_cancel_balance(fs_info)) {
+			ret = -ECANCELED;
+			goto out;
+		}
 	}
 	WARN_ON(nr != cluster->nr);
 out:
@@ -4259,6 +4263,14 @@ static noinline_for_stack int relocate_block_group(struct reloc_control *rc)
 	backref_cache_cleanup(&rc->backref_cache);
 	btrfs_block_rsv_release(fs_info, rc->block_rsv, (u64)-1);
 
+	/*
+	 * Even in the case when the relocation is cancelled, we should all go
+	 * through prepare_to_merge() and merge_reloc_roots().
+	 *
+	 * For error (including cancelled balance), prepare_to_merge() will
+	 * mark all reloc trees orphan, then queue them for cleanup in
+	 * merge_reloc_roots()
+	 */
 	err = prepare_to_merge(rc, err);
 
 	merge_reloc_roots(rc);