On Mon, 5 Feb 2007 14:12:04 +0100
Jan Kara [off-list ref] wrote:

quoted

Move the blocks on the temporary inode to the original inode
by a page.
1. Read the file data from the old blocks to the page
2. Move the block on the temporary inode to the original inode
3. Write the file data on the page into the new blocks

  I have one thing - it's probably not good to use page cache for
defragmentation.

Then it is no longer online defragmentation.  The issues with maintaining
correctness and coherency with ongoing VFS activity would be truly ghastly.

If we're worried about pagecache pollution then it would be better to control
that from userspace via fadvise().