On Thu, Jun 17, 2021 at 09:24:02AM +0100, David Howells wrote:

With transparent huge pages, in the future, write_begin() and write_end()
may be passed a length parameter that, in combination with the offset into
the page, exceeds the length of that page.  This allows
grab_cache_page_write_begin() to better choose the size of THP to allocate.

While this is all true, it's really not necessary at this point in time.
That change will come with a conversion of these functions to work with
folios and basically every line you change here will change again.