On Sat, Jul 17, 2021 at 07:40:41PM +0100, Matthew Wilcox wrote:

Well, either sense of a WARN_ON is wrong.

For a file which is PAGE_SIZE + 3 bytes in size, to_iomap_page() will
be NULL.  For a file which is PAGE_SIZE/2 + 3 bytes in size,
to_iomap_page() will not be NULL.  (assuming the block size is <=
PAGE_SIZE / 2).

I think we need a prep patch that looks something like this:

Something like this is where we should eventually end up, but it
also affects the read from disk case so we need to be careful.