Re: [RFC PATCH V3] iomap: add support to track dirty state of sub pages

[Matthew Wilcox]

On Fri, Aug 21, 2020 at 07:12:34AM +0100, Christoph Hellwig wrote:
> iomap sets PagePrivate if a iomap_page structure is allocated.  Right
> now that means for all pages on a file system with a block size smaller
> than the page size, although I hope we reduce that scope a little.

I was thinking about that.  Is there a problem where we initially allocate
the page with a contiguous extent larger than the page, then later need
to write the page to a pair of extents?

If we're doing an unshare operation, then we know our src and dest iomaps
and can allocate the iop then.  But if we readahead, we don't necessarily
know our eventual dest.  So the conditions for skipping allocating an
iop are tricky to be sure we'll never need it.