Merge patch series "iomap: zero range folio batch support"

Brian Foster <bfoster@redhat.com> says:

This adds folio batch support for iomap. This initially only targets
zero range, the use case being zeroing of dirty folios over unwritten
mappings. There is potential to support other operations in the future:
iomap seek data/hole has similar raciness issues as zero range, the
prospect of using this for buffered write has been raised for granular
locking purposes, etc.

The one major caveat with this zero range implementation is that it
doesn't look at iomap_folio_state to determine whether to zero a
sub-folio portion of the folio. Instead it just relies on whether the
folio was dirty or not. This means that spurious zeroing of unwritten
ranges is possible if a folio is dirty but the target range includes a
subrange that is not.

The reasoning is that this is essentially a complexity tradeoff. The
current use cases for iomap_zero_range() are limited mostly to partial
block zeroing scenarios. It's relatively harmless to zero an unwritten
block (i.e. not a correctness issue), and this is something that
filesystems have done in the past without much notice or issue. The
advantage is less code and this makes it a little easier to use a
filemap lookup function for the batch rather than open coding more logic
in iomap. That said, this can probably be enhanced to look at ifs in the
future if the use case expands and/or other operations justify it.

WRT testing, I've tested with and without a local hack to redirect
fallocate zero range calls to iomap_zero_range() in XFS. This helps test
beyond the partial block/folio use case, i.e. to cover boundary
conditions like full folio batch handling, etc. I recently added patch 7
in spirit of that, which turns this logic into an XFS errortag. Further
comments on that are inline with patch 7.

* patches from https://lore.kernel.org/20251003134642.604736-1-bfoster@redhat.com:
  xfs: error tag to force zeroing on debug kernels
  iomap: remove old partial eof zeroing optimization
  xfs: fill dirty folios on zero range of unwritten mappings
  xfs: always trim mapping to requested range for zero range
  iomap: optional zero range dirty folio processing
  iomap: remove pos+len BUG_ON() to after folio lookup
  filemap: add helper to look up dirty folios in a range

Signed-off-by: Christian Brauner <brauner@kernel.org>