Merge patch series "iomap: incremental per-operation iter advance"

Brian Foster <bfoster@redhat.com> says:

This is a first pass at supporting more incremental, per-operation
iomap_iter advancement. The motivation for this is folio_batch support
for zero range, where the fs provides a batch of folios to process
in certain situations. Since the batch may not be logically contiguous,
processing loops require a bit more flexibility than the typical offset
based iteration.

The current iteration model basically has the operation _iter() handler
lift the pos/length wrt to the current iomap out of the iomap_iter,
process it locally, then return the result to be stored in
iter.processed. The latter is overloaded with error status, so the
handler must decide whether to return error or a partial completion
(i.e. consider a short write). iomap_iter() then uses the result to
advance the iter and look up the next iomap.

The updated model proposed in this series is to allow an operation to
advance the iter itself as subranges are processed and then return
success or failure in iter.processed. Note that at least initially, this
is implemented as an optional mode to minimize churn. This series
converts operations that use iomap_write_begin(): buffered write,
unshare, and zero range.

The main advantage of this is that the future folio_batch work can be
plumbed down into the folio get path more naturally, and the
associated codepath can advance the iter itself when appropriate rather
than require each operation to manage the gaps in the range being
processed. Some secondary advantages are a little less boilerplate code
for walking ranges and more clear semantics for partial completions in
the event of errors, etc.

* patches from https://lore.kernel.org/r/20250207143253.314068-1-bfoster@redhat.com:
  iomap: advance the iter directly on zero range
  iomap: advance the iter directly on unshare range
  iomap: advance the iter directly on buffered writes
  iomap: support incremental iomap_iter advances
  iomap: export iomap_iter_advance() and return remaining length
  iomap: lift iter termination logic from iomap_iter_advance()
  iomap: lift error code check out of iomap_iter_advance()
  iomap: refactor iomap_iter() length check and tracepoint
  iomap: split out iomap check and reset logic from iter advance
  iomap: factor out iomap length helper

Link: https://lore.kernel.org/r/20250207143253.314068-1-bfoster@redhat.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

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