[PATCH v3 0/7] iomap: zero range folio batch support

Brian Foster
2025-07-14T20:37:50Z

Hi all,

Quick update.. This series was held up by testing work on my end. I
don't have the custom test to go along with patch 7 yet, but hch was
asking for updates, I have vacation looming, and realistically I wasn't
going to get to that beforehand. So I'm posting v2 without the
additional test and reviewers can decide if/how to proceed in the
meantime. Either way, I'll pick up where this leaves off.

Zero range is still obviously functionally testable. We just don't yet
have the enhanced coverage I was hoping for via the errortag knobs.
There are also a couple small fstests failures related to to tests that
explicitly expect unwritten extents in cases where this now decides to
perform zeroing (generic/009, xfs/242). I don't consider these
functional regressions, but the tests need to be fixed up to accommodate
behavior. Again, I'll get back to this stuff either way, it's just going
to be a couple weeks or so at least at this point. Thanks.

Brian

--- Original cover letter ---

Hi all,

Here's a first real v1 of 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.

Thoughts, reviews, flames appreciated.

Brian

v3:
- Update commit log description in patch 2.
- Improve comments in patch 7.
v2: https://lore.kernel.org/linux-fsdevel/20250714132059.288129-1-bfoster@redhat.com/
- Move filemap patch to top. Add some comments and drop export.
- Drop unnecessary BUG_ON()s from iomap_write_begin() instead of moving.
- Added folio mapping check to batch codepath, improved comments.
v1: https://lore.kernel.org/linux-fsdevel/20250605173357.579720-1-bfoster@redhat.com/
- Dropped most prep patches from previous version (merged separately).
- Reworked dirty folio lookup to use find_get_entry() loop (new patch
for filemap helper).
- Misc. bug fixes, code cleanups, comments, etc.
- Added (RFC) prospective patch for wider zero range test coverage.
RFCv2: https://lore.kernel.org/linux-fsdevel/20241213150528.1003662-1-bfoster@redhat.com/
- Port onto incremental advance, drop patch 1 from RFCv1.
- Moved batch into iomap_iter, dynamically allocate and drop flag.
- Tweak XFS patch to always trim zero range on EOF boundary.
RFCv1: https://lore.kernel.org/linux-fsdevel/20241119154656.774395-1-bfoster@redhat.com/

Brian Foster (7):
filemap: add helper to look up dirty folios in a range
iomap: remove pos+len BUG_ON() to after folio lookup
iomap: optional zero range dirty folio processing
xfs: always trim mapping to requested range for zero range
xfs: fill dirty folios on zero range of unwritten mappings
iomap: remove old partial eof zeroing optimization
xfs: error tag to force zeroing on debug kernels

fs/iomap/buffered-io.c | 116 +++++++++++++++++++++++++----------
fs/iomap/iter.c | 6 ++
fs/xfs/libxfs/xfs_errortag.h | 4 +-
fs/xfs/xfs_error.c | 3 +
fs/xfs/xfs_file.c | 26 ++++++--
fs/xfs/xfs_iomap.c | 38 +++++++++---
include/linux/iomap.h | 4 ++
include/linux/pagemap.h | 2 +
mm/filemap.c | 58 ++++++++++++++++++
9 files changed, 210 insertions(+), 47 deletions(-)

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2.50.0