reflink and atomic write bugs

This series did a few rounds privately, but looks ready for the list now. Carlos has accepted patches 1-4, but I've kept them here so the whole series can still be tested.

Except for Christoph's patch, these were all found and fixed by an LLM.

The meat of the series is fixing reflink corner cases that incorrectly let you write into the source of a reflink. LLMs also tripped over a few ways to use atomic writes to expose stale blocks on the drive, and a few races with realtime flag and extent size hint manipulations.

v7->v8:

Christoph pointed out that only page faults can cope with the -EAGAIN from "xfs: restart faults racing with realtime flag changes". pNFS is the one other caller that can really get there, so now faults get -EAGAIN and everyone else gets -EINVAL.

Sashiko had two complaints about the same patch. The stale reservation is fixed by the two "resample" patches. Those are the rest of the 3 additional patches I promised in v7, I just managed to leave them out of the posting.

The second complaint turned out to be on disk corruption for realtime filesystems with extents bigger than one block, with nothing returning an error. That's the new "xfs: test for an extsize hint again under the ILOCK".

While checking what a DAX fault does with the -EAGAIN, we found that DAX and zoned inodes don't work together at all. With a zoned filesystem on pmem and dax=always, the first store into a mapped file oopses on 7.4-rc2, and so does ftruncate(). "xfs: don't enable DAX on zoned inodes" turns DAX off for zoned inodes.

Added Christoph's review to "xfs: test for always COW after the dio write checks".

v6->v7:

Sashiko noticed a few pre-existing corners that I had ignored, so we have 3 additional patches dealing with races between FS_IOC_FSSETXATTR and decisions about file writes.

"xfs: restart faults racing with realtime flag changes" changed enough that I dropped Christoph's review.

Chris Mason (14):
xfs: don't skip the flush before shifting extents
xfs: flush the shifted range before changing the file
xfs: don't change the block map under a dirty folio
iomap: don't punch delalloc under a dirty EOF block
fs: validate atomic writes after O_APPEND and limits
xfs: recheck direct write alignment after write checks
xfs: test for reflink after the dio write checks
xfs: test for always COW after the dio write checks
xfs: refuse fallocate racing with realtime flag changes
xfs: restart faults racing with realtime flag changes
xfs: test for an extsize hint again under the ILOCK
xfs: resample the extsize hint after taking the ILOCK
xfs: resample the COW extsize hint after cycling ILOCK
xfs: don't enable DAX on zoned inodes

Christoph Hellwig (1):
xfs: refuse writes to files racing with realtime inode flag changes

 fs/ext4/file.c         |   4 -
 fs/iomap/buffered-io.c |  34 ++++++++-
 fs/read_write.c        |  13 ++++
 fs/xfs/xfs_bmap_util.c |  19 ++++-
 fs/xfs/xfs_file.c      | 167 ++++++++++++++++++++++++++++++++---------
 fs/xfs/xfs_ioctl.c     |   8 +-
 fs/xfs/xfs_iomap.c     |  73 ++++++++++++++++--
 fs/xfs/xfs_iops.c      |   4 +
 fs/xfs/xfs_reflink.c   |  16 +++-
 9 files changed, 282 insertions(+), 56 deletions(-)