[PATCH v4 0/2] fuse: zero the partial EOF page when extending a file

[Jimmy Zuber <jamz@amazon.com>]

Extending a fuse file past a non-page-aligned EOF does not zero the tail of
the old last page.  When that page is cached and has been mmap-dirtied beyond
the old EOF, the now in-bounds tail is served to later reads as stale data
rather than zeros, which violates POSIX file-extension semantics.

Some file systems get this zeroing automatically at writeback time
(block_write_full_folio() / iomap_writeback_handle_eof() zero the tail of the
folio straddling i_size).  A non-writeback caching fuse file system uses neither
path, so it has to zero the tail itself from the size-extending paths, like
XFS (xfs_file_write_zero_eof()) and ext4 (ext4_block_zero_eof()) do.

Patch 1 calls truncate_pagecache_range() over the newly-exposed range up front
from the three paths that extend a file: buffered write, setattr, and
fallocate.  Patch 2 adds a self-contained raw /dev/fuse selftest covering each
path.

Tested by booting the patched kernel under User-Mode Linux and running the new
selftest, and reproduced against libfuse's passthrough_ll example daemon.

Changes since v3 [3]:
- Drop the fuse_zero_partial_eof_folio() helper and call
  truncate_pagecache_range(inode, from, to - 1) from the three extend paths
  instead.  Miklos pointed out the helper's folio_mkclean()/folio_mark_dirty()
  dance is only needed by file systems using buffer heads (as Jan noted), and
  that truncate_pagecache_range() already does the rest of the zeroing.  The
  selftest is unchanged and still passes with patch 1 (and fails without it).

Changes since v1 [1]:
- Zero from the size-extending paths up front, keyed on the write starting
  beyond EOF, rather than after the fact from fuse_write_update_attr(), so a
  write that lands inside the old EOF folio is preserved.  (v2 [2] carried this
  change but went out with malformed recipient headers; it was resent as
  v3 [3].)

[1] https://lore.kernel.org/all/20260731203842.540798-1-jamz@amazon.com/
[2] https://lore.kernel.org/all/20260820122627.156961-1-jamz@amazon.com/
[3] https://lore.kernel.org/all/20260820123533.190470-1-jamz@amazon.com/

Jimmy Zuber (2):
  fuse: zero the partial EOF page when extending a file
  selftests/fuse: test post-EOF page zeroing when a file is extended

 fs/fuse/dir.c                                 |   3 +
 fs/fuse/file.c                                |   9 +
 .../selftests/filesystems/fuse/.gitignore     |   1 +
 .../selftests/filesystems/fuse/Makefile       |   3 +
 .../filesystems/fuse/write_extend_eof_test.c  | 368 ++++++++++++++++++
 5 files changed, 384 insertions(+)
 create mode 100644 tools/testing/selftests/filesystems/fuse/write_extend_eof_test.c


base-commit: 7d87a5a284bb34edb3f4e7e312ef403b3385a7b7
-- 
2.50.1