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68ae56eb74
The unit tests added in bf3279f09 built sparse files by writing only
at one offset and assuming the surrounding pages stayed unallocated.
That breaks on shmem/tmpfs with huge=within_size: kernel 6.10+ added
large-folio support to shmem, and on first write the kernel allocates
one folio whose order is the largest power-of-two number of pages
that fits inside the file size (capped at PMD-size). For a 64 KiB
test file the very first pwrite anywhere allocates a 64 KiB folio
covering the whole file, so SEEK_HOLE never reports a hole and
written_pages_show_as_data_extents,
sparse_file_yields_extents_at_written_positions, and
single_extent_at_zero_offset all fail. memfd_create lives on shmem
too and inherits the same THP policy from
/sys/kernel/mm/transparent_hugepage/shmem_enabled, so the problem is
not /tmp-specific.
Fix the fixtures, not the production code: build each test file via
a new sparse_layout() helper that writes the requested data extents
and then fallocate(FALLOC_FL_PUNCH_HOLE)s every gap. PUNCH_HOLE is
the explicit "deallocate these pages" syscall and is honored by every
Linux filesystem we run tests on (tmpfs, ext4, xfs, btrfs); the
kernel splits any large folio overlapping the punched range. The
resulting SEEK_DATA/SEEK_HOLE map matches the spec exactly regardless
of folio/THP policy.
For single_extent_at_zero_offset the dst side still loses to the
folio allocator -- writing 8 KiB into a 64 KiB tmpfs file allocates
a 64 KiB folio whether we want it or not -- so the previous
meta.blocks()-based sparseness assertion (which tested the filesystem,
not our code) is replaced with a sentinel pre-fill: dst starts filled
with 0xFE and the post-condition is that bytes outside the
source-data extent are still 0xFE. That directly verifies
write_region_sparse only touched the data extent without depending on
dst-side hole reporting.
Side effect: extent_at_non_zero_src_offset,
two_regions_in_same_destination_file_at_dst_offset, and
round_trip_sparse_write_then_read previously passed by accident on
hosts with mTHP-on-shmem -- their src memfds reported the whole file
as data so write_region_sparse silently fell into a dense copy of
zeros + data. With sparse_layout() the sources are genuinely sparse
and those tests now exercise the sparse path on every host.
Tested on tmpfs (huge=within_size) and ext4 (TMPDIR=/var/tmp); all 9
tests pass on both with no skips.
Assisted-by: Claude:Opus-4.7
Signed-off-by: Dylan Reid <dgreid@fb.com>