v2:
- Simplify condition to if (!s->needs_alignment) in patch 1 [Vladimir]

This series fixes a bug I introduced in commit 5634622bcb33 ("file-posix: allow
BLKZEROOUT with -t writeback"). The Linux fallocate(2) and ioctl(BLKZEROOUT)
syscalls require logical block size alignment of the offset and length, even
when the file is opened in buffered I/O mode where read/write operations do not
require alignment.

The fix is to populate the pwrite_zeroes_alignment block limits field and to
use that limit in create_file_fallback_zero_first_sector().

One issue I want to raise is that pwrite_zeroes_alignment is an "optimal
alignment" hint. Hence create_file_fallback_zero_first_sector() had to be
modified to honor the limit explicitly. The block layer doesn't automatically
apply padding in order to align requests. This is different from how QEMU's
block layer pwrite/pread works, where it does automatically apply padding and
read/modify/write as necessary. If you want consistency, please let me know.

Stefan Hajnoczi (3):
  file-posix: populate pwrite_zeroes_alignment
  block: use pwrite_zeroes_alignment when writing first sector
  iotests: add Linux loop device image creation test

 include/system/block-backend-io.h             |  1 +
 block.c                                       |  3 +-
 block/block-backend.c                         | 11 ++++
 block/file-posix.c                            | 16 +++++
 tests/qemu-iotests/tests/loop-create-file     | 59 +++++++++++++++++++
 tests/qemu-iotests/tests/loop-create-file.out |  8 +++
 6 files changed, 97 insertions(+), 1 deletion(-)
 create mode 100755 tests/qemu-iotests/tests/loop-create-file
 create mode 100644 tests/qemu-iotests/tests/loop-create-file.out

-- 
2.51.0