ext4: base unaligned DIO lock decision on partial block zeroing

For unaligned DIO writes, the previous ext4_overwrite_io() required the
entire range to fall within a single written extent.  This was overly
conservative: the DIO layer only performs partial block zeroing for the
head and tail blocks when they are partially covered by the write.
Middle blocks that are fully covered are written as whole blocks
without any zeroing, so they are safe regardless of extent state.

Therefore exclusive lock is only required when partial block zeroing
will actually happen:
 - The head partial block (if any) lands on a hole or unwritten extent.
 - The tail partial block (if any) lands on a hole or unwritten extent.

Middle full-cover blocks can be in any state (hole, unwritten, or
written) - block allocation under shared lock is safe per the previous
patch's analysis (inode_dio_begin + i_data_sem protection).

Replace ext4_overwrite_io() with ext4_dio_needs_zeroing(), which
directly answers the question driving the lock decision.  It uses at
most two ext4_map_blocks() calls: one for the head partial block (also
catching the case where it spans through the tail), and one for the
tail partial block if not already covered.

This enables shared lock for previously-rejected scenarios such as:
 - Unaligned write spanning written extent + mid-range hole + written
   extent at the tail.
 - Unaligned write where the partial blocks land on written extents but
   the middle has unwritten extents.

Performance:

Hardware: /dev/sda (rotational disk, ~1 GB/s sustained write)
Filesystem: ext4 default mkfs

Unaligned DIO writes (14336 bytes at +512 within each 16K stripe).
Each stripe is laid out as [written][unwritten][unwritten][written],
so the head and tail partial blocks land on written extents but the
middle is unwritten.  Metric: IOPS.

  JOBS      Before        After    speedup
  ----    --------    ---------    -------
     1      15,547       17,381      1.12x
     2      15,910       34,172      2.15x
     4      15,014       57,567      3.83x
     8      15,022       81,947      5.46x
    16      14,586       99,126      6.80x
    32      14,047       92,519      6.59x

Wall time at JOBS=32: 149.3s (Before) -> 22.7s (After), 6.58x faster.

Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Baokun Li <libaokun@linux.alibaba.com>
Link: https://patch.msgid.link/20260629113827.4074335-5-libaokun@linux.alibaba.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

0 files changed, 0 insertions, 0 deletions