ext4: skip overwrite check for aligned non-extending DIO writes

Currently, ext4_dio_write_checks() calls ext4_overwrite_io() to
determine if a write is a pure overwrite, and upgrades to exclusive
i_rwsem if not. However, ext4_overwrite_io() uses a single
ext4_map_blocks() call which only returns the first contiguous extent of
the same type. A write spanning multiple pre-allocated extents (e.g.
written + unwritten, or two physically discontiguous written extents)
produces a false negative, forcing an unnecessary exclusive lock upgrade.

After commit 5d87c7fca2c1 ("ext4: avoid starting handle when dio
writing an unwritten extent") and commit 012924f0eeef ("ext4: remove
useless ext4_iomap_overwrite_ops"), ext4_iomap_begin()'s fast path
accepts both EXT4_MAP_MAPPED and EXT4_MAP_UNWRITTEN without starting a
journal transaction. The iomap iteration naturally handles multi-extent
ranges: each call returns the mapping for the current segment, and
unwritten-to-written conversion is deferred to ext4_dio_write_end_io().
This means the common case of mixed written/unwritten extents never
reaches ext4_iomap_alloc() at all.

Even for the less common case where the range contains a hole and
ext4_iomap_alloc() is needed, exclusive i_rwsem is still unnecessary for
aligned non-extending writes:

 - truncate/punch_hole are kept out: they require exclusive i_rwsem
   (blocked by our shared lock during allocation), and inode_dio_begin()
   keeps their inode_dio_wait() blocked until in-flight bios complete.
 - i_data_sem write-lock inside ext4_map_blocks() serializes concurrent
   extent tree modifications (parallel writers to the same hole).
 - The journal handle is per-thread and does not require i_rwsem
   exclusion.
 - i_disksize and orphan list are not involved in non-extending writes.

Skip the ext4_overwrite_io() check entirely for aligned writes by
initializing overwrite to true and only calling ext4_overwrite_io() for
unaligned writes. Unaligned writes still need the extent state check
because concurrent partial block zeroing in the DIO layer requires
exclusive serialization unless the range is a pure written-extent
overwrite.

Performance:

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

Aligned 8K DIO writes spanning written+unwritten extent boundaries.
Each thread writes its own 1G region sequentially; the file is rebuilt
between runs so every block is written exactly once. Metric: IOPS.

  JOBS      Before        After    speedup
  ----    --------    ---------    -------
     1      42,322       43,329      1.02x
     2      68,516       70,677      1.03x
     4      62,489       97,072      1.55x
     8      58,701      110,819      1.89x
    16      58,569      116,392      1.99x
    32      60,860      117,244      1.93x

Wall time at JOBS=32: 69.2s (Before) -> 35.4s (After), 1.96x 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/20260629113827.4074335-4-libaokun@linux.alibaba.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

0 files changed, 0 insertions, 0 deletions