Re: [RFC PATCH] ext4: fix false-negative overwrite check for DIO spanning extent boundaries

[Baokun Li <libaokun@linux.alibaba.com>]

On 2026/6/7 20:49, Peng Wang wrote:
> ext4_overwrite_io() decides whether a direct I/O write is an overwrite
> (all target blocks already allocated) so the write can proceed under a
> shared inode lock.  It calls ext4_map_blocks() once and returns false
> if the mapped length is shorter than the requested length.
>
> ext4_map_blocks() maps at most one extent per call.  When a write
> straddles two extents (e.g. a written extent and an adjacent unwritten
> extent created by fallocate), the single call returns only the first
> extent's length.  ext4_overwrite_io() then mis-classifies the write as
> non-overwrite and forces the caller to cycle i_rwsem from shared to
> exclusive.

For the aligned case, the overwrite check can now be skipped entirely.

For non-aligned cases, you can optimistically hold the read lock and then
use the IOMAP_DIO_OVERWRITE_ONLY flag to upgrade to a write lock if needed.