A one-line fix coming in Linux 7.2 restores Btrfs Direct I/O (DIO) write parallelism that was accidentally broken in 2023 during the migration to the new kernel mount API. The bug caused DIO writes to the same file to be serialized by always acquiring an exclusive inode lock, due to the SB_NOSEC flag not being set. Meta engineer Mark Harmstone identified the regression, and the fix yields roughly 59% throughput improvement (826 MB/s to ~1311 MB/s) on his hardware, with Intel's Kernel Test Robot also confirming ~12% gains. The patch is in the '-next' tree and expected to land in the Linux 7.2 merge window in June.
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