On Mon, Mar 25, 2019 at 01:24:48PM -0400, Brian Foster wrote: > XFS applies more strict serialization constraints to unaligned > direct writes to accommodate things like direct I/O layer zeroing, > unwritten extent conversion, etc. Unaligned submissions acquire the > exclusive iolock and wait for in-flight dio to complete to ensure > multiple submissions do not race on the same block and cause data > corruption. > > This generally works in the case of an aligned dio followed by an > unaligned dio, but the serialization is lost if I/Os occur in the > opposite order. If an unaligned write is submitted first and > immediately followed by an overlapping, aligned write, the latter > submits without the typical unaligned serialization barriers because > there is no indication of an unaligned dio still in-flight. This can > lead to unpredictable results. > > To provide proper unaligned dio serialization, require that such > direct writes are always the only dio allowed in-flight at one time > for a particular inode. We already acquire the exclusive iolock and > drain pending dio before submitting the unaligned dio. Wait once > more after the dio submission to hold the iolock across the I/O and > prevent further submissions until the unaligned I/O completes. This > is heavy handed, but consistent with the current pre-submission > serialization for unaligned direct writes. > > Signed-off-by: Brian Foster <bfoster@xxxxxxxxxx> > Reviewed-by: Allison Henderson <allison.henderson@xxxxxxxxxx> looks good. Reviewed-by: Dave Chinner <dchinner@xxxxxxxxxx> -- Dave Chinner david@xxxxxxxxxxxxx
