Dan Williams <dan.j.williams@xxxxxxxxx> writes: > On Wed, Nov 20, 2019 at 9:26 AM Jeff Moyer <jmoyer@xxxxxxxxxx> wrote: >> >> Pankaj Gupta <pagupta@xxxxxxxxxx> writes: >> >> > Remove logic to create child bio in the async flush function which >> > causes child bio to get executed after parent bio 'pmem_make_request' >> > completes. This resulted in wrong ordering of REQ_PREFLUSH with the >> > data write request. >> > >> > Instead we are performing flush from the parent bio to maintain the >> > correct order. Also, returning from function 'pmem_make_request' if >> > REQ_PREFLUSH returns an error. >> > >> > Reported-by: Jeff Moyer <jmoyer@xxxxxxxxxx> >> > Signed-off-by: Pankaj Gupta <pagupta@xxxxxxxxxx> >> >> There's a slight change in behavior for the error path in the >> virtio_pmem driver. Previously, all errors from virtio_pmem_flush were >> converted to -EIO. Now, they are reported as-is. I think this is >> actually an improvement. >> >> I'll also note that the current behavior can result in data corruption, >> so this should be tagged for stable. > > I added that and was about to push this out, but what about the fact > that now the guest will synchronously wait for flushing to occur. The > goal of the child bio was to allow that to be an I/O wait with > overlapping I/O, or at least not blocking the submission thread. Does > the block layer synchronously wait for PREFLUSH requests? You *have* to wait for the preflush to complete before issuing the data write. See the "Explicit cache flushes" section in Documentation/block/writeback_cache_control.rst. -Jeff
