On Wed, Nov 20, 2019 at 11:23 PM Dan Williams <dan.j.williams@xxxxxxxxx> wrote: > > 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? If not I > think a synchronous wait is going to be a significant performance > regression. Are there any numbers to accompany this change? Why not just swap the parent child relationship in the PREFLUSH case?
