On Fri, Nov 22, 2019 at 8:09 AM Jeff Moyer <jmoyer@xxxxxxxxxx> wrote: > > 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. I'm not debating the ordering, or that the current implementation is obviously broken. I'm questioning whether the bio tagged with PREFLUSH is a barrier for future I/Os. My reading is that it is only a gate for past writes, and it can be queued. I.e. along the lines of md_flush_request().
