Re: [PATCH] virtio pmem: fix async flush ordering

Jeff Moyer
2019-11-22T16:08:58Z

Dan Williams <dan.j.williams@intel.com> writes:

> On Wed, Nov 20, 2019 at 9:26 AM Jeff Moyer <jmoyer@redhat.com> wrote:
>>
>> Pankaj Gupta <pagupta@redhat.com> 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@redhat.com>
>> > Signed-off-by: Pankaj Gupta <pagupta@redhat.com>
>>
>> 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