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

[Jeff Moyer]

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