On Thu, Nov 21, 2019 at 8:38 PM Pankaj Gupta <pagupta@xxxxxxxxxx> wrote: > > > > > > > > > > > > > > > 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? > > > > > > I we are already inside parent bio "make_request" function and we create > > > child > > > bio. How we exactly will swap the parent/child relationship for PREFLUSH > > > case? > > > > > > Child bio is queued after parent bio completes. > > > > Sorry, I didn't quite mean with bio_split, but issuing another request > > in front of the real bio. See md_flush_request() for inspiration. > > o.k. Thank you. Will try to post patch today to be considered for 5.4. > I think it is too late for v5.4-final, but we can get it in the -stable queue. Let's take the time to do it right and get some testing on it.
