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<1617854972.35808055.1574323227395.JavaMail.zimbra@redhat.com> In-Reply-To: <1617854972.35808055.1574323227395.JavaMail.zimbra@redhat.com> From: Dan Williams Date: Thu, 21 Nov 2019 08:09:02 -0800 Message-ID: Subject: Re: [PATCH] virtio pmem: fix async flush ordering To: Pankaj Gupta Message-ID-Hash: YP2GML3G3EIZK3A5KALGP4GNZD5HLPOF X-Message-ID-Hash: YP2GML3G3EIZK3A5KALGP4GNZD5HLPOF X-MailFrom: dan.j.williams@intel.com X-Mailman-Rule-Misses: dmarc-mitigation; no-senders; approved; emergency; loop; banned-address; member-moderation; nonmember-moderation; administrivia; implicit-dest; max-recipients; max-size; news-moderation; no-subject; suspicious-header CC: linux-nvdimm , Linux Kernel Mailing List , Linux ACPI , "Rafael J. Wysocki" , Len Brown X-Mailman-Version: 3.1.1 Precedence: list List-Id: "Linux-nvdimm developer list." Archived-At: List-Archive: List-Help: List-Post: List-Subscribe: List-Unsubscribe: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit On Thu, Nov 21, 2019 at 12:00 AM Pankaj Gupta 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 > > > > > Signed-off-by: Pankaj Gupta > > > > > > > > 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. _______________________________________________ Linux-nvdimm mailing list -- linux-nvdimm@lists.01.org To unsubscribe send an email to linux-nvdimm-leave@lists.01.org