On Thu, Jun 28, 2018 at 05:42:34PM +0000, Kani, Toshi wrote: > On Tue, 2018-06-26 at 16:04 -0600, Ross Zwisler wrote: > > On Tue, Jun 26, 2018 at 02:51:52PM -0700, Dan Williams wrote: > > > On Tue, Jun 26, 2018 at 2:31 PM, Kani, Toshi <toshi.kani@xxxxxxx> wrote: > > > > On Tue, 2018-06-26 at 14:28 -0700, Dan Williams wrote: > > > > > On Tue, Jun 26, 2018 at 2:23 PM, Kani, Toshi <toshi.kani@xxxxxxx> wrote: > > > > > > On Tue, 2018-06-26 at 14:02 -0700, Dan Williams wrote: > > > > > > > On Tue, Jun 26, 2018 at 1:54 PM, Kani, Toshi <toshi.kani@xxxxxxx> wrote: > > > > > > > > > > [..] > > > > > > > > When this dm change was made, the pmem driver supported DAX for both raw > > > > > > > > and memory modes (note: sector mode does not use the pmem driver). I > > > > > > > > think the issue was introduced when we dropped DAX support from raw > > > > > > > > mode. > > > > > > > > > > > > > > Still DAX with raw mode never really worked any way. It was also > > > > > > > something that was broken from day one. So what happens to someone who > > > > > > > happened to avoid all the problems with page-less DAX and enabled > > > > > > > device-mapper on top? That failure mode detail needs to be added to > > > > > > > this changelog if we want to propose this for -stable. > > > > > > > > > > > > My point is that the behavior should be consistent between pmem and > > > > > > device-mapper. When -o dax succeeds on a pmem, then it should succeed > > > > > > on a device-mapper on top of that pmem. > > > > > > > > > > > > Has the drop of dax support from raw mode made to -stable back to the > > > > > > baseline accepted 545ed20e6df6? It will introduce inconsistency, > > > > > > otherwise. > > > > > > > > > > That commit, 569d0365f571 "dax: require 'struct page' by default for > > > > > filesystem dax", has not been tagged for -stable. > > > > > > > > Then, Fixes tag should be set to 569d0365f571 to keep the behavior > > > > consistent. > > > > > > Sure, and the failure mode is...? I'm thinking the commit log should say: > > > > > > "Starting with commit 569d0365f571 "dax: require 'struct page' by > > > default for filesystem dax", dax is no longer supported for page-less > > > configurations. However, device-mapper sees the QUEUE_FLAG_DAX still > > > being set and falsely assumes that DAX is enabled, this leads to > > > <insert user visible failure mode details here>" > > > > Dan is correct that there is no user visible change for this. It is the right > > thing to do for consistency and sanity, but it doesn't actually have user > > visible behavior that needs to be backported to stable. > > > > Toshi is correct that this change is only for raw mode namespaces, not btt > > namespaces. > > > > I'll adjust the changelog and remove the stable flag for v5, and I'll add a > > Fixes: tag for patch 2. > > Hi Ross, > > Your patches look good. But I am still not clear about the Fixes & > stable handling. Talking about user visible behavior, I do not think we > had any issue until dax support was dropped from raw mode. Until then, > the pmem driver supported dax for all modes, and the check for > direct_access worked. I agree that the fsdax + raw mode failure mode I mentioned in my cover letter only started when we restricted filesystem DAX to having struct page, but I think that the other failure mode, fsdax + some random block driver (I used brd) was present in DM from the beginning. In any case, I think both are fixed with the patches, and I think it's fine that all 3 get thrown at stable. Thanks, Mike, for the help.
