> On Thu, May 31, 2018 at 11:26:43AM -0700, Dan Williams wrote: > > On Thu, May 31, 2018 at 10:46 AM, Darrick J. Wong > > <darrick.wong@xxxxxxxxxx> wrote: > > > On Thu, May 31, 2018 at 09:29:15AM -0700, Dan Williams wrote: > > >> On Thu, May 31, 2018 at 8:07 AM, Ross Zwisler > > >> <ross.zwisler@xxxxxxxxxxxxxxx> wrote: > > >> > On Thu, May 31, 2018 at 11:27:33AM +0900, Yasunori Goto wrote: > > >> >> Hello, > > >> >> > > >> >> > > >> >> I would like to know about the Experimental message of Filesystem DAX. > > >> >> -------------------------------------------------------- > > >> >> DAX enabled. Warning: EXPERIMENTAL, use at your own risk > > >> >> -------------------------------------------------------- > > >> >> > > >> >> AFAIK, the final issue of Filesystem DAX is metadata update problem, > > >> >> and it is(will be?) solved by great effort of MAP_SYNC and > > >> >> "fix dma vs truncate/hole-punch" patch set. > > >> >> So, I suppose that the Experimental message can be removed, > > >> >> but I'm not sure. > > >> >> > > >> >> Is it possible? > > >> >> Otherwise, are there any other issues in Filesystem DAX yet? > > >> >> > > >> >> If this is silly question, sorry for noise.... > > >> >> > > >> >> Thanks, > > >> >> --- > > >> >> Yasunori Goto > > >> > > > >> > Adding in the XFS and ext4 developers, as it's really their call when to > > >> > remove this notice. > > >> > > > >> > We've talked about this off and on for a long while, but IMHO we should remove > > >> > the EXPERIMENTAL warning. The last few things that we had on our TODO list > > >> > before this was removed were: > > >> > > > >> > 1) Get consistent handling of the DAX mount option. We currently have this, > > >> > as both filesystems will behave the same and fall back and remove the DAX > > >> > mount option if it is unsupported by the block device, etc. > > > > > > <nod> > > > > > > As an aside, I wonder if Christoph's musings about "just have the kernel > > > determine the appropriate dax/non-dax setting from the acpi tables and > > > skip the inode flag entirely" ever got resolved? > > > > > >> > 2) Get consistent handling of the DAX inode option. We currently have this, > > >> > as all DAX behavior now happens through the mount option. If/when we > > >> > re-enable the per-inode DAX flag we should do it consistently for all DAX > > >> > enabled filesystems. > > > > > > The behavior of the inode flag isn't all that consistent. ext4 doesn't > > > support it at all. On XFS, you can set or clear FS_XFLAG_DAX on a > > > directory which will propagate the setting to any files created in that > > > directory. > > > > > > However, if you set or clear it on a file we update the on-disk inode > > > but we can't change the in-core state flag (S_DAX) until the next > > > in-core inode instantiation. It's weird that users can change the flag > > > but the intended behavior changes won't happen until some ... time ... > > > in the future?? > > > > > >> > 3) Make DAX work with other XFS features like reflink, etc. This one isn't > > >> > done, but we at least disallow DAX with XFS features like reflink where it > > >> > could be an issue. Darrick, do you still feel like we need to get these > > >> > working together to remove EXPERIMENTAL, or are you happy enough that we're > > >> > keeping them separated and that we're keeping user data safe? > > > > > > Yes, reflink and dax still need to work together. I've not heard any > > > good arguments for why page sharing + copy on write are fundamentally > > > incompatible with the dax model, or why dax users will never, ever > > > require reflink. > > > > Right, but that's separate from DAX being scream in your face > > "EXPERIMENTAL!". It's just an additional feature that can be added on > > once all the normal expectations of a userspace mapping work. I think > > reliable rmap is the last of those requirements. > > > > > The recent thread between Jan and Dan make me wonder if making mappings > > > share struct pages is going to be a nightmare to add to the mm code, > > > though... > > > > It's going to be a bit messy because a singular page->mapping > > association is fundamentally incompatible with DAX. Perhaps a linked > > list of mapping "siblings"? > > > > > Also: ideally XFS would also be able to consume poison event > > > notifications from the pmem so that it can try to deal with metadata > > > loss, but that's probably a separate effort. > > > > Right, not a gating item for declaring DAX ready for prime time. > > Yep, I think that the very loud EXPERIMENTAL message is essentially telling > users "your data is at risk if you use this". I totally agree that we still > have lots of work to do. However, I don't think that these feature > enhancements should gate removal of the EXPERIMENTAL notice. IMHO that > should only exist as long as we have issues that we know could corrupt data, > crash the box, etc. As far as I know those are basically the 2 items on Dan's > list from a few mails ago (poison recovery & DMA vs truncate). Everyone, Thank you very much for your information/opinions. Not only about "experimental", I could understand what is still to do. Thanks a lot! --- Yasunori Goto
