On Wed, Feb 06, 2019 at 04:22:16PM -0800, Dan Williams wrote: > On Wed, Feb 6, 2019 at 3:41 PM Jason Gunthorpe <j...@ziepe.ca> wrote: > [..] > > > You're describing the current situation, i.e. Linux already implements > > > this, it's called Device-DAX and some users of RDMA find it > > > insufficient. The choices are to continue to tell them "no", or say > > > "yes, but you need to submit to lease coordination". > > > > Device-DAX is not what I'm imagining when I say XFS--. > > > > I mean more like XFS with all features that require rellocation of > > blocks disabled. > > > > Forbidding hold punch, reflink, cow, etc, doesn't devolve back to > > device-dax. > > True, not all the way, but the distinction loses significance as you > lose fs features. > > Filesystems mark DAX functionality experimental [1] precisely because > it forbids otherwise typical operations that work in the nominal page > cache case. An approach that says "lets cement the list of things a > filesystem or a core-memory-mangement facility can't do because RDMA > finds it awkward" is bad precedent.
I'm not saying these rules should apply globaly. I'm suggesting you could have a FS that supports gup_longterm by design, and a FS that doesn't. And that is OK. They can have different rules. Obviously the golden case here is to use ODP (which doesn't call gup_longterm at all) - that works for both. Supporting non-ODP is a trade off case - users that want to run on limited HW must accept limited functionality. Limited functionality is better than no-funtionality. Linux has many of these user-choose tradeoffs. This is how it supports such a wide range of HW capabilities. Not all HW can do all things. Some features really do need HW support. It has always been that way. Jason
