get_user_pages(), pinned pages, and DAX

I think anything done with leases has to interact with GUP - so the lease is not fully released until any GUP on the constituent DAX pages of the block device is also released. Otherwise we have user-triggered data corruption problems if the lease goes away while DMA is in progress, and real apps could unknowingly hit these corner cases..

ie without a linkage process exit might mis-order lease release and DMA fence in the kernel creating data corruption races!

IMHO this brings us right back to the start of the discussion where the FS is blocked on progress because of GUP - just the blocked progress is lease revoke now.

Dan's long-ago original idea of having the GUP caller provide an in-kernel revoke GUP callback is, IMHO, the only way to make userspace leases work. Keep revoke in the kernel.

The problem with Dan's idea is that we couldn't find any way to actually implement revoke in vfio and rdma that wasn't also data corrupting. :(

RDMA *might* have some hope here, at least for some cases, on a driver by driver basis, but it requires someone to convince the popular driver vendors to implement a special rereg MR verb.

Which is where I disagree with DaveC's assessment - IMHO, the real disagreement is that the two good solutions requires either XFS or RDMA to do something *really hard*.

Boaz was right, the FS could solve this by orphaning GUP'd pages, Dan said this was very very hard for XFS.

Dan is right, RDMA could solve this by supporting ODP or MR revoke on all hardware, however this requires new standards and HW.