Re: [LSF/MM TOPIC] Filesystem-DAX, page-pinning, and RDMA

The get_user_pages_longterm() api was recently added as a stop-gap measure to prevent applications from growing dependencies on the ability to to pin DAX-mapped filesystem blocks for RDMA indefinitely with no ongoing coordination with the filesystem. This 'longterm' pinning is also problematic for the non-DAX VMA case where the core-mm needs a time bounded way to revoke a pin and manipulate the physical pages. While existing RDMA applications have already grown the assumption that they can pin page-cache pages indefinitely, the fact that we are breaking this assumption for filesystem-dax presents an opportunity to deprecate the 'indefinite pin' mechanisms and move to a general interface that supports pin revocation. While RDMA may grow an explicit Infiniband-verb for this 'memory registration with lease' semantic, it seems that this problem is bigger than just RDMA. At LSF/MM it would be useful to have a discussion between fs, mm, dax, and RDMA folks about addressing this problem at the core level. Particular people that would be useful to have in attendance are Michal Hocko, Christoph Hellwig, and Jason Gunthorpe (cc'd).