On Wed, Jan 24, 2018 at 8:01 PM, Bart Van Assche [off-list ref] wrote:

On Wed, 2018-01-24 at 19:56 -0800, Dan Williams wrote:

quoted

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).

Is on demand paging sufficient as a solution for your use case...

No, in 3 dimensions since there is a need to support non-ODP RDMA
hardware, hypervisors want to coordinate DMA for guests, and non-RDMA
hardware also pins memory indefinitely like V4L2. So it's bigger than
RDMA, but that will likely be the first consumer of this 'longterm
pin' mechanism.

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