Re: [PATCH v2 1/4] mm: introduce get_user_pages_longterm

[Andrew Morton]

On Tue, 14 Nov 2017 11:56:34 -0800 Dan Williams <dan.j.williams@xxxxxxxxx> wrote:

> Until there is a solution to the dma-to-dax vs truncate problem it is
> not safe to allow long standing memory registrations against
> filesytem-dax vmas. Device-dax vmas do not have this problem and are
> explicitly allowed.
> 
> This is temporary until a "memory registration with layout-lease"
> mechanism can be implemented for the affected sub-systems (RDMA and
> V4L2).

Sounds like that will be unpleasant.  Do we really need it to be that
complex?  Can we get away with simply failing the get_user_pages()
request?  Or are there significant usecases for RDMA and V4L to play
with DAX memory?