Re: [RFC PATCH 3/3] mm/map_contig: Add mmap(MAP_CONTIG) support

[Christopher Lameter]

On Fri, 13 Oct 2017, Michal Hocko wrote:

> On Fri 13-10-17 10:20:06, Cristopher Lameter wrote:
> > On Fri, 13 Oct 2017, Michal Hocko wrote:
> [...]
> > > I am not really convinced this is a good interface. You are basically
> > > trying to bypass virtual memory abstraction and that is quite
> > > contradicting the mmap API to me.
> >
> > This is a standardized posix interface as described in our presentation at
> > the plumbers conference. See the presentation on contiguous allocations.
>
> Are you trying to desing a generic interface with a very specific and HW
> dependent usecase in mind?

There is a generic posix interface that could we used for a variety of
specific hardware dependent use cases.

> > The contiguous allocations are particularly useful for the RDMA API which
> > allows registering user space memory with devices.
>
> then make those devices expose an implementation of an mmap which does
> that. You would get both a proper access control (via fd), accounting
> and others.

There are numerous RDMA devices that would all need the mmap
implementation. And this covers only the needs of one subsystem. There are
other use cases.