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

[Michal Hocko]

On Fri 13-10-17 10:42:37, Cristopher Lameter wrote:
> 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.

Yes you wrote that already and my counter argument was that this generic
posix interface shouldn't bypass virtual memory abstraction.

> > > 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.

That doesn't prevent providing a library function which could be reused
by all those drivers. Nothing really too much different from
remap_pfn_range.
-- 
Michal Hocko
SUSE Labs