From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christopher Lameter <cl@linux.com>
Subject: Re: [RFC PATCH 3/3] mm/map_contig: Add mmap(MAP_CONTIG) support
Date: Fri, 13 Oct 2017 10:42:37 -0500 (CDT)
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To: Michal Hocko <mhocko@kernel.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-api@vger.kernel.org, Marek Szyprowski <m.szyprowski@samsung.com>, Michal Nazarewicz <mina86@mina86.com>, "Aneesh Kumar K . V" <aneesh.kumar@linux.vnet.ibm.com>, Joonsoo Kim <iamjoonsoo.kim@lge.com>, Guy Shattah <sguy@mellanox.com>, Anshuman Khandual <khandual@linux.vnet.ibm.com>, Laura Abbott <labbott@redhat.com>, Vlastimil Babka <vbabka@suse.cz>
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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.