linux-kernel - Re: [RFC PATCH 3/3] mm/map_contig: Add mmap(MAP

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Date:   Fri, 13 Oct 2017 17:47:47 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Christopher Lameter <cl@...ux.com>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Michal Nazarewicz <mina86@...a86.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Guy Shattah <sguy@...lanox.com>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        Laura Abbott <labbott@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [RFC PATCH 3/3] mm/map_contig: Add mmap(MAP_CONTIG) support

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