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

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Date:   Mon, 16 Oct 2017 10:18:04 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Pavel Machek <pavel@....cz>
Cc:     Christopher Lameter <cl@...ux.com>,
        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 Sun 15-10-17 08:58:56, Pavel Machek wrote:
> Hi!
> 
> > 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.
> 
> So you'd suggest using ioctl() for allocating memory?

Why not using standard mmap on the device fd?
 
> That sounds quite ugly to me... mmap(MAP_CONTIG) is not nice, either, but better than
> each driver inventing custom interface...

As already pointed out elsewhere, I do not really see a different to
remap_pfn_range from the API point of view. A driver has some
requirements to the memory so those can be reflected in the mmap
implementation for the driver. I really do not see how that would be a
general interface without a lot of headache in future. Contiguous memory
is a hard requirement to guarantee or give out without risks.

-- 
Michal Hocko
SUSE Labs