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

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Date:   Sun, 15 Oct 2017 10:50:29 +0300
From:   Guy Shattah <sguy@...lanox.com>
To:     Michal Hocko <mhocko@...nel.org>,
        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>,
        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 13/10/2017 19:17, Michal Hocko wrote:
> On Fri 13-10-17 10:56:13, Cristopher Lameter wrote:
>> On Fri, 13 Oct 2017, Michal Hocko wrote:
>>
>>>> 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.
>> It does do that? In what way?
> availability of the virtual address space depends on the availability of
> the same sized contiguous physical memory range. That sounds like the
> abstraction is gone to large part to me.
In what way? userspace users will still be working with virtual memory.

>
>>>> 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.
>> And then in all the other use cases as well. It would be much easier if
>> mmap could give you the memory you need instead of havig numerous drivers
>> improvise on their own. This is in particular also useful
>> for numerous embedded use cases where you need contiguous memory.
> But a generic implementation would have to deal with many issues as
> already mentioned. If you make this driver specific you can have access
> control based on fd etc... I really fail to see how this is any
> different from remap_pfn_range.
Why have several driver specific implementation if you can generalize 
the idea and implement
an already existing POSIX standard?
--
Guy