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

[Guy Shattah]

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