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:

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On Fri, 13 Oct 2017, Michal Hocko wrote:

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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.

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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

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