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:

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On Fri 13-10-17 10:20:06, Cristopher Lameter wrote:

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On Fri, 13 Oct 2017, Michal Hocko wrote:

[...]

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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.

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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

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