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

On 10/17/2017 07:20 AM, Guy Shattah wrote:

quoted

On Tue, Oct 17 2017, Guy Shattah wrote:

quoted

Are you going to be OK with kernel API which implements contiguous
memory allocation?  Possibly with mmap style?  Many drivers could
utilize it instead of having their own weird and possibly non-standard
way to allocate contiguous memory.  Such API won't be available for
user space.

What you describe sounds like CMA.  It may be far from perfect but it’s there
already and drivers which need contiguous memory can allocate it.

1. CMA has to preconfigured. We're suggesting mechanism that works 'out of the box'
2. Due to the pre-allocation techniques CMA imposes limitation on maximum 
   allocated memory. RDMA users often require 1Gb or more, sometimes more.
3. CMA reserves memory in advance, our suggestion is using existing kernel memory
     mechanisms (THP for example) to allocate memory. 

I would not totally rule out the use of CMA.  I like the way that it reserves
memory, but does not prohibit use by others.  In addition, there can be
device (or purpose) specific reservations.

However, since reservations need to happen quite early it is often done on
the kernel command line.  IMO, this should be avoided if possible.  There
are interfaces for arch specific code to make reservations.  I do not know
the system initialization sequence well enough to know if it would be
possible for driver code to make CMA reservations.  But, it looks doubtful.

-- 
Mike Kravetz

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