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

On 10/17/2017 04:20 PM, 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. 

You can already use THP, right? madvise(MADV_HUGEPAGE) increases your
chances to get the huge pages. Then you can mlock() them if you want.
And you get the TLB benefits. There's no guarantee of course, but you
shouldn't require a guarantee for MMAP_CONTIG anyway, because it's for
performance reasons, not functionality. So either MMAP_CONTIG would have
to fallback itself, or the userspace caller. Or would your scenario
rather fail than perform suboptimally?

Guy

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