Re: [RFC] mmap(MAP_CONTIG)

On Thu, 5 Oct 2017, Vlastimil Babka wrote:

On 10/04/2017 01:56 AM, Mike Kravetz wrote:

quoted

At Plumbers this year, Guy Shattah and Christoph Lameter gave a presentation
titled 'User space contiguous memory allocation for DMA' [1].  The slides

Hm I didn't find slides on that link, are they available?

I just added Guy's slides to the entry.

As Michal N. noted, the drivers might have different requirements. Is
contiguity (without extra requirements) so common that it would benefit
from a userspace API change?

Yes.

Also how are the driver-specific allocations done today? mmap() on the
driver's device? Maybe we could provide some in-kernel API/library to
make them less "ad-hoc". Conversion to MAP_ANONYMOUS would at first seem
like an improvement in that userspace would be able to use a generic
allocation API and all the generic treatment of anonymous pages (LRU
aging, reclaim, migration etc), but the restrictions you listed below
eliminate most of that?
(It's likely that I just don't have enough info about how it works today
so it's difficult to judge)

Contemporary devices typically can address all of memory. Moreover the
device used actually can trigger faults to page in 4k pages if they are
not present (ODP in RDMA layer). There is no need for driver specific
allocation in those drivers.