On Mon, 2015-04-27 at 11:48 -0500, Christoph Lameter wrote: > On Mon, 27 Apr 2015, Rik van Riel wrote: > > > Why would we want to avoid the sane approach that makes this thing > > work with the fewest required changes to core code? > > Becaus new ZONEs are a pretty invasive change to the memory management and > because there are other ways to handle references to device specific > memory. ZONEs is just one option we put on the table. I think we can mostly agree on the fundamentals that a good model of such a co-processor is a NUMA node, possibly with a higher distance than other nodes (but even that can be debated). That gives us a lot of the basics we need such as struct page, ability to use existing migration infrastructure, and is actually a reasonably representation at high level as well. The question is how do we additionally get the random stuff we don't care about out of the way. The large distance will not help that much under memory pressure for example. Covering the entire device memory with a CMA goes a long way toward that goal. It will avoid your ordinary kernel allocations. It also provides just what we need to be able to do large contiguous "explicit" allocations for use by workloads that don't want the transparent migration and by the driver for the device which might also need such special allocations for its own internal management data structures. We still have the risk of pages in the CMA being pinned by something like gup however, that's where the ZONE idea comes in, to ensure the various kernel allocators will *never* allocate in that zone unless explicitly specified, but that could possibly implemented differently. Maybe a concept of "exclusive" NUMA node, where allocations never fallback to that node unless explicitly asked to go there. Of course that would have an impact on memory pressure calculations, nothign comes completely for free, but at this stage, this is the goal of this thread, ie, to swap ideas around and see what's most likely to work in the long run before we even start implementing something. Cheers, Ben. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>