[$] Improving support for large, contiguous allocations

Allocating chunks of memory that are both large and physically contiguoushas long been a difficult thing to do in the kernel. But there are times wherethere is no alternative. Two sessions in the memory-management track ofthe 2018 Linux Storage, Filesystem, and Memory-Management Summit exploredways of making those allocations more reliable. It turns out that some usecases have a rather larger value of "large" than others.