[$] Minimizing the use of tail pages

Compound pages are created by the kernel asa way of combining a number of small pages into a single, larger unit.Such pages are implemented as a single "head page" at the beginning,followed by a number of "tail pages". Matthew Wilcox has concluded thatit would be beneficial to minimize the use of tail pages in the kernel; heran a session during the memory-management track at the 2019 Linux Storage,Filesystem, and Memory-Management Summit to explore how that could be done.The discussion ranged widely, veering into the representation of DMA I/Ooperations, but few hard conclusions were reached.