[$] Sharing memory for shared file extents

On the second day of the2022 Linux Storage,Filesystem, Memory-management and BPF Summit (LSFMM), Goldwyn Rodriguesled a combined filesystem and memory-management session on saving memory whenreading files that share extents. That kind of sharing can occur withcopy-on-write (COW) filesystems, reflinks, snapshots, and other featuresof that sort. When reading those files, memory is wasted because multiplecopies of the same data is stored in the page cache, so he wanted toexplore adding a cachespecifically to handle that.