@dragon all of these work by moving around references to pages in kernel memory and if they were ever a valid optimization it was predicated upon performance considerations from back in the 90s that made copies really slow. i found this page to be very helpful regarding the concept of the page cache https://blog.pr4tt.com/2016/02/02/BSD-virtual-memory/ and particularly this dissertation on the uvm virtual memory system describes it thusly http://chuck.cranor.org/p/diss.pdf

One major problem with traditional operating systems is that they require unnecessary data copies. Data copies are expensive for a number of reasons. First, the bandwidth of main memory is limited. Each copy made of the data is effected by this. Second, to copy data, the CPU must move it word-by-word from the source buffer to the destination. While the CPU is busy moving data it is unavailable to the user application. Finally, data copying often flushes useful information out of the cache. since the CPU accesses main memory through the cache, the process of copying data fills the cache with the data being copied — displacing potentially useful data that was resident in the cache before the copy.