It is definitely common during run time. It was seen in practice enough
to be reproducible and get fixed for the non-ramdisk case.
The big underlying question is how which ramdisk usage case are we
shooting for. Keeping the ram disk pages off the LRU can certainly help
the VM if larger ramdisks used at runtime are very common.
Otherwise, I'd say to keep it as simple as possible and use Eric's
patch. By simple I'm not counting lines of code, I'm counting overall
readability between something everyone knows (page cache usage) and
something specific to ramdisks (Nick's patch).