It isn't needed at all. When a cached page is written, it's trying hard to replace the page in the cache by a new copy,

to return the previously cached page. Instead, I copy the cached page and return the copy, which is what it already

does in another instance. In the current version, not yet available (but soon), I do away with the page hash

table entirely, so there's nowhere for the page hash chains to accumulate. Page is smaller and simpler, with

a reference count but no lock.