PAGE_CACHE_* macros were introduced long time ago in hope to implement
page cache with larger chunks than one page in future.
In fact it was never done.
Some code paths assume PAGE_CACHE_SIZE <= PAGE_SIZE. E.g. we use
zero_user_segments() to clear stale parts of page on cache filling, but
the function is implemented only for individual small page.
It's unlikely that global switch to PAGE_CACHE_SIZE > PAGE_SIZE will never
happen since it will affect to much code at once.
I think support of larger chunks in page cache can be in implemented in
some form of THP with per-fs enabling.
Is it time to get rid of PAGE_CACHE_* macros?
I can prepare patchset if it's okay.