[Libguestfs] [PATCH libnbd v3 0/3] copy: Preserve the host page cache.

This is a combination and evolution of two previous series: https://listman.redhat.com/archives/libguestfs/2021-February/msg00091.html https://listman.redhat.com/archives/libguestfs/2021-February/msg00094.html I made a few enhancements: - Separate macros for the platforms where we enhance reading (#ifdef PAGE_CACHE_MAPPING), vs writing (#ifdef EVICT_WRITES). This was mainly about renaming stuff. - When writing we now use 8 windows instead of 2 [both per thread]. This reduces the overhead from about 25% to about 10%. It's possible to reduce it further by using more windows, but you trade the amount of cruft added to the page cache. - This patch series is much more well-tested than before, including combining it with TLS, and testing it from the nbdkit test suite. Rich.