Andreas Kling c624d3875e Kernel: Use a shared physical page for zero-filled pages until written ...

This patch adds a globally shared zero-filled PhysicalPage that will
be mapped into every slot of every zero-filled AnonymousVMObject until
that page is written to, achieving CoW-like zero-filled pages.

Initial testing show that this doesn't actually achieve any sharing yet
but it seems like a good design regardless, since it may reduce the
number of page faults taken by programs.

If you look at the refcount of MM.shared_zero_page() it will have quite
a high refcount, but that's just because everything maps it everywhere.
If you want to see the "real" refcount, you can build with the
MAP_SHARED_ZERO_PAGE_LAZILY flag, and we'll defer mapping of the shared
zero page until the first NP read fault.

I've left this behavior behind a flag for future testing of this code.

2020-02-15 13:17:40 +01:00

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