MetaCommon is now gone. The back end must provide a SlabMetadata,
which must be a subtype of MetaSlab (i.e. MetaSlab or a subclass of
MetaSlab). It may add additional state here.
The MetaEntry is now templated on the concrete subclass of MetaSlab that
the back-end uses. The MetaEntry still stores this as a `uintptr_t` to
allow easier toggling of the boundary bit but the interfaces are all in
terms of stable types now.
Also some tidying of names (SharedStateHandle is now called Backend).
In a follow-on PR, we can then remove the chunk field from the
BackendMetadata in the non-CHERI back end and allow back ends that don't
require extra state to use MetaSlab directly.
Other cleanups:
- Remove backend/metatypes, define the types that the front end expects
in mem/metaslab. The back end may extend them but these types define
part of the contract between the front and back ends.
- Remove FrontendMetaEntry and fold its methods into MetaEntry.
- For example purposes, the default back end now extends MetaEntry.
This also ensures that nothing in the front end depends on the
specific type of MetaEntry.
- Some things now have more sensible names.
The meta entry now operates in one of three modes:
- When owned by the front end, it stores a pointer to a remote, a
pointer to some MetaSlab subclass, and a sizeclass.
- When owned by the back end, it stores two back-end defined values
that must fit in the bits of `uintptr_t` that are not reserved for
the MetaEntry itself.
- When not owned by either, it can be queried as if owned by the front
end.
The red-black tree has been refactored to allow the holder to be a
wrapper type, removing all of the Holder* and Holder& uses and treating
it uniformly as a value type that can be used to access the contents.
The chunk field is fone from the slab medatada.
This will need to be added back in the CHERI back ends, but it's a
back-end policy. The back end can choose to use it or not, depending on
whether it can safely convert between an Alloc-bounded pointer and a
Chunk-bounded pointer.
The term 'metaslab' originated in snmalloc 1 to mean a slab of slabs.
In the snmalloc2 branch it was repurposed to mean metadata about a
slab. To make this clearer, all uses of metaslab are now gone and have
been renamed to slab metadata. The frontend metadata classes are all
prefixed Frontend and some extra invariants are checked with
`static_assert`.
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