# Small changes before rewrite
* Additional bit in remote allocator to prevent type confusion with the backend.
* Move Chunk allocator to backend.
* Improvements to RedBlack tree
* Expose message from Pal
# Complete backend rewrite
This provides two key changes:
* We use buddy allocators to allow memory to reconsolidated
* The backend is factored into a series of small operations that
allocate and deallocate memory.
The backend now uses "Ranges", there are two ranges that don't require a
parent range:
* EmptyRange - Never returns any memory
* PalRange - Returns memory from the platform.
All other ranges require a parent range to supply memory to them. Some
ranges support both allocation and deallocation, and some just
deallocation. For instance, CommitRange supports both, and maps
requests to the parent range, but will Commit and Decommit the memory.
As the ranges perform only a single task, they are generally small and
easy to follow. The two exceptions to this are the two BuddyRanges
(Large and Small). Large is for CHUNK_SIZE and above blocks, while
Small is for below CHUNK_SIZE blocks. Both are implemented with a buddy
allocator, but the SmallBuddyRange uses in place meta-data, while the
LargeBuddyRange uses the pagemap for its meta-data. This means the
LargeBuddyRange can keep the majority of memory it is managing
decommitted.
The Backend glues together the various ranges to support the appropriate
way to manage memory on the platform.
This is the set of changes required for snmalloc2 to be usable by the
process sandboxing code and incorporates some API changes that reduce
the amount of code required to embed snmalloc. Highlights:
- Merge the config and back-end classes.
- Everything in config is now global (all methods are static)
- The GlobalState class is gone (all global state is managed by global
methods on the config class)
- LocalState is now a member of the config class, all methods are
instance methods.
- Not every configuration needs to use the lazy initialisation hooks.
They now need to be provided only if they are used. If the
configuration does not provide an `ensure_init` method, it is not
called. If it does not provide an `is_initialised` method then the
global initialisation state is not checked.
- There is now an `snmalloc::Options` class that default initialises
itself to the default behaviour. Every configuration must provide a
`constexpr` instance of this class. Each flag can be separately
overridden and new flags can be added without breaking any existing
API consumers.
The config classes are moved into the backend directory.
# Pagemap
The Pagemap now stores all the meta-data for the object allocation. The meta-data in the pagemap is effectively a triple of the sizeclass, the remote allocator, and a pointer to a 64 byte block of meta-data for this chunk of memory. By storing the pointer to a block, it allows the pagemap to handle multiple slab sizes without branching on the fast path. There is one entry in the pagemap per 16KiB of address space, but by using the same entry in the pagemap for 4 adjacent entries, then we can treat a 64KiB range can be treated as a single slab of allocations.
This change also means there is almost no capability amplification required by the implementation on CHERI for finding meta-data. The only amplification is required, when we change the way a chunk is used to a size of object allocation.
# Backend
There is a second major aspect of the refactor that there is now a narrow API that abstracts the Pagemap, PAL and address space management. This should better enable the compartmentalisation and makes it easier to produce alternative backends for various research directions. This is a template parameter that can be used to specialised by the front-end in different ways.
# Thread local state
The thread local state has been refactored into two components, one (called 'localalloc') that is stored directly in the TLS and is constant initialised, and one that is allocated in the address space (called 'coreallloc') which is lazily created and pooled.
# Difference
This removes Superslabs/Medium slabs as there meta-data is now part of the pagemap.
