See src/snmalloc/README.md for an explanation of the layers.
Some other cleanups on the way:
Fine-grained stats support is now gone.
It's been broken for two years, it depends on iostream (which then
causes linker failures with libstdc++) and it's collecting the wrong
stats for the new design. After discussion with @mjp41, it's better to
remove it and introduce new stats support later, rather than keep broken
code in the main branch.
Tracing was controlled with a preprocessor macro, now there's also a
CMake option.
These encapsulate the wildly powerful reinterpret_cast<> operator where one side
is a uintptr_t and the other is a native pointer. In both cases we require the
pointer type to be explicitly given.
# 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.
- Make GlobalPagemapTemplate and ExternalGlobalPagemap generic in the type of
the pagemap they're encapsulating.
We're going to want to use these for other kinds of pagemaps in the near
future.
- Rename snmalloc_pagemap_global_get to snmalloc_chunkmap_global_get.
- Rename GlobalPagemap to GlobalChunkmap.
