# 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.
MSVC has strong opinions on implicit conversions as used in CI, while Clang both
locally and in CI has weaker opinions. In an effort to avoid subsequent
roundtrips through CI, make clang more strict. Adding -Wconversion definitely
increases the strength of clang's opinions, apparently to include frowning on
some that even MSVC considers OK, so go make explicit the current implicit
behavior.
Summary of changes:
- Add a new PAL that doesn't allocate memory, which can be used with a
memory provider that is pre-initialised with a range of memory.
- Add a `NoAllocation` PAL property so that the methods on a PAL that
doesn't support dynamically reserving address space will never be
called and therefore don't need to be implemented.
- Slightly refactor the memory provider class so that it has a narrower
interface with LargeAlloc and is easier to proxy.
- Allow the address space manager and the memory provider to be
initialised with a range of memory.
This may eventually also remove the need for (or, at least, simplify)
the Open Enclave PAL.
This commit also ends up with a few other cleanups:
- The `malloc_useable_size` CMake test that checks whether the
parameter is const qualified was failing on FreeBSD where this
function is declared in `malloc_np.h` but where including
`malloc.h` raises an error. This should now be more robust.
- The BSD aligned PAL inherited from the BSD PAL, which does not
expose aligned allocation. This meant that it exposed both the
aligned and non-aligned allocation interfaces and so happily
accepted incorrect `constexpr` if blocks that expected one or
the other but accidentally required both to exist. The unaligned
function is now deleted so the same failures that appear in CI should
appear locally for anyone using this PAL.
* Remote dealloc refactor.
* Improve remote dealloc
Change remote to count down to 0, so fast path does not need a constant.
Use signed value so that branch does not depend on addition.
* Inline remote_dealloc
The fast path of remote_dealloc is sufficiently compact that it can be
inlined.
* Improve fast path in Slab::alloc
Turn the internal structure into tail calls, to improve fast path.
Should be no algorithmic changes.
* Refactor initialisation to help fast path.
Break lazy initialisation into two functions, so it is easier to codegen
fast paths.
* Minor tidy to statically sized dealloc.
* Refactor semi-slow path for alloc
Make the backup path a bit faster. Only algorithmic change is to delay
checking for first allocation. Otherwise, should be unchanged.
* Test initial operation of a thread
The first operation a new thread takes is special. It results in
allocating an allocator, and swinging it into the TLS. This makes
this a very special path, that is rarely tested. This test generates
a lot of threads to cover the first alloc and dealloc operations.
* Correctly handle reusing get_noncachable
* Fix large alloc stats
Large alloc stats aren't necessarily balanced on a thread, this changes
to tracking individual pushs and pops, rather than the net effect
(with an unsigned value).
* Fix TLS init on large alloc path
* Add Bump ptrs to allocator
Each allocator has a bump ptr for each size class. This is no longer
slab local.
Slabs that haven't been fully allocated no longer need to be in the DLL
for this sizeclass.
* Change to a cycle non-empty list
This change reduces the branching in the case of finding a new free
list. Using a non-empty cyclic list enables branch free add, and a
single branch in remove to detect the empty case.
* Update differences
* Rename first allocation
Use needs initialisation as makes more sense for other scenarios.
* Use a ptrdiff to help with zero init.
* Make GlobalPlaceholder zero init
The GlobalPlaceholder allocator is now a zero init block of memory.
This removes various issues for when things are initialised. It is made read-only
to we detect write to it on some platforms.
This is useful as codegen is nicer if we use size_t, but the semantics
is uint8_t, and is stored as that in many places in the metadata.
Ultimately should introduce a wrapper to check this invariant.
This introduces a new `address_t` type and two new casts: `pointer_cast`
and `address_cast` for casting between an `address_t` and a pointer.
These should make it easier to audit the codebase for casts between
pointers and integers. In particular, the remaining `reinterpret_cast`s
and `pointer_cast`s should be the only places where we could perform
invalid pointer arithmetic.
Also adds a `pointer_offset` helper that adds an offset (in bytes) to a
pointer, preserving its original type. This is a sufficiently common
pattern that it seemed worthwhile to centralise it.
