# 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.
* Sparc support proposal.
* Tweaks from feedback.
Shift to integer/pointers cast like other archs.
Assembly reworks.
Note apparently reading the register once is sufficient to provoke
a pause in the cpu adding a clobber tough.
The POSIX PAL and in some configurations the Windows PAL overallocate
address space. If address space has become exhausted then this can lead
to issues, where the PAL fails, even though there is enough aligned
address space to satisfy the underlying request.
This commit tries increasingly smaller overallocation sizes in an
attempt to succeed in more cases.
* Reduce code duplication.
The Apple and Haiku PALs both had *almost* identical code to the POSIX
PAL, differing only in some small argument variables. This is fragile
and easy to accidentally get out of sync. For example, the changes to
`reserve_at_least` involved copying identical code into multiple PALs
and it's easy to accidentally miss one.
This change introduces two optional fields on POSIX-derived PALs:
- `default_mmap_flags` allows a PAL to provide additional `MAP_*`
flags to all `mmap` calls.
- `anonymous_memory_fd` allows the PAL to override the default file
descriptor used for memory mappings.
If a PAL does not provide these, default values are used.
Fixes#219
This change brings in a new approach to managing address space.
It wraps the Pal with a power of two reservation system, that
guarantees all returned blocks are naturally aligned to their size. It
either lets the Pal perform aligned requests, or over allocates and
splits into power of two blocks.
* Defensive code for alloc/dealloc during TLS teardown
If an allocation or deallocation occurs during TLS teardown, then it is
possible for a new allocator to be created and then this is leaked. On
the mimalloc-bench mstressN benchmark this was observed leading to a
large memory leak.
This fix, detects if we are in the TLS teardown phase, and if so,
the calls to alloc or dealloc must return the allocator once they have
perform the specific operation.
Uses a separate variable to represent if a thread_local's destructor has
run already. This is used to detect thread teardown to put the
allocator into a special slow path to avoid leaks.
* Added some printing first operation to track progress
* Improve error messages on posix
Flush errors, print assert details, and present stack traces.
* Detect incorrect use of pool.
* Clang format.
* Replace broken LL/SC implementation
LL/SC implementation was broken, this replaces it with
a locking implementation. Changes the API to support LL/SC
for future implementation on ARM.
* Improve TLS teardown.
* Make std::function fully inlined.
* Factor out PALLinux stack trace.
* Add checks for leaking allocators.
* Add release build of Windows Clang
The PAL API previously allowed for returning more memory than asked for.
This was when the PAL performed the alignment work, now this is done in
large alloc, so removing from the PAL.
Pull out a generic POSIX PAL as a superclass for the Linux and generic
BSD PALs. Now we have FreeBSD and Linux adding OS-specific behaviour,
OpenBSD as a named subclass of the generic BSD PAL that doesn't add any
behaviour.
I believe a NetBSD PAL should now be identical to the OpenBSD one -
patches welcome if anyone wants to test one!
