Before this change, we were hard-coding 4 KiB. This meant that systems
with a 16 KiB native page size were wasting 12 KiB per HeapBlock on
nothing, leading to worse locality and more mmap/madvise churn.
We now query the system page size on startup and use that as the
HeapBlock size.
The only downside here is that some of the pointer math for finding the
base of a HeapBlock now has to use a runtime computed value instead of a
compile time constant. But that's a small price to pay for what we get.
NetBSD and FreeBSD get upset when we don't set the fd to an invalid
value when using a non-shared mapping.
Reported-By: Thomas Klausner <wiz@gatalith.at>
Instead of returning HeapBlock memory to the kernel (or a non-type
specific shared cache), we now keep a BlockAllocator per CellAllocator
and implement "deallocation" by basically informing the kernel that we
don't need the physical memory right now.
This is done with MADV_FREE or MADV_DONTNEED if available, but for other
platforms (including SerenityOS) we munmap and then re-mmap the memory
to achieve the same effect. It's definitely clunky, so I've added a
FIXME about implementing the madvise options on SerenityOS too.
The important outcome of this change is that GC types that use a
type-specific allocator become immune to use-after-free type confusion
attacks, since their virtual addresses will only ever be re-used for
the same exact type again and again.
Fixes#22274
This is the coarsest grained ASAN instrumentation possible for the LibJS
heap. Future instrumentation could add red-zones to heap block
allocations, and poison the entire heap block and only un-poison used
cells at the CellAllocator level.
The previous VERIFY() call checked that aligned_alloc() didn't return
MAP_FAILED. When out of memory aligned_alloc() returns a null pointer
so let's check for that instead.
This patch adds a BlockAllocator to the GC heap where we now cache up to
64 HeapBlock-sized mmap's that get recycled when allocating HeapBlocks.
This improves test-js runtime performance by ~35%, pretty cool! :^)
