[chenyan2002]

Page size 1 memory is not supported with -fPIC and in the native dlmalloc. There are two different code paths depending on the target:

• For wasip2, the default allocator is the wasi-libc's malloc (dlmalloc). Rust's allocator calls malloc/free. In wasi-libc, sbrk is the only code that touches memory.grow and memory.size. We use our custom crate libc-alloc to replace sbrk with a version that grows by bytes. Everything else stays stock.
  • We tried to use talc as the Rust #[global_allocator], with a malloc/free shim on top of it. But it's not enough. cabi_realloc comes from wasip2 and allocates canonical ABI buffers through __rust_alloc. wasi-libc frees some of those buffers with free(), but two allocators disagree about the block layout, so free trapped with an out-of-bounds access.
  • We need one allocator shared by Rust, C and cabi_realloc. Rust's system allocator on wasi already calls malloc. So the easiest fix is to keep wasi-libc's malloc and fixing only sbrk.
• For wasm-unknown-unknown, there is no wasi-libc. The default allocator is std's own dlmalloc. The built-in memory source hardcodes 64K pages.
  • We create a custom source OneBytePage in assets/one_byte_page_alloc.rs. Its alloc(size) call memorry.grow(size). page_size() still reports 64K, which dlmalloc only uses for rounding.
  • We could also use talc as the global allocator, but it either requires a fixed-size arena, or the custom memory source has to grow memory by enough to cover talc's private per-heap overhead.