Misaligned metadata

[wks]

On i686-unknown-linux-gnu, the metadata spec mmtk::util::metadata::side_metadata::spec_defs::MS_FREE (aliased as mmtk::policy::marksweepspace::native_ms::block::Block::FREE_LIST_TABLE) has the offset.rel_offset field holding the value 150081 (0x24a41) which is not 4-byte aligned. As a result, its meta_addr is 0x40024a41 on the DummyVM. It will be an error when loading the free list pointer (usize which is 4 bytes) from that metadata using unaligned pointer. The error passed silently on Rust 1.66.1, but will panic on 1.71.0 because of unaligned pointer access.

If the MS_FREE metadata is intended to store 4-byte elements, the metadata itself should be 4-byte aligned. Since the metadata specs are constants, the bug should be in the computing of the relative offset or the starting address of the metadata.

[wks]

The reason is that the CHUNK_MARK local side metadata occupies only one byte per chunk, and local metadata are allocated in a per-chunk basis. That caused subsequent local side metadata to be misaligned.

I have added a fix in #887