Mimalloc pages are data structures that contain contiguous allocations of the same block size. Note that they are distinct from operating system pages. Mimalloc pages are contained in segments.

When a thread exits, it abandons any segments and contained pages that have live allocations. These segments and pages may be later reclaimed by another thread. To support GC and certain thread-safety guarantees in free-threaded builds, we want pages to only be reclaimed by the corresponding heap in the claimant thread. For example, we want pages containing GC objects to only be claimed by GC heaps.

This allows heaps and pages to be tagged with an integer tag that is used to ensure that abandoned pages are only claimed by heaps with the same tag. Heaps can be initialized with a tag (0-15); any page allocated by that heap copies the corresponding tag.

• Issue: mimalloc: additional integration and changes for --disable-gil builds #112532