Jemalloc 5.4.0 Released with Portability Improvements and Bug Fixes

Technical Debt and Portability Focus

Jemalloc 5.4.0 includes over 160 commits, primarily addressing technical debt through refactorings, bug fixes, and improved test coverage. The release also enhances portability, resolving issues reported upstream.

New Features Introduced

New features include EXTENT_ALLOC_FLAG_PINNED, allowing custom extent-allocation hooks to mark non-reclaimable mappings like HugeTLB pages for preferential reuse outside the decay and purge pipeline. Corresponding mallctl interfaces have been added to report pinned-memory usage and mutex statistics. The update also enables resuming per-CPU arena selection via `thread.arena` and improves alignment for human-readable and JSON malloc statistics. The `experimental_infallible_new` option has been replaced with a compile-time equivalent, `--enable-cxx-infallible-new`, which enables compiler-level optimizations and fixes the `new(std::nothrow)` contract.

Incompatible Changes to Tcache Management

The release introduces incompatible changes by adapting tcache fill and retention targets based on observed demand between GC events, replacing the previous fixed refill/flush policy. This change removes seven legacy non-experimental controls: `lg_tcache_nslots_mul`, `tcache_nslots_small_min`, `tcache_nslots_small_max`, `tcache_nslots_large`, `tcache_gc_delay_bytes`, `lg_tcache_flush_small_div`, and `lg_tcache_flush_large_div`. Corresponding `malloc_conf` settings are now silently ignored, and `opt.*` mallctls return `ENOENT`.

Key Bug Fixes

Several bugs have been resolved, including preserving `errno` across `free`, `free_sized`, `free_aligned_sized`, and `process_madvise`-based page purging. Numeric overflow checks in size classes have been fixed, and `NULL` is now accepted in `free_sized()` and `free_aligned_sized()` for C23 correctness. TSD lifecycle edge cases were addressed by initializing thread-cache bins before enabling the cache and avoiding TSD recreation for late deallocations. The release also fixes the use of `O_CLOEXEC` when opening the THP sysfs file and resolves duplicate `opt.stats_print` and `opt.stats_print_opts` fields.