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Linux ARM64 kernels can be configured with 4KB, 16KB, or 64KB page sizes. Previously, the default CeilingOnPageSize for Linux ARM64 was 16KB, which caused an immediate RELEASE_ASSERT crash on systems with 64KB pages (e.g., RHEL aarch64, Oracle Linux, Ubuntu generic-64k). The existing USE(64KB_PAGE_BLOCK) workaround disables both JIT and bmalloc/libpas, which is a significant performance regression. This change raises CeilingOnPageSize to 64KB for all Linux ARM64 builds, and correspondingly updates libpas page/granule sizes to be at least 64KB on Linux ARM64. This allows Bun and other WebKit embedders to run on 64KB page kernels with full JIT and libpas support. Changes: - WTF/PageBlock.h: Set CeilingOnPageSize=64KB for Linux ARM64 - libpas/pas_internal_config.h: Raise small page (16KB->64KB), small bitfit page (16KB->64KB), medium page (128KB->256KB), and granule size (16KB->64KB) for Linux ARM64 - libpas/jit_heap_config.h: Raise JIT small page/granule (16KB->64KB) and JIT medium page/granule (128KB->256KB / 16KB->64KB) - libpas/pas_expendable_memory.h: Raise expendable memory page size (16KB->64KB) for Linux ARM64 - bmalloc/BPlatform.h: Add BUSE_PRECOMPUTED_CONSTANTS_VMPAGE64K These changes propagate correctly through: - ConfigSizeToProtect (via std::max(CeilingOnPageSize, 16*KB)) - OpcodeConfigSizeToProtect (same pattern) - MarkedBlock::blockSize (via std::max(16*KB, CeilingOnPageSize)) - All libpas heap configs that use PAS_*_DEFAULT_SIZE constants - mprotect/mmap calls that use runtime pageSize() Co-Authored-By: Claude <noreply@anthropic.com>
The previous hardcoded 2048 assumed PAS_SMALL_PAGE_DEFAULT_SIZE=16KB. With 64KB pages, num_alloc_bits = page_size >> min_align_shift can reach 8192 (utility heap, PAS_INTERNAL_MIN_ALIGN_SHIFT=3), causing pas_baseline_allocator_attach_directory to PAS_ASSERT when a thread hits the baseline allocator slow path.
…anule use-count under uint8_t DECOMMITTED sentinel
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WalkthroughUpdated compile-time page-size configurations for Linux ARM64 across WTF and bmalloc subsystems, changing default allocator page sizes from 16 KiB to 64 KiB. Modified PageBlock.h selection logic and added platform-specific constants in multiple configuration headers. Changes
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Summary
Support Linux ARM64 kernels with 64KB pages (RHEL aarch64, Oracle Linux, Ubuntu
generic-64k) without disabling JIT or libpas. Previously these systems hitRELEASE_ASSERTinWTF::pageSize()at startup.Upstream's
USE_64KB_PAGE_BLOCKoption disables JIT and switches to mimalloc — this patch keeps both enabled.Changes
Three commits, the first two rebased from the reverted PR #165:
0e6a06612— raise compile-time page ceilingsCeilingOnPageSize16K → 64K forOS(LINUX) && CPU(ARM64)PAS_EXPENDABLE_MEMORY_PAGE_SIZE: 16K → 64K82cc18638— derivePAS_MAX_OBJECTS_PER_PAGEfrom page sizeWas hardcoded
2048, which assumed 16K pages. With 64K pages the utility heap hasnum_alloc_bits = 64K >> 3 = 8192, overflowing the baseline allocator's bit array when a thread hitspas_baseline_allocator_attach_directoryon the slow path. This is the bug that causedthe #165 revert. Now derived:
PAS_SMALL_PAGE_DEFAULT_SIZE >> PAS_INTERNAL_MIN_ALIGN_SHIFT.d56ecd41— bumpJIT_MEDIUM_BITFIT_MIN_ALIGN_SHIFT8 → 9New bug found by static analysis.
pas_page_granule_use_countisuint8_twith 255 reserved as theDECOMMITTEDsentinel. Max objects per granule isgranule_size >> min_align_shift + 1. At64K >> 8 + 1 = 257the counter overflows; at64K >> 9 + 1 = 129it's safe.Crash-reproduced: buggy build SIGTRAPs after ~500-1000 JIT-compiled functions; fixed build passes.
Verified
granule ≥ os_page,use_count < 255,num_alloc_bits ≤ PAS_MAX_OBJECTS_PER_PAGE,num_granules ≤ PAS_MAX_GRANULESfor OS page sizes {4K, 16K, 64K}0e6a06612(both fixes absent) SIGTRAPs in phase 2Trade-off
Changes are unconditional for Linux ARM64 — a single binary runs on 4K/16K/64K kernels. On 4K-page systems (most distros, AWS Graviton):
MarkedBlock::blockSize16K → 64K (4× GC block overhead)CeilingOnPageSizeis used inalignas()andconstexpr— can't be runtime-selected without restructuringMarkedBlock.