test(heapStats-mimalloc): read VM tags from the kernel instead of vmmap labels

[robobun]

Problem

• heapStats-mimalloc.test.ts > arena memory is tagged as application memory, not IOAccelerator fails on macOS 27 beta (26A5416b): expect(appTag).toBeGreaterThan(64), Received: 0. It passes on macOS 14, 15 and 26. Build 103104, lane darwin 27 aarch64.
• The test sums vmmap --summary lines that start with Memory Tag 24 / Memory Tag 25. vmmap on macOS 27 prints tag 240 as App-Specific Tag 1 (255 as App-Specific Tag 16), so nothing matches. The kernel still stores tag 240 on every mimalloc mapping. Bun has no regression.

Fix

• The child reads the tags from the kernel with mach_vm_region(VM_REGION_EXTENDED_INFO) through bun:ffi and sums the region sizes per user_tag. ioaccelerator is the total for tag 100, appTag the total for tags 240 to 255. The assertions do not change.
• This is the same quantity as before (vmmap's VIRTUAL column is the sum of region sizes per tag), without the label table that Apple renames between releases.
• Verified on the macOS 27 box with bun 1.4.0: the same walk reports tag 240 at 1042.4 MB, tag 100 at 0 MB. On Linux the test is skipped. The other four tests in the file pass with bun bd test.

Background

• A VM tag is an 8-bit label on each kernel map entry. On Darwin, mmap takes it in the fd argument as VM_MAKE_TAG(tag) when MAP_ANON is set. mimalloc passes 240 (VM_MEMORY_APPLICATION_SPECIFIC_1) since Update mimalloc to the upstream dev3 (v3.4.3) sync #36431. The old value, 100, is VM_MEMORY_IOACCELERATOR, so Instruments showed Bun's heap as GPU memory.
• vmmap turns tags into display names with a private table. mach_vm_region returns the raw tag in vm_region_extended_info.user_tag, the second 32-bit field of the struct.

Fixes #40037

Notes

Data from the macOS 27 box (darwin-arm64-bingus, xnu-13432.1.9~3), collected with a plain C probe and with the same probe through bun:ffi in bun 1.4.0.

Kernel view, one mapping per tag through mmap(fd=VM_MAKE_TAG(tag)) and mach_vm_allocate(VM_FLAGS_ANYWHERE | VM_MAKE_TAG(tag)), read back with mach_vm_region:

mmap              requested=100  stored=100
mmap              requested=240  stored=240
mach_vm_allocate  requested=240  stored=240
...
mmap              requested=255  stored=255

vmmap labels on macOS 27 for the same mappings:

App-Specific Tag 1 .. 16   tags 240 .. 255   (macOS 26: "Memory Tag 240" ..)
Rosetta Tag 10             tag 239
VM_MEMORY_150              tag 150           (macOS 26: "Memory Tag 150")
Sanitizer                  tag 99
IOAccelerator              tag 100           (unchanged)
Untagged                   tag 0             (macOS 26: "VM_ALLOCATE")

bun 1.4.0 on macOS 27, vmmap --summary of the test's child (96 x 1 MiB latin1 strings kept alive):

App-Specific Tag 1               260.4M   146.9M   121.2M   ...   9
App-Specific Tag 1 (reserved)    768.0M       0K       0K   ...   1   reserved VM address space (unallocated)

The same process, kernel walk via bun:ffi (what the test now does):

tag 100: (absent)
tag 240: virtual=    1042.4M resident=   146.9M regions=17

bun 1.3.13 (before the tag change) on macOS 27 shows the arena as IOAccelerator 132.3M plus IOAccelerator (reserved) 896.0M, so the ioaccelerator assertion still catches the old default on 27.

macOS 27's vmmap also retitles the other rows (Malloc Small, Stack Guard, Guard, OS Alloc Once) and splits unallocated address space into <label> (reserved) rows. None of that matters once the test stops parsing the summary.

The gate cannot exercise this test: it is skipIf(!isMacOS) and there is no src/ change. The darwin CI lanes run it on macOS 14, 15 and 26. The macOS 27 lane from #40019 runs it once that PR lands.

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no test proof · iteration 0 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/bun/jsc/heapStats-mimalloc.test.ts

[coderabbitai]

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Walkthrough

The macOS arena-memory test now uses bun:ffi and libSystem.B.dylib to inspect VM regions directly. It aggregates memory by kernel user tag instead of parsing vmmap --summary labels.

Changes

macOS VM tag inspection

Layer / File(s)	Summary
VM region tag aggregation test/js/bun/jsc/heapStats-mimalloc.test.ts	The test calls task_self_trap and mach_vm_region, then aggregates bytes for IOAccelerator tag 100 and application tags 240–255.

Merge Risk: 🔵 Low · up to 4bda7

The test now reads VM tags directly from the kernel, with a bounded resource leak in the short-lived test child during region scanning. This is mergeable with owner awareness or follow-up.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly describes the main change: reading VM tags from the kernel instead of parsing vmmap labels.
Description check	✅ Passed	The description explains the problem, fix, background, and verification details, although it does not use the template headings exactly.
Linked Issues check	✅ Passed	The change directly addresses issue #40037 by avoiding macOS 27 vmmap label changes while preserving VM-tag validation.
Out of Scope Changes check	✅ Passed	The changes are limited to the affected heapStats-mimalloc test and support the linked issue objectives.

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[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@test/js/bun/jsc/heapStats-mimalloc.test.ts`:
- Around line 80-94: Release each nonzero object_name port returned by
mach_vm_region in the heap-region iteration: load mach_port_deallocate from
libSystem.B.dylib, then call it with task and object[0] after reading the tag
and updating bytesByTag, before the next mach_vm_region call.

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• test/js/bun/jsc/heapStats-mimalloc.test.ts

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[claude]

LGTM — test-only fix that swaps fragile vmmap label parsing for the raw user_tag from mach_vm_region, with the assertions unchanged. The inline nit about VM_REGION_EXTENDED_INFO_COUNT (10 vs the header's 9) is cosmetic; the kernel accepts any count ≥ 9 and the info buffer is oversized, so feel free to land as-is or tweak on the way in.

Extended reasoning...

Overview

This PR touches a single test file, test/js/bun/jsc/heapStats-mimalloc.test.ts, rewriting one macOS-only test case that verifies mimalloc's arena mmaps carry an app-reserved VM tag (240–255) rather than the old default of 100 (VM_MEMORY_IOACCELERATOR). The old test parsed vmmap --summary output by matching label prefixes like Memory Tag 24 / Memory Tag 25; macOS 27 renames those rows to App-Specific Tag N, so the test read 0 MB and failed. The new test walks the process's VM regions via mach_vm_region(VM_REGION_EXTENDED_INFO) through bun:ffi and sums region sizes by the raw user_tag field — the same quantity vmmap's VIRTUAL column reports, without the release-specific label table in the way.

Security risks

None. This is test-only code that runs in a spawned child, calls read-only Mach introspection APIs on its own task, and touches no production paths. No src/ changes.

Level of scrutiny

Low. It's a CI-flake fix for a platform-specific test, with no change to the assertions (ioaccelerator === 0, appTag > 64). The PR description includes empirical verification on macOS 14–27, including a demonstration that bun 1.3.13 (pre-tag-change) still shows tag 100 on macOS 27 — so the negative assertion still catches the regression it was written for. The FFI signatures, struct offset (info[1] for user_tag), and walk termination (non-KERN_SUCCESS return once address passes the last region) all check out against xnu headers.

Other factors

The one finding is a nit: VM_REGION_EXTENDED_INFO_COUNT is set to 10 where the header macro evaluates to 9. vm_map_region in xnu only rejects *count < VM_REGION_EXTENDED_INFO_COUNT, and the info buffer here is 16 u32s, so this has no runtime effect on any macOS release — it's purely a magic-number accuracy point. Not a reason to hold the merge. The loop's address[0] += size[0] advance and Number(size[0]) accumulation are fine (region sizes fit comfortably in a double). No prior review activity on the PR.

[robobun]

^{Updated 10:19 PM PT - Aug 21st, 2026}

✅ @robobun, your commit 4bda74de9c1fac2d8c9b9454f2c678c222a03dd4 passed in Build #103230! 🎉

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🧪   To try this PR locally:

bunx bun-pr 40044

That installs a local version of the PR into your bun-40044 executable, so you can run:

bun-40044 --bun

[claude]

LGTM — the VM_REGION_EXTENDED_INFO_COUNT nit is addressed and the info buffer is now sized from it.

Checked the FFI signatures against the Mach headers (task_self_trap → u32, mach_vm_region → i32 with the seven-arg shape), the region-walk loop terminates on the non-zero KERN_INVALID_ADDRESS return, and info[1] is the correct offset for user_tag after the 4-byte protection field. Summing tags 240–255 matches the old test's startsWith("Memory Tag 24"/"Memory Tag 25") coverage. The object_name port question was already resolved in-thread (xnu returns IP_NULL).

Extended reasoning...

Overview

Test-only change to test/js/bun/jsc/heapStats-mimalloc.test.ts. The macOS-only "arena memory is tagged as application memory" test previously parsed vmmap --summary text output, prefix-matching label strings that Apple renamed in macOS 27 ("Memory Tag 240" → "App-Specific Tag 1"). The rewrite walks VM regions directly via mach_vm_region(VM_REGION_EXTENDED_INFO) through bun:ffi, aggregating bytes by the raw user_tag field, and keeps the same assertions (ioaccelerator === 0, appTag > 64).

Security risks

None. Test-only, runs in a spawned child that exits immediately, reads its own task's memory map, no network, no user input.

Level of scrutiny

Low-to-medium. This is a CI de-flake for a platform-specific test with no src/ changes; the assertions are unchanged so the invariant it protects (mimalloc uses tag 240–255, not 100) is still enforced. The FFI code is dense but small enough to verify field-by-field against the xnu headers, which the PR description and follow-up commit did carefully.

Other factors

• My previous inline nit (VM_REGION_EXTENDED_INFO_COUNT was 10, header value is 9) was addressed in 4bda74d: the constant is now 9 with a comment deriving it, and the info buffer is sized from the constant so a future struct revision can't overrun it.
• CodeRabbit's mach_port_deallocate concern was correctly resolved: xnu's vm_map_region sets *object_name = IP_NULL for all flavors, and the child process exits right after the walk anyway.
• Verified the new tag-range sum (240–255) is equivalent to the old prefix-match on "Memory Tag 24"/"Memory Tag 25", which via startsWith covered 240–249 and 250–255.
• The loop advances address[0] += size[0] and breaks on non-zero kern_return_t, which is the standard Mach region-walk pattern; Number(size[0]) is safe since region sizes are well under 2^53.

[dylan-conway]

@robobun it's expected bun prints different values just because it's on macos 27? sounds like a bun bug if true, and shouldnt' need a test change

[robobun]

Bun prints the same thing on 26 and 27. The numbers that changed come from vmmap, Apple's debugging tool, which the old test spawned and text-parsed.

What Bun does: mimalloc passes VM_MAKE_TAG(240) to mmap for every arena. That is unchanged. On the macOS 27 box the kernel reports user_tag = 240 for Bun's heap (mach_vm_region: tag 240, 1042 MB virtual for a bun 1.4.0 child). A plain C probe gives the same answer for all of 240 to 255, through both mmap and mach_vm_allocate.

What changed: vmmap's private label table. Up to macOS 26 it prints tag 240 as Memory Tag 240. On 27 it prints App-Specific Tag 1, Apple's own name for VM_MEMORY_APPLICATION_SPECIFIC_1 (240). The old test did startsWith("Memory Tag 24") on vmmap's output, found nothing, and summed 0. A test that greps a tool's display strings breaks whenever Apple renames them.

So the test was checking the right property through the wrong channel. This PR reads the tag from the kernel with mach_vm_region and keeps both assertions as they were (tag 100 total is 0, tags 240 to 255 total more than 64 MB). The minimal alternative is to also match App-Specific Tag, but that stays coupled to vmmap's table and needs a patch on the next rename.