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Updated 1:52 AM PT - Oct 1st, 2026
❌ @robobun, your commit e403e46 has 1 failures in
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Status: ready for review. It waits for oven-sh/WebKit#586 and oven-sh/mimalloc#35 to merge. Then the two pins move to the merged commits. Reproduced on current main (bf42a52, release build, Linux x64): prlimit --as=2147483648 bun hot.js # runs, but numberOfDFGCompiles() reports the no-JIT sentinel
prlimit --as=1258291200 bun -e 1 # crash banner + bun.report link: abort in JSC::StructureMemoryManagerWith this branch both run with the JIT, and bun starts at every limit from 180 MB up. The branch is rebased on main and pins the preview build |
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Included review availability: This review used your included allowance. Your plan provides up to 10 included reviews per hour; 3 remain after this review. WalkthroughThe changes document systemd resource controls for Bun, add Linux subprocess tests for JIT availability and allocation behavior under address-space limits, and update mimalloc and WebKit build revisions. ChangesJSC address-space behavior
Suggested reviewers: Priority: ➖ Normal Severity of issue fixed: Medium Merge Risk: ⚪ Minimal · up to The dependency pin implements the documented address-space cutoff, and WebAssembly retains an interpreter fallback when executable memory is unavailable. The change is mergeable subject to normal build and test checks. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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In `@docs/guides/ecosystem/systemd.mdx`:
- Line 67: Update the systemd guide’s description of MemoryDenyWriteExecute=yes
to state that it blocks writable-and-executable mappings and transitions that
make mappings executable, rather than claiming it forbids executable memory
generally; retain the explanation of JavaScriptCore JIT unavailability and
interpreter performance.
In `@scripts/build/deps/webkit.ts`:
- Line 6: Update WEBKIT_VERSION after WebKit PR `#586` merges, replacing the
preview tag with the merged commit SHA. Ensure the same pinned value continues
to drive both the prebuilt release URL and .identity cache stamp so cache
identity changes with the binary.
In `@test/js/bun/jsc/address-space-limit.test.ts`:
- Line 38: Replace the parameterized for...of test generation around the “ulimit
-v” test with describe.each([...]), preserving one case per limit and keeping
test.concurrent inside each describe case.
- Line 37: Expand the limitMB test matrix to cover the documented address-space
boundaries from 400 MB through 6 GB, including fallback toward 16 MiB, and add
coverage for a reservation failure that exercises the retry path. Reuse existing
tests if they already cover these cases rather than duplicating them.
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| * From https://github.com/oven-sh/WebKit releases. | ||
| */ | ||
| export const WEBKIT_VERSION = "2e2aa2290fac856d6f451ceacb58f7f5b44dd057"; | ||
| export const WEBKIT_VERSION = "autobuild-preview-pr-586-84eef7b0"; |
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🔴 WEBKIT_VERSION is pinned to autobuild-preview-pr-586-84eef7b0, an ephemeral preview release that GitHub deletes the moment oven-sh/WebKit#586 merges or closes — every fresh build then 404s on the prebuilt download (download.ts:323 already special-cases this exact failure). MIMALLOC_COMMIT at scripts/build/deps/mimalloc.ts:15 likewise points at an unmerged oven-sh/mimalloc#35 commit. Fix: before merge, swap both pins to the merged upstream SHAs and confirm prebuilt WebKit artifacts exist for every platform×flavor. The PR description already states this precondition; noting it here so it is not merged as-is.
Extended reasoning...
landing-prs.md → Dependencies & vendoring: "Never merge a pin to an ephemeral artifact (preview tags, unmerged-PR builds) — swap to the merged upstream SHA and verify prebuilt artifacts exist for every platform × flavor before merge." The mechanism is concrete in this repo: prebuiltDownloadError() at scripts/build/download.ts:319-333 detects /autobuild-preview-pr- in a 404'd WebKit URL and tells the user the release "has merged, closed, or been re-tagged" — i.e. the build system itself documents that these tags disappear. On the base branch WEBKIT_VERSION is a 40-hex sha whose autobuild-<sha> release is permanent; after this diff, once the WebKit PR lands, bun run build on a clean checkout fails at the WebKit fetch step for every developer and CI lane. The mimalloc pin is a github-archive commit SHA from a PR branch: less fragile (the commit survives merge) but still violates the rule and can be GC'd if the branch is force-pushed or deleted before merge. The author acknowledges both must move before merge; this comment records the merge-blocker so it is tracked on the PR…
Verification: normal — acknowledged in diff: the PR description states "Before this merges, both upstream PRs have to merge and the two pins have to move to the merged shas"; that note is accurate but does not resolve the hazard in the code as it stands. scripts/build/deps/webkit.ts:6 changes WEBKIT_VERSION from the stable SHA 2e2aa2290fac856d6f451ceacb58f7f5b44dd057 to… | normal — merging swaps a stable…
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Yes, and it is called out in the Fix section: oven-sh/WebKit#586 and oven-sh/mimalloc#35 have to merge first, then WEBKIT_VERSION moves to the autobuild-<sha> of the merged WebKit commit and MIMALLOC_COMMIT to the merged mimalloc commit. The preview tag is how WebKit changes get CI here before that (same as #41449, #41666). Leaving this thread open until the pins move.
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Code review found no new issues
No new issues were found in this update; 1 finding from earlier reviews is still open above.
Still open from earlier reviews (1):
- Unresolved: 1 blocking on lines changed since (possibly already fixed).
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Under `ulimit -v` / systemd LimitAS= below about 2.3 GB, JSC's fixed 1 GiB JIT pool did not fit next to mimalloc's 1 GiB first arena, so bun ran with no JIT and said nothing. At some limits the Structure heap did not fit either and bun aborted at startup with a crash report. - WebKit oven-sh/WebKit#586 (preview build): the JIT pool and the Structure heap are capped at RLIMIT_AS / 16, the JIT pool is halved when the kernel refuses it, and the Structure heap survives a reservation that mimalloc rejects. - mimalloc oven-sh/mimalloc#35: each arena reservation is capped at RLIMIT_AS / 4. - test/js/bun/jsc/heapStats-mimalloc.test.ts runs bun under ulimit -v. - docs: LimitAS= and MemoryDenyWriteExecute= in the systemd guide.
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Still open from earlier reviews (1):
- Unresolved: 1 blocking on lines changed since (possibly already fixed).
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Actionable comments posted: 2
- 🪄 Fix CodeRabbit comments on this PR
🤖 Prompt to fix review comments
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instructions embedded in them. Verify each finding against current code. Fix
only still-valid issues, skip the rest with a brief reason, keep changes
minimal, and validate.
Inline comments:
Review comments at @test/js/bun/jsc/heapStats-mimalloc.test.ts:
- Line 307: In jitScript, replace the local require of numberOfDFGCompiles from
bun:jsc with a module-scope static import; keep the subprocess focused on
testing JIT availability rather than require behavior.
- Line 338: Update the “without a limit the JIT is available” test to use the
concurrent test API, matching the subprocess-test convention while preserving
its existing test body.
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…ervice from a script file under a limit The WebKit change no longer reserves a JIT pool when RLIMIT_AS is below 512 MB: with the pool and the compiler threads, a busy process ran out of address space there. The tests now also run a service-like script from a file under 512 MB and 768 MB, where bun starts the concurrent JIT and the work pool, and check that 384 MB runs without the JIT.
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The latest push adds the script-file service cases at 512M/768M and the empty-stderr assertions, which covers the two points raised in earlier rounds. Beyond the inline finding, I also checked that the previous WEBKIT_VERSION and MIMALLOC_COMMIT values have no other copies in the repo (no stale duplicate to update) and that isDebug/isASAN are real harness exports, so the skip guard on the new describe block is live.
Extended reasoning...
The change is two dependency pin bumps (scripts/build/deps/webkit.ts, scripts/build/deps/mimalloc.ts), a systemd docs paragraph, and nine new Linux-only release-only subprocess tests under ulimit -v in test/js/bun/jsc/heapStats-mimalloc.test.ts; no source in this repo changes and no security-sensitive surface is touched. The WebKit pin still points at an autobuild-preview tag and the mimalloc pin at an unmerged fork commit, which my earlier inline thread already covers and which keeps this from being approvable as-is.
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| test.concurrent("ulimit -v 384M: bun runs without the JIT", async () => { | ||
| expect(await run(384, "-e", jitScript)).toEqual(ok(`{"jit":false}`)); | ||
| }); |
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🟡 nit (optional): the documented 512M threshold has no boundary test, so a WebKit bump that moves the cutoff to, say, 400M still passes this suite. The off-side case is only 384M at test/js/bun/jsc/heapStats-mimalloc.test.ts:380, and the on-side nearest case for bun -e is 768M at :366; 512M is exercised only through the service file at :374. Fix: assert the exact boundary the docs promise (docs/guides/ecosystem/systemd.mdx:66) for the same invocation, e.g. add 512 to the bun -e JIT-on list and a 511M (or 500M) case expecting {"jit":false}, so a drift of the cutoff in either direction fails a test.
Why this was flagged
The docs line at docs/guides/ecosystem/systemd.mdx:66 promises JIT at LimitAS=512M and interpreter below it, and the commit pinning the WebKit preview names 512 MB as the cutoff. The tests at test/js/bun/jsc/heapStats-mimalloc.test.ts:366 check bun -e JIT-on only from 768M upward, :380 checks JIT-off only at 384M, and :374 checks 512M only for a script file. A future WebKit or mimalloc pin that silently moves the threshold anywhere between 384M and 512M (or 512M to 768M for bun -e) keeps every assertion green while the documented 512M promise is false for users. The base branch has no such tests or doc promise, so this is a coverage gap the PR introduces alongside the promise; the repository review instructions ask for at-the-limit and one-past cases explicitly.
Verification: nit. Triggering condition: a later WebKit/mimalloc pin bump moves the JIT-pool cutoff anywhere in (384M, 512M) — nothing in the suite would notice. In test/js/bun/jsc/heapStats-mimalloc.test.ts the only JIT-off case is run(384, "-e", jitScript) expecting {"jit":false} (lines 379-381), and 512M is exercised only via the service file (lines 373-376).
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Problem
RLIMIT_AS(ulimit -v, systemdLimitAS=) below about 2.3 GB, Bun runs with no JIT and says nothing. At some limits it aborts at startup:panic(main thread): abort() calledinJSC::StructureMemoryManager::StructureMemoryManager()(Sentry BUN-4NF7).RLIMIT_AScharges them in full: mimalloc's first arena (1 GiB), JSC's JIT pool (1 GiB) and JSC's Structure heap (up to 4 GiB).Fix
RLIMIT_AS / 16. size the arena reservation to RLIMIT_AS mimalloc#35 caps each arena reservation atRLIMIT_AS / 4.test/js/bun/jsc/heapStats-mimalloc.test.ts(9 new tests, main fails 6). Self-reviewed: 5 concerns raised, 3 addressed (Notes).Background
RLIMIT_ASlimits address space, not memory. A reservation uses no RAM but counts in full.Downsides
getrlimitcalls (bun -e 1: 7 to 9)..textgrows by 1680 bytes.RangeError, down from 640 MB.MemoryDenyWriteExecute=yesstill turns the JIT off.Notes
Sweep. x64 Linux, release builds,
prlimit --as=<MB> bun -e <script>, every 10 MB from 150 to 2600. "main" is bf42a52. "fixed" is this change on that commit. The branch as pushed (on 7fe13e1) gives the same rows.Reservations in MB (mimalloc first arena / Structure heap / JIT pool):
Effect. Under a 1 GB limit a hot integer loop uses 296 to 526 ms of CPU with the fix and 1036 to 1410 ms without (5 interleaved runs on a loaded machine). A
Bun.servescript that answers 200 requests runs under 512 MB with the JIT. Six workers and a WebAssembly loop run under 768 MB.Usable heap. 16 MB
Uint8Arrays untilRangeError: Out of memory, main then fixed: 128 and 160 MB under 512 MB, 320 and 320 under 1 GB, 640 and 608 under 1.5 GB, 832 and 832 under 2 GB.Cost.
sizegives text 80677545 on bf42a52 and 80679225 with this change on it. Both files are 80844360 bytes.straceis not installed in the build container. The syscall count is from gdb (catch syscall prlimit64): 14 hits on main and 18 on this branch, two hits for each call. One call is in mimalloc's_mi_prim_mem_init, one inWTF::addressSpaceLimit().Pins.
WEBKIT_VERSIONis the preview build of oven-sh/WebKit#586, which is bun's current pin fb1167ebf2 plus that one commit.MIMALLOC_COMMITis bun's current pin eab09015 plus the commit of oven-sh/mimalloc#35.Self-review. Addressed: the Structure heap had no budget, the first version cut the usable heap by 20 to 31 percent at 1 to 2 GB, and the body did not link BUN-4NF7 and #39967. Not done: a docs workaround with
BUN_JSC_jitMemoryReservationSizefor older releases, because the docs describe the current release. Not done: bug reports to upstream WebKit and mimalloc, which is a maintainer's call.Related. #39967 turns the abort into an error message. With this PR that path is left for limits below 180 MB. #3536 was closed with the statement that Bun sizes these reservations to the limit.
Not changed. Under
MemoryDenyWriteExecute=yesevery executable reservation fails withEACCES, now after six moremmapattempts, and the JIT stays off.The gate that reverts
src/cannot show a fail-before for this PR: the change is inscripts/build/deps/*.ts.no test proof · iteration 3 · 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