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…y the bytecode mapped On a cache hit the whole entry file is mapped so the bytecode blob can be handed to JSC in place. The header and stored post-transpile source in front of the blob are only read once, by the byte compare against the current source, but the mapping kept them resident (and paged in by the memcmp) for the rest of the process, next to the live source string holding the same text. After validation, munmap the page-aligned prefix in front of the blob and record only the remaining tail in Backing::Map. The blob offset is a multiple of 128 and the retained base is a page boundary, so the decoder's alignment requirement still holds; if the partial unmap fails the whole mapping is kept and released on drop as before.
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Status: closed by the maintainer, not merged. The unmapped prefix is clean file-backed page cache, so on a hit this only changes RSS accounting (phys_footprint on macOS is unchanged), not actual memory use. See the discussion below. |
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ChangesCompile-cache mapping
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Actionable comments posted: 3
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In `@src/jsc/NodeCompileCache.rs`:
- Around line 74-80: Update the mapping ownership documentation near the
AlignedBlob/Backing::Map contract to state both outcomes: after successful
unmapping, base is the page boundary at or before the blob; if sys::munmap fails
in AlignedBlob::from_mapping, Backing::Map retains ownership of the full mapping
and base remains the file start.
In `@test/js/node/module/node-module-module.test.js`:
- Around line 265-273: Strengthen the mapping assertions in the test around
run(basename) by parsing the cache entry’s code-size header and deriving the
aligned bytecode offset. Assert that mapped offset equals the preceding page
boundary for the bytecode start, within the supported 64 KiB page-size bound,
while preserving the existing coverage-through-end checks.
- Line 247: Update the Bun.spawn invocation to pass an absolute script path
using path.join(String(dir), "main.js") instead of the relative "main.js"
command entry. Retain cwd only if the test explicitly needs to validate child
working-directory behavior.
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I reviewed this PR and didn't find any bugs. Because it introduces a new unsafe partial-munmap path whose bookkeeping governs the lifetime of a blob JSC reads in place across FFI, a human look at the memory-safety reasoning would still be worthwhile.
What was reviewed:
from_mappingarithmetic:prefix_lenis page-aligned and<= blob_off <= map_len, so both the prefixmunmapand the tailmunmapon Drop get page-aligned addresses and in-bounds lengths; the blob's virtual address is unchanged so 128-alignment is preserved.- Caller safety:
bytes/blob_bytes(views into the unmapped prefix) are not read afterfrom_mapping; theelseheap-copy branch that does readblob_bytesnever unmaps. - Windows:
sys::mmapreturns ENOTSUP somap_is_alignedis false andfrom_mappingis unreachable there. - Test reads
/proc/self/mapsfor the specific entry file and asserts offset > 0 and coverage through EOF; verified it would fail pre-fix (mapping at offset 0).
Extended reasoning...
Overview
This PR modifies src/jsc/NodeCompileCache.rs to reduce resident memory on NODE_COMPILE_CACHE hits. Previously, an accepted cache entry kept the entire file mapping (header + stored source + bytecode blob) alive for the process because JSC reads the bytecode blob in place. The change adds AlignedBlob::from_mapping, which unmaps the page-aligned prefix (header + stored source) after validation and records only the tail in Backing::Map. A Linux-only test in test/js/node/module/node-module-module.test.js inspects /proc/self/maps in a child to verify the mapping starts at the blob's page rather than file offset 0.
Security risks
None identified. The change is a memory-footprint optimization on a read-only MAP_PRIVATE mapping of a cache file the process itself wrote; no new inputs are parsed and no trust boundary is crossed. The existing header/hash validation and byte-compare are unchanged.
Level of scrutiny
High. This is new unsafe Rust doing pointer arithmetic and a partial munmap on a mapping whose tail is later handed to JSC as a raw span kept for the process lifetime. Per the repo's review guidance, native memory safety is the most-blocked category, and getting the (base, map_len) bookkeeping wrong here would be a use-after-unmap the next time JSC decodes the blob. I traced the arithmetic (prefix is page-aligned and bounded by blob_off; tail base is page-aligned; map_len - prefix_len cannot underflow because blob_off + cache_size == map_len was validated), confirmed the blob pointer itself does not move so the 128-byte alignment check upstream is unaffected, and confirmed the fallback (prefix in first page, or munmap refused) keeps the whole mapping so nothing leaks or dangles. On Windows the map path is unreachable because sys::mmap returns ENOTSUP.
Other factors
The SAFETY comment at the call site correctly notes that bytes/blob_bytes — &[u8] views spanning the prefix — are not read after the unmap; I verified this by inspection (only finish(line, ...) and entry.blob = Some(blob) follow). The test is well-constructed (256 KiB source spans several pages even at 64 KiB page size, asserts exactly one mapping of the entry file, checks both that the offset moved forward and that the tail still covers EOF) and the PR description reports the rest of the file plus the 14 test-compile-cache-*.js parallel tests pass. Still, partial munmap of a mapping that backs an FFI-held pointer is exactly the kind of change a maintainer should sign off on rather than auto-approve.
…hole-mapping case
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Thanks for the fix in 7c341fe — deriving bytecodeOffset as file size - blob size from the entry's own header is cleaner than hardcoding HEADER_SIZE and matches how read_cache_file validates the entry, so the concern is fully addressed. I didn't find any further issues. Because this is unsafe native code doing partial munmap of a live mapping whose tail is handed to JSC for the process lifetime, a human look would still be worthwhile.
Checked: bytes/blob_bytes (raw-pointer-derived views over the whole mapping) are not read after from_mapping unmaps the prefix — only the map_is_aligned branch reaches the unmap and it does not touch blob_bytes; the closure passed to finish captures only cache_size.
Checked: Windows never reaches from_mapping (sys::mmap returns ENOTSUP there so map_guard.0 is always None); the prefix_len == 0 and munmap-failure paths both retain the whole mapping and Drop releases it once.
Checked: the retained (base, map_len) after a successful prefix unmap is exactly the tail — Drop's munmap(tail, map_len - prefix_len) covers what remains and nothing more; ptr = base + blob_off is unchanged so the 128-byte alignment JSC checks still holds.
Extended reasoning...
Overview
This PR changes src/jsc/NodeCompileCache.rs to unmap the header + stored-source prefix of an accepted NODE_COMPILE_CACHE entry file after validation, keeping only the pages containing the JSC bytecode blob mapped. It adds AlignedBlob::from_mapping (an unsafe fn that takes ownership of a whole-file mapping, munmaps its page-aligned prefix, and records the tail in Backing::Map), updates the Backing::Map doc, and rewires the accept path in read_cache_file to call it. A Linux-only test in test/js/node/module/node-module-module.test.js reads /proc/self/maps and /proc/self/auxv to assert the exact retained {offset, length} for a 256 KiB module's cache entry.
Security risks
None identified. The mapping is PROT_READ/MAP_PRIVATE over a cache file the process itself wrote (mode 0600, dir owner-checked). Partial munmap of a page-aligned prefix is POSIX-defined behavior. No new inputs are parsed and no trust boundary is crossed.
Level of scrutiny
High. This is unsafe Rust doing raw pointer arithmetic and partial munmap on a mapping whose tail JSC's bytecode decoder reads in place for the rest of the process. REVIEW.md flags native memory safety as the most-blocked category. The correctness relies on (a) the SAFETY contract that bytes/blob_bytes borrows over the whole mapping are not used after the prefix is unmapped — which the borrow checker cannot enforce here because the slice comes from from_raw_parts — and (b) POSIX partial-munmap semantics on both Linux and macOS. I traced both and they hold, but this is exactly the kind of change a maintainer familiar with the JSC provider lifetime should sign off on.
Other factors
My earlier inline finding (the test hardcoded a 108-byte header vs the real 76-byte HEADER_SIZE) was addressed in 7c341fe by a better approach: the test now computes bytecodeOffset = fileSize - blobSize from the entry's third header u32, which is the same identity read_cache_file validates (blob_off + cache_size == total), so no format constant is duplicated in the test. All CodeRabbit and comment-cop threads are resolved. The test is precise (exact offset/length via AT_PAGESZ), hermetic, drains subprocess pipes concurrently, and follows the file's existing conventions. The fallback paths (blob in first page; munmap failure) degrade to the pre-PR behavior, so nothing leaks.
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Updated 8:38 PM PT - Aug 16th, 2026
✅ @robobun, your commit 7c341fed8ea7f3493b5cb6582c11afbf5c8fe002 passed in 🧪 To try this PR locally: bunx bun-pr 39384That installs a local version of the PR into your bun-39384 --bun |
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Closing: the pages this unmaps are clean MAP_PRIVATE file-backed pages that only share the page cache. munmap removes them from this process's RSS/Pss accounting but does not release the memory (the page cache keeps them until there is pressure, and the memcmp fault-in cost is unchanged), so this is an accounting change rather than a real footprint reduction. Confirmed locally on macOS: Bun.unsafe.memoryFootprint (phys_footprint) is identical before/after on a cache hit; only rss moves. |
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Agreed. The prefix is clean file-backed pages, so on a hit the munmap only drops this process's PTEs for page cache that exists either way (and is already excluded from phys_footprint on macOS); the memcmp fault-in is unchanged too. The only non-accounting effect is that the pages become plain reclaimable cache instead of mapped pages, which is marginal. The real duplicate copy in this area is on the miss path, where |
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Problem
NODE_COMPILE_CACHEhit,read_cache_filemaps the whole entry file, byte-compares the stored source against the current post-transpile source, and then keeps the entire mapping alive for the process because the bytecode blob at the end of the file is handed to JSC in place (src/jsc/NodeCompileCache.rs,Backing::Map).Fix
AlignedBlob::from_mappingunmaps the page-aligned prefix in front ofblob_file_offsetand records only the remaining tail (base= last page boundary at or before the blob) inBacking::Map;Dropreleases that tail.munmapof the prefix fails, the whole mapping is likewise kept and released at once on drop, so nothing leaks.Boxcopy of the source kept until persist) is unchanged; that is a separate, larger change.test/js/node/module/node-module-module.test.js("compile cache keeps only the bytecode pages of an accepted entry mapped"): loads a 256 KiB module twice and reads the child's/proc/self/maps. It takes the blob offset from the entry itself (file size minus the blob size stored in the header), the page size fromAT_PAGESZ, and asserts the entry's single mapping is exactly{ offset: pageFloor(blobOffset), length: pageCeil(fileSize) - pageFloor(blobOffset) }. Before: offset 0 / length 528384 (whole file). After: offset 262144 / length 266240. Linux-only because the layout is read from/proc/self/maps; the unmap itself is plain POSIX and runs on macOS too.test/js/node/test/parallel/test-compile-cache-*.jstests pass with the debug build.BUN_JSC_verboseDiskCache=1reports[Disk Cache] Cache hitand evaluates to the same result, so JSC decodes the blob from the truncated mapping.Background
HEADER_SIZE: three u32s for magic, code size and blob size, plus two sha256s), the module's post-transpile source, zero padding up to the next multiple of 128, then the JSC bytecode blob (blob_file_offset), which runs to the end of the file.AlignedBlob) instead of being read into a temporary buffer.munmapworks on any page-aligned sub-range of a mapping, so unmapping the leading pages simply shrinks the mapping; the kernel keeps the file's old inode alive under the remaining range, which is what makes entry rewrites (tmpfile + rename) safe, as before.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/node/module/node-module-module.test.js