Stream: git-wasmtime

Topic: wasmtime / PR #14357 pooling allocator: complete the page...

Wasmtime GitHub notifications bot (Sep 19 2026 at 02:38):

HasanH47 opened PR #14357 from HasanH47:pagemap-reset-complete-traversal to bytecodealliance:main:

reset_with_pagemap collects dirty regions into a stack buffer of 32 and
treats the kernel's walk_end as the end of the memory it will ever reset
in place: everything after it is decommitted with madvise(DONTNEED).

Two consequences we hit running a QuickJS-based guest with a fresh
instance per request (pooling allocator, memory_init_cow, pagemap_scan):

1. Throughput. A JS heap dirties far more than 32 disjoint runs, so
   the scan stopped early on every reset and the rest of the heap was
   decommitted. Each new instance then re-faulted its heap (95–315 minor
   faults per request measured with perf stat), and the madvise +
   mprotect storm capped multi-core scaling with TLB-shootdown IPIs.
   Resuming the scan from walk_end whenever the buffer filled, and
   stopping early only when the keep_resident page budget is spent (which
   is what walk_end is for), brought a request from 2.1 ms to 1.0 ms on
   one core and ×5 at 16 concurrent instances on a 16-core host, with 0
   faults per request.

2. Freshness. The mask requires PRESENT. A dirty page the kernel has
   swapped out is WRITTEN | SWAPPED, not PRESENT; it is neither reset in
   place nor decommitted (it sits before walk_end), so the next instance in
   the slot reads the previous instance's bytes. We reproduced this with a
   swapfile and MADV_PAGEOUT between two instantiations of the same slot:
   the second instance observed the first's heap and trapped in the
   allocator. Requiring WRITTEN and not PFNZERO/FILE regardless of
   residency fixes it — resetting a swapped page pages it in and overwrites
   it, which is the cost of correctness. With 32 regions the accidental
   decommit of everything after walk_end hid most of this; with a complete
   traversal it would be exposed on every host with swap, so the two changes
   belong together.

The patch keeps the fixed stack buffer (64 regions) and loops the ioctl,
so there is still no allocation on the reset path. Unit tests in
pagemap.rs (they skip where PAGEMAP_SCAN is unavailable):

• reset_resumes_past_the_region_buffer: 200 disjoint dirty pages in a
  400-page mapping, keep_resident = everything — all 200 reset in place,
  nothing decommitted, the mapping reads zero;

• reset_stops_at_the_page_budget: same mapping, keep_resident = 100
  pages — exactly 100 reset, the rest decommitted;

• reset_covers_dirty_pages_that_were_paged_out: 64 dirty pages,
  MADV_PAGEOUT, then the reset — every page reset (on a host with swap
  this fails with the old PRESENT mask; elsewhere it is the ordinary
  reset).

Wasmtime GitHub notifications bot (Sep 19 2026 at 02:38):

HasanH47 requested dicej for a review on PR #14357.

Wasmtime GitHub notifications bot (Sep 19 2026 at 02:38):

HasanH47 requested wasmtime-core-reviewers for a review on PR #14357.

Wasmtime GitHub notifications bot (Sep 19 2026 at 02:56):

HasanH47 updated PR #14357.

Wasmtime GitHub notifications bot (Sep 19 2026 at 04:46):

github-actions[bot] added the label wasmtime:api on PR #14357.

Last updated: Sep 20 2026 at 18:08 UTC