[1.14] Advertise wait ack bit in free page hinting#14
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Whenever free-page hinting is enabled, also advertise the new VIRTIO_BALLOON_F_HINT_WAIT_ON_ACK feature bit (6). When negotiated, the guest driver waits for the device to signal-used each hint buffer before pushing the just-hinted page onto vb->free_page_list, closing a stale-hint data-loss race where the shrinker could recycle a page back to the buddy allocator before discard_range completed on the host. Guests without kernel support for bit 6 simply do not negotiate it (the driver self-clears the bit if VIRTIO_BALLOON_F_FREE_PAGE_HINT is not also negotiated), so this is forward-compatible with stock guests. No host-side protocol change is required: process_free_page_hinting_queue already calls signal_used_queue once per drain, which serves as the ACK the guest waits on. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com> Signed-off-by: Nikita Kalyazin <nikita.kalyazin@e2b.dev>
Adds a guest-side check that the negotiated balloon features in
/sys/bus/virtio/devices/virtioN/features include bit 3 (FREE_PAGE_HINT)
and bit 6 (HINT_WAIT_ON_ACK) when free_page_hinting is enabled.
The test is gated on a new dedicated marker, requires_patched_kernel,
which is registered in tests/pytest.ini and added to the default -m
exclusion filter so the test is auto-skipped by every CI run (regular
and nightly). To run it, replace the 6.1 artifact vmlinux with a build
that carries Jack Thomson's wait-on-ACK patch and invoke:
tools/devtool -y test -- -m requires_patched_kernel \
tests/integration_tests/functional/test_balloon_wait_on_ack.py
If the kernel is not patched, the bit-6 assertion fails with a clear
"did you replace the kernel?" message.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Signed-off-by: Nikita Kalyazin <nikita.kalyazin@e2b.dev>
Add a subsection under free_page_hinting describing the behaviour of VIRTIO_BALLOON_F_HINT_WAIT_ON_ACK: always advertised alongside FPH, self-cleared by guests without the supporting kernel patch, no separate config knob, and a note on the per-buffer round-trip cost on supported guests. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com> Signed-off-by: Nikita Kalyazin <nikita.kalyazin@e2b.dev>
PR SummaryMedium Risk Overview The only end-to-end coverage for negotiation is a new integration test that is excluded from CI and depends on a manually swapped, out-of-tree patched kernel artifact, so regressions/compatibility issues are easy to miss. Documentation is updated to describe the new bit and its guest-kernel dependency. Reviewed by Cursor Bugbot for commit b4fbe13. Bugbot is set up for automated code reviews on this repo. Configure here. |
kalyazin
merged commit f0a35a1
into
e2b-dev:firecracker-v1.14-direct-mem
This is a known and accepted gap. We will be working on improving test coverage. |
2 participants
Changes
Advertise VIRTIO_BALLOON_F_HINT_WAIT_ON_ACK unconditionally when free page hinting is enabled.
The feature support in the guest kernel: e2b-dev/fc-kernels#19 .
Reason
This is to address the race condition under memory pressure in the guest. See more in the kernel patch.