[1.14] Advertise wait ack bit in free page hinting#14
Open
kalyazin wants to merge 3 commits intoe2b-dev:firecracker-v1.14-direct-memfrom
Open
[1.14] Advertise wait ack bit in free page hinting#14kalyazin wants to merge 3 commits intoe2b-dev:firecracker-v1.14-direct-memfrom
kalyazin wants to merge 3 commits intoe2b-dev:firecracker-v1.14-direct-memfrom
Conversation
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>
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
1 participant
Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.
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.