Bound working-set recording to a per-clone window after handshake - #862
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A serve unions every clone's faulted pages into the snapshot's working-set sidecar for the clone's whole life, so one long-lived clone turns the restore hint into its lifetime footprint (issue #858: hours of uptime grew a 128 GiB guest's hint from 6.5 GiB to 122 GiB, and a day of serving grew it to 84 GiB, which also consumed the hugepage pool headroom the next clone's reservation needed). Each clone now gets a recording deadline anchored at its UFFD handshake. Faults past the deadline are served exactly as before but no longer recorded, so the sidecar converges to the union of restore windows. Replay is unchanged. Knob: --uffd-prefetch-record-window <secs> on snapshot serve, or FCVM_UFFD_PREFETCH_RECORD_WINDOW for the implicit server inside snapshot run. 0 records nothing. The 300 s default is a safety bound pending empirical tuning: the slowest restore-to-guest-ACK in the issue is 28.5 s, so the default sits more than 10x past it and cannot truncate an honest restore working set; the 128 GiB workload measurement is tracked in the issue. A window cannot repair a sidecar already poisoned on disk, so serve startup now warns when a loaded hint covers more than half the guest (honest restore sets measured 5 to 11 percent of guest memory, the two poisoned ones 66 and 95 percent) and names the sidecar to delete. Tests, each watched failing before the gate and the predicate were added (cargo test --lib -p fcvm uffd:: on the ungated code, 75 passed 3 failed): a_fault_past_the_recording_window_is_not_recorded panicked at src/uffd/server.rs: a fault resolved past the recording window must not be recorded observations_past_the_window_contribute_no_learned_pages panicked at src/uffd/server.rs: faults past the window must not become learned pages in the store a_hint_covering_most_of_the_guest_is_flagged_as_suspect panicked at src/uffd/working_set.rs: assertion failed: hint_covers_most_of_the_image(84 * GIB, guest) With the fix: cargo test --lib -p fcvm uffd:: reports 78 passed, 0 failed. cargo check --all-targets -p fcvm and cargo fmt -p fcvm -- --check pass.
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📝 WalkthroughWalkthroughThe PR adds configurable UFFD prefetch recording windows, applies them to explicit and implicit snapshot servers, and limits recording per clone after the UFFD handshake. It also warns when loaded working-set hints cover more than half of the guest image. ChangesUFFD prefetch recording and working-set validation
Estimated code review effort: 4 (Complex) | ~45 minutes Merge Risk: 🔵 Low · up to The change adds bounded working-set recording and oversized-hint warnings, but an extreme-size threshold calculation can incorrectly suppress that warning. The PR is otherwise mergeable with owner awareness and a small follow-up to use an overflow-free comparison. Sequence Diagram(s)sequenceDiagram
participant UffdServer
participant CloneVmState
participant WorkingSetRecorder
UffdServer->>CloneVmState: complete UFFD handshake
CloneVmState->>CloneVmState: set record_until deadline
UffdServer->>CloneVmState: handle demand fault
CloneVmState->>WorkingSetRecorder: record fault while deadline is active
CloneVmState-->>UffdServer: serve fault after deadline without recording
Possibly related PRs
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… guidance Two fixes on PR #862. CI lint: UffdServer::new grew an 8th parameter (record_window), tripping clippy::too_many_arguments. Reproduced locally: cargo clippy --all-targets -p fcvm -- -D warnings failed with "this function has too many arguments (8/7)" at src/uffd/server.rs:574. The three serving-behavior values (backing, prefetch, record_window) now travel as one ServeShape struct, following the RestoreParams/CreateSnapshotParams grouping pattern. Both call sites in src/commands/snapshot.rs construct it; no allow attribute. The same clippy command now passes. Review finding (working_set.rs suspect-hint warning): the warning told the operator to delete the sidecar, but deleting it under a running serve does not repair that server. The suspect set stays in WorkingSetStore::known, later clones still replay it, and the persistence worker can rewrite the sidecar, racing the deletion. The warning now gives the safe sequence: stop the serve, delete the sidecar, restart the serve, and says why deletion alone does not help. A synchronized in-server removal operation is out of scope for this change. Red watched first: suspect_hint_warning_orders_stop_before_delete_before_restart captures the emitted warning through a scoped tracing subscriber and failed on the old text with "guidance must start with stopping the serve: ... Delete the sidecar to re-record." With the new text it passes. cargo test --lib -p fcvm: 479 passed, 0 failed (uffd:: subset 79 passed). cargo clippy --all-targets -p fcvm -- -D warnings and cargo fmt -p fcvm -- --check pass.
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Inline comments:
In `@src/uffd/working_set.rs`:
- Around line 751-753: Update hint_covers_most_of_the_image to compare
hint_bytes against mem_len / 2 instead of multiplying hint_bytes, preserving
correct behavior without overflow or saturation; add a test covering hint_bytes
= u64::MAX / 2 + 1 with mem_len = u64::MAX.
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src/cli/args.rssrc/commands/snapshot.rssrc/uffd/mod.rssrc/uffd/server.rssrc/uffd/working_set.rs
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hint_covers_most_of_the_image doubled hint_bytes with saturating_mul, so at the boundary (a hint over half of a u64::MAX-byte image) the product saturated to u64::MAX, the strict comparison failed, and a more-than-half hint was called safe. Divide the image size instead. Tested: the boundary assertion hint_covers_most_of_the_image(u64::MAX / 2 + 1, u64::MAX) added to a_hint_covering_most_of_the_guest_is_flagged_as_suspect fails on the multiply form and passes on the division; uffd::working_set suite 20 passed, cargo fmt and clippy --all-targets -D warnings clean.
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NOT-A-DEFECT: the review bodies are summaries and auto-replies; the two inline findings are dispositioned in their threads, both RED-VERIFIED: the suspect-hint guidance now orders stop, delete, restart (8fdbbba) and the threshold comparison is overflow-free (c7d43b2), each with its test named and watched red first. The CI Lint failure on the first head was clippy too-many-arguments, fixed by grouping the serve-shape parameters into ServeShape in the same commit.
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A serve unions every clone's faulted pages into the snapshot's working-set
sidecar for the clone's whole life, so one long-lived clone turns the restore
hint into its lifetime footprint (issue #858: hours of uptime grew a 128 GiB
guest's hint from 6.5 GiB to 122 GiB, and a day of serving grew it to 84 GiB,
which also consumed the hugepage pool headroom the next clone's reservation
needed).
Each clone now gets a recording deadline anchored at its UFFD handshake.
Faults past the deadline are served exactly as before but no longer recorded,
so the sidecar converges to the union of restore windows. Replay is unchanged.
Knob: --uffd-prefetch-record-window on snapshot serve, or
FCVM_UFFD_PREFETCH_RECORD_WINDOW for the implicit server inside snapshot run.
0 records nothing. The 300 s default is a safety bound pending empirical
tuning: the slowest restore-to-guest-ACK in the issue is 28.5 s, so the
default sits more than 10x past it and cannot truncate an honest restore
working set; the 128 GiB workload measurement is tracked in the issue.
A window cannot repair a sidecar already poisoned on disk, so serve startup
now warns when a loaded hint covers more than half the guest (honest restore
sets measured 5 to 11 percent of guest memory, the two poisoned ones 66 and
95 percent) and names the sidecar to delete.
Tests, each watched failing before the gate and the predicate were added
(cargo test --lib -p fcvm uffd:: on the ungated code, 75 passed 3 failed):
a_fault_past_the_recording_window_is_not_recorded
panicked at src/uffd/server.rs: a fault resolved past the recording
window must not be recorded
observations_past_the_window_contribute_no_learned_pages
panicked at src/uffd/server.rs: faults past the window must not become
learned pages in the store
a_hint_covering_most_of_the_guest_is_flagged_as_suspect
panicked at src/uffd/working_set.rs: assertion failed:
hint_covers_most_of_the_image(84 * GIB, guest)
With the fix: cargo test --lib -p fcvm uffd:: reports 78 passed, 0 failed.
cargo check --all-targets -p fcvm and cargo fmt -p fcvm -- --check pass.
Summary by CodeRabbit
FCVM_UFFD_PREFETCH_RECORD_WINDOW.