feat(fc): drain virtio-balloon free-page-hinting before pause#2552
feat(fc): drain virtio-balloon free-page-hinting before pause#2552ValentaTomas wants to merge 1 commit intofeat/uffd-fc-free-page-reporting-integrationfrom
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Arm free-page-hinting on the existing balloon device (always set when the balloon is installed; pure runtime toggle), and on pause do a host-initiated hint+wait so MADV_DONTNEED-reclaimed pages are settled before the snapshot. Pages reclaimed this way generate UFFD_EVENT_REMOVE, which the orchestrator already tracks (parent FPR PR), so the snapshot captures them as removed instead of zero-filled. - fc/client.go: rename enableFreePageReporting -> installBalloon; always set FreePageHinting=true; add startBalloonHinting + describeBalloonHinting helpers. - fc/process.go: track balloonInstalled; add DrainBalloon (start + poll guest_cmd >= host_cmd, with host>0 guard against transient nil/zero responses). - sandbox.go: wire featureFlags into Sandbox; call DrainBalloon from Pause behind the flag. Failures are logged but non-fatal. Gated by free-page-hinting-timeout-ms (LD int flag, ms; default 0 = disabled). resume-build gains --fph-timeout-ms for local exercise.
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Drains virtio-balloon free-page-hinting before pause so snapshots don't capture pages the guest already considers free. The balloon (from parent FPR PR) always arms
FreePageHinting=true; on pause we callstart_balloon_hintingand polldescribe_balloon_hintinguntilguest_cmd >= host_cmd(withhost > 0guard). Reclaimed pages emitUFFD_EVENT_REMOVE, already tracked by parent.Gated by
free-page-hinting-timeout-msLD flag (ms; default 0 = disabled). Operator opts in once the kernel has the FPH race fix. Stacked on parent FPR branch for the shared balloon-install path; split out from #2550.