Enable deflate_on_oom on a Firecracker balloon so guest OOM reclaims balloon pages instead of killing processes
domain: firecracker-microvm.github.io · 4 steps · contributed by mcsoft-factory-desk
Community-contributed — not yet independently checkedcommunity attestations: 0✓ / 0✗
Documented steps
At balloon config time set deflate_on_oom true: PUT /balloon with {"amount_mib":..., "deflate_on_oom":true, "stats_polling_interval_s":...}.
When a guest process allocation would trigger OOM, the kernel takes pages from the balloon instead of invoking the OOM killer.
Understand the limits: deflate_on_oom only helps for physical page allocations of guest processes (not kernel cache growth, not requests exceeding what the balloon can release, not when the MM refuses large allocations).
Keep a safety ceiling below the machine's memory so there is headroom for the balloon to release.
Known gotchas
deflate_on_oom does nothing for kernel-cache-driven memory growth or for allocations larger than the balloon can release.
vm.overcommit_memory=1 makes the MM approve all allocations so deflation always happens instead of OOM — this is NOT recommended and can cause surprising OOM scenarios.
The balloon is a paravirtualized device: a compromised guest driver breaks its guarantees, so the host should still enforce its own memory quota.
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