Kernel memory_tension_drop Causes Silent Packet Loss Before Conserve Mode
Linux kernel page allocation failures in the FortiOS dataplane silently discard customer packets while total system memory remains below the 88% red conserve mode threshold, leaving network monitoring dashboards green during severe outages.
Technical Root Cause Analysis
FortiOS relies on dual memory supervision layers: user-space conserve mode monitors (which poll total RAM every second against 88% Red / 95% Extreme thresholds) and kernel-level atomic page allocators (which serve fast-path session table updates and sk_buff packet buffers).
When slab cache structures (such as kmalloc-512 or radix_tree_node) suffer severe internal fragmentation, the kernel allocator encounters atomic memory allocation failures. Instead of queuing packets or entering formal conserve mode, the session driver silently discards packets and increments the internal counter memory_tension_drop. Because total RAM may only be at 72–84%, no SNMP trap or conserve mode event log is emitted.
Affected Firmware & Blast Radius Matrix
| FortiOS Branch | Vulnerable Builds | Confirmed Clean Build | Status & Workaround |
|---|---|---|---|
| FortiOS 7.0 | 7.0.0 – 7.0.13 | 7.0.14+ | Tune slab timers & reduce session TTLs |
| FortiOS 7.2 | 7.2.0 – 7.2.7 | 7.2.8+ | Lower extreme threshold to 90% or patch |
| FortiOS 7.4 | 7.4.0 – 7.4.3 | 7.4.4+ | Apply mini ISDB and session cache pruning |
Platform Impact: High-throughput perimeter appliances and branch units handling high rates of short-lived TCP microservice calls or DNS resolution bursts.
Step 01: Free Verification CLI (Safe Read-Only)
Execute these commands to expose active memory tension packet drops and check kernel slab cache sizing:
Diagnostic Commands
# 1. Check if the kernel is dropping transit packets due to memory tension
diagnose sys session stat | grep memory_tension_drop
# 2. Compare against active conserve mode thresholds
diagnose hardware sysinfo conserve
get system performance status
# 3. Inspect kernel slab cache usage
diagnose hardware sysinfo slabRemediation & Workaround Steps (Teaser Preview)
Unlock the complete multi-source triage workflow on the Ferrite interactive platform:
Frequently Asked Questions
Dataplane memory page fragmentation triggers atomic allocation failures in the Linux kernel before user-space daemons recognize high total memory utilization.
Kernel page drops happen in real-time in the network driver layer, bypassing user-space event log generators.
Reclaiming socket buffers by pruning TCP timewait sessions and switching heavy inspection profiles to flow mode restores slab availability.