CVE-2026-89756
Low risk· EPSS 8%Exploitation Probability (EPSS)
Low risk8th percentile - higher than 8% of all known CVEs
Summary
In the Linux kernel, migrate_pages_batch() unmaps each folio, which on KVM hosts triggers expensive MMU notifier callbacks and keeps the CPU busy for a long time. The loop only calls cond_resched(), which is a no-op on PREEMPTION kernels, so the migrating task (e.g. kcompactd) never reports a Tasks-RCU quiescent state and stalls the Tasks-RCU grace period for minutes.
Risk Assessment
Prolonged stalling of the Tasks-RCU grace period can cause RCU stall warnings, delayed resource release, and stability issues on KVM hosts. Under heavy memory migration load this may lead to performance degradation and false alarms.
Recommendation
Update the Linux kernel to a version containing the fix that replaces cond_resched() with cond_resched_tasks_rcu_qs() in migrate_pages_batch(). After updating, verify that rcu_tasks stall messages no longer appear.
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Original NVD description (English source)
In the Linux kernel, the following vulnerability has been resolved: mm/migrate: report RCU-tasks quiescent states in migrate_pages_batch() migrate_pages_batch() unmaps each folio before moving it, and every unmap runs the mmu_notifier invalidate callbacks. On KVM hosts try_to_migrate() ends up in kvm_mmu_notifier_invalidate_range_start() -> tdp_mmu_zap_leafs(), which is expensive, so unmapping a large batch keeps the CPU busy for a long time. The loop already calls cond_resched(), but on PREEMPTION kernels that is a no-op, and involuntary preemption is not a Tasks-RCU quiescent state. A long batch therefore never reports a quiescent state, and the migrating task (e.g. kcompactd) becomes a Tasks-RCU holdout, stalling the Tasks-RCU grace period for minutes, which is common at Meta fleet: INFO: rcu_tasks detected stalls on tasks: 0000000055349ecc: .. nvcsw: 1157401/1157401 holdout: 1 idle_cpu: -1/56 task:kcompactd0 state:R running task Call Trace: tdp_mmu_zap_leafs tdp_mmu_next_root gfn_to_pfn_cache_invalidate_start kvm_mmu_notifier_invalidate_range_start __mmu_notifier_invalidate_range_start try_to_migrate_one try_to_migrate migrate_pages_batch migrate_pages compact_zone compact_node kcompactd kthread Use cond_resched_tasks_rcu_qs() so a quiescent state is reported even when cond_resched() does nothing. This has also been discussed at [1]
Vulnerability data from NVD (NIST) · CISA KEV · EPSS