# CVE-2026-89756

## Summary

- **CVE ID:** CVE-2026-89756
- **Severity:** UNKNOWN
- **CVSS Score:** 0
- **CWE:** N/A
- **Published:** Sep 11, 2026
- **Last Modified:** Sep 16, 2026

## Description

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]

## Affected Products

- Linux — Linux (1da177e4c3f41524e886b7f1b8a0c1fc7321cac2)
- Linux — Linux (0)
- Linux — Linux (6.12.109)
- Linux — Linux (6.18.50)
- Linux — Linux (7.2.4)
- Linux — Linux (7.3-rc1)
- Linux — Linux (8315f42295d2667a7f942f154b73a86fd7cb2227)
- Linux — Linux (3.18)
- Linux — Linux (6.1.188)
- Linux — Linux (6.6.157)

## References

- [CNA](https://git.kernel.org/stable/c/4996a7bc01ef35570664854dac2530c604981039)
- [CNA](https://git.kernel.org/stable/c/5dc0daff0341c6baba19c38f47d299ac831d7e99)
- [CNA](https://git.kernel.org/stable/c/66734981b4d3c105223a13c827c9c73be18d91ad)
- [CNA](https://git.kernel.org/stable/c/efe8f86c0916f0f74eea74ae21a3b37f728c6bad)
- [CNA](https://git.kernel.org/stable/c/8c6d63d434ebb85c6cf3dac1e70a171b183c6614)
- [CNA](https://git.kernel.org/stable/c/4757542649af56d894e25e30f57cd497dffad53f)

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.19%
- **EPSS Percentile:** 9.0

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_Exported from OnDuty AI Vulnerability Intelligence on 2026-09-17._