mm: don't try to NUMA-migrate COW pages that have other uses

CVE-2022-48797

Summary

A vulnerability has been identified in the Linux kernel related to memory management that specifically impacts the handling of Copy-On-Write (COW) pages when NUMA balancing is enabled. Under certain conditions, including the use of page pinning for DMA operations, this issue can lead to data corruption. The root of the problem lies in how page references are managed during the NUMA migration process, indicating that changes in writability of pages can occur unexpectedly, leading to potential stability and reliability issues in systems utilizing these kernel features. The correct implementation requires revisions to existing code handling page counts to prevent any spurious behavior triggered by the NUMA balancing process.

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