Claude review: drm/xe: Do not blindly copy system CCS pages when shrinking

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Overall Series Review

Subject: drm/xe: Do not blindly copy system CCS pages when shrinking
Author: Matthew Brost <matthew.brost@intel.com>
Patches: 1
Reviewed: 2026-05-05T10:06:13.444811

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This is a single-patch optimization for the Xe driver's memory shrinker path on integrated (non-DGFX) GPUs. It avoids CCS (Compression Control Surface) page copies during BO shrinking when the BO has never been bound with a compressed PAT index. The commit message cites a ~6.5:1 ratio of eligible-but-unused CCS BOs and roughly 50% reduction in CCS copies — a meaningful optimization for memory pressure scenarios.

The approach is fundamentally sound: add a one-way `ccs_used` flag that transitions false-to-true when a VMA with a compressed PAT binds the BO, and skip the `xe_migrate_copy` in the move path when CCS was never used. However, there are a few issues worth addressing around locking documentation, error-path interaction, and the correctness of skipping the copy entirely (stub fence) versus just skipping the CCS portion.

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