Claude review: drm/i915/gem: Drop check for changed VM in EXECBUF

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Overall Series Review

Subject: drm/i915/gem: Drop check for changed VM in EXECBUF
Author: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Patches: 2
Reviewed: 2026-04-12T11:46:34.835044

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This is a single, well-motivated cleanup patch that removes a dead VM-equality check in the execbuffer VMA lookup path. The commit message and Sima's included analysis clearly trace the history: the check was originally needed when `gem_ctx->vm` could change at runtime (introduced in e0695db7298e), but since the proto-context rework (d4433c7600f7), `ctx->vm` is set once at context creation and never modified afterward. The VMA entries in `handles_vma` are always created via `i915_vma_instance(obj, vm, NULL)` using the same `vm`, so the `vma->vm == vm` check is indeed tautologically true.

The change is correct and trivially safe. It removes one comparison per cached VMA lookup on the fast path of execbuffer, which is a minor but nice cleanup.

One observation: the v3 changelog mentions adding a `WARN_ON_ONCE`, but the v4 patch does not include one. The v4 changelog says "Focus only on latest mainline codebase," suggesting the WARN was intentionally dropped. This is worth noting — a `WARN_ON_ONCE(vma && vma->vm != vm)` would have provided runtime validation of the invariant while keeping the fast path essentially the same. However, if the maintainers are confident the invariant is well-established (and it has been since 2022), omitting the WARN to keep the code clean is a reasonable choice.

**Verdict:** The patch looks correct and ready to merge.

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