Claude review: drm/shmem-helper: Fix huge page mapping in fault handler

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Overall Series Review

Subject: drm/shmem-helper: Fix huge page mapping in fault handler
Author: Pedro Demarchi Gomes <pedrodemargomes@gmail.com>
Patches: 2
Reviewed: 2026-03-22T04:58:23.495148

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This is a single-patch fix for a real race condition in `drm_gem_shmem_helper.c` where two concurrent page faults within the same PMD range can conflict — one installing a PMD mapping via `vmf_insert_pfn_pmd()` while the other attempts a PTE mapping via `vmf_insert_pfn()`, triggering a BUG in `walk_to_pmd()`.

The fundamental approach is correct: adding a `.huge_fault` callback separates PMD-level faults from PTE-level faults, letting the MM core route them through the proper path (`create_huge_pmd()` at mm/memory.c:6417 only calls `.huge_fault` when `pmd_none()` is true). However, this patch is based on an older tree and **does not apply to drm-next**, which has already refactored this code significantly (adding `drm_gem_shmem_try_insert_pfn_pmd`, `folio_mark_accessed`, `pfn_mkwrite` callback, etc.). The patch needs to be rebased.

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