LLMs made my breakfast, lunch, and dinner. Not really. They served as an assistive tool while I performed the debugging, testing, and analysis needed to isolate the root cause in core MM while fixing a known DRM SVM issue involving THP allocation failures in the CPU fault-to-device page migration path.
When a CPU faults on a device private PMD and the driver cannot allocate a compound destination folio, the source THP has to be split. That path is broken: the CPU fault reference makes the split always fail, and it demotes the PMD only in the faulting VMA, leaving any other VMA mapping the folio pointing a huge PMD at an order-0 page. The latter is memory corruption, previously masked by the former. The DRM side had its own problems in the same fallback: there was no order-0 fallback at all despite a TODO saying one was needed, the error path computed folio_order() after put_page(), and once the destination is demoted to order-0 the source page array has to be populated per page rather than per folio head, or the copy stops after one page. Validation was performed using xe_exec_system_allocator. The issue was initially discovered on systems configured with an artificially constrained memory footprint (mem=8G), where failures occurred intermittently. Error injection was then introduced to reliably reproduce the failure condition, enabling thorough validation of the fix. Results were confirmed through pass/fail A/B testing. Matt v2:: - Add assert in 'Fix folio allocation fallback and use-after-put' for THP placement invariant which Sashiko hallucinated as a bug [1] - Add 'Clear MIGRATE_PFN_MIGRATE on all sub-folios of a split THP' (Sashiko) - Fix checkpatch issues (CI) - Swap cache issue flagged by Sashiko [1] not fixed as this code doesn't appear reachable (i.e., dead code). Can address in a follow up if needed [1] https://sashiko.dev/#/patchset/20260805113338.3742178-1-matthew.brost%40intel.com Matthew Brost (5): mm/migrate_device: Clear MIGRATE_PFN_MIGRATE on all sub-folios of a split THP mm/migrate_device: Fix THP splitting of a CPU faulted device private folio mm/migrate_device: Apply the fault reference to the correct folio drm/pagemap: Fix folio allocation fallback and use-after-put drm/pagemap: Add fault injection for higher-order RAM folio allocation drivers/gpu/drm/drm_pagemap.c | 164 ++++++++++++++++++++++++++++------ mm/migrate_device.c | 129 ++++++++++++++++++++++---- 2 files changed, 251 insertions(+), 42 deletions(-) -- 2.34.1
