[PATCH v2 0/5] Fix device page migration in low memory fallback

[Matthew Brost <matthew.brost@intel.com>]

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