[PATCH 0/4] Fix device page migration in low memory fallback

[Matthew Brost]

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

Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Lorenzo Stoakes <ljs@xxxxxxxxxx>
Cc: Zi Yan <ziy@xxxxxxxxxx>
Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
Cc: Liam R. Howlett <liam@xxxxxxxxxxxxx>
Cc: Nico Pache <nico.pache@xxxxxxxxx>
Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
Cc: Dev Jain <dev.jain@xxxxxxx>
Cc: Barry Song <baohua@xxxxxxxxxx>
Cc: Lance Yang <lance.yang@xxxxxxxxx>
Cc: Usama Arif <usama.arif@xxxxxxxxx>
Cc: Joshua Hahn <joshua.hahnjy@xxxxxxxxx>
Cc: Rakie Kim <rakie.kim@xxxxxx>
Cc: Byungchul Park <byungchul@xxxxxx>
Cc: Gregory Price <gourry@xxxxxxxxxx>
Cc: Ying Huang <ying.huang@xxxxxxxxxxxxxxxxx>
Cc: Alistair Popple <apopple@xxxxxxxxxx>
Cc: Balbir Singh <balbirs@xxxxxxxxxx>
Cc: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>
Cc: Maxime Ripard <mripard@xxxxxxxxxx>
Cc: Thomas Zimmermann <tzimmermann@xxxxxxx>
Cc: David Airlie <airlied@xxxxxxxxx>
Cc: Simona Vetter <simona@xxxxxxxx>
Cc: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
Cc: Francois Dugast <francois.dugast@xxxxxxxxx>
Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
Cc: linux-mm@xxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Assisted-by: GitHub Copilot:claude-opus-5
Signed-off-by: Matthew Brost <matthew.brost@xxxxxxxxx>

Matthew Brost (4):
  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 | 150 ++++++++++++++++++++++++++++------
 mm/migrate_device.c           | 120 +++++++++++++++++++++++----
 2 files changed, 230 insertions(+), 40 deletions(-)

-- 
2.34.1