mm/migrate_device: pin large folios before splitting

Browse source

migrate_vma_collect_pmd() can detect a large folio while holding the PTE
lock, then drop the PTE lock before calling migrate_vma_split_folio(). 
The split helper took its own reference, but only after the lock had
already been dropped.

One way to hit this is device migration over a range that contains a large
folio.  The walker reads the PTE while holding the PTE lock and derives
the folio either from a present PTE via vm_normal_page(), or from a
non-present PTE that encodes a device-private softleaf entry.  It then has
to drop the PTE lock because split_folio() can block.  Before
migrate_vma_split_folio() gets a folio reference, concurrent reclaim,
migration, or truncation can replace or clear the entry and drop the last
reference to the folio.  The split helper would then take a reference and
lock on a stale folio pointer.

Take a temporary reference before dropping the PTE lock and pass that
reference into migrate_vma_split_folio().  The helper consumes the
reference, so split_folio() still sees only the expected caller pin
instead of an extra pin that could make the split fail.

[usama.arif@linux.dev: condense comment about folio reference]
  Link: https://lore.kernel.org/87bbf335-648f-4065-abc8-3eaab5a3beeb@linux.dev
Link: https://lore.kernel.org/20260701140638.840773-1-usama.arif@linux.dev
Fixes: 022a12deda ("mm/migrate_device: handle partially mapped folios during collection")
Signed-off-by: Usama Arif <usama.arif@linux.dev>
Reported-by: sashiko-bot <sashiko-bot@kernel.org>
Link: https://sashiko.dev/#/patchset/20260630164143.1595669-1-usama.arif%40linux.dev
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Reviewed-by: SJ Park <sj@kernel.org>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Byungchul Park <byungchul@sk.com>
Cc: Gregory Price <gourry@gourry.net>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

...

This commit is contained in:

Usama Arif 2026-07-01 07:06:38 -07:00 • committed by Andrew Morton

parent 67a06fdfbf

commit 78b0ca26d0

1 changed files with 10 additions and 3 deletions

Show all changes Ignore whitespace when comparing lines Ignore changes in amount of whitespace Ignore changes in whitespace at EOL

Download patch file Download diff file Expand all files Collapse all files

13

mm/migrate_device.c

View file