mm/migrate_device: pin large folios before splitting

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: 022a12deda53 ("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>

0 files changed, 0 insertions, 0 deletions