[merged mm-stable] mm-migrate_device-pin-large-folios-before-splitting.patch removed from -mm tree

[merged mm-stable] mm-migrate_device-pin-large-folios-before-splitting.patch removed from -mm tree

[Andrew Morton]

The quilt patch titled
     Subject: mm/migrate_device: pin large folios before splitting
has been removed from the -mm tree.  Its filename was
     mm-migrate_device-pin-large-folios-before-splitting.patch

This patch was dropped because it was merged into the mm-stable branch
of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

------------------------------------------------------
From: Usama Arif <usama.arif@linux.dev>
Subject: mm/migrate_device: pin large folios before splitting
Date: Wed, 1 Jul 2026 07:06:38 -0700

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>
---

 mm/migrate_device.c |   13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

--- a/mm/migrate_device.c~mm-migrate_device-pin-large-folios-before-splitting
+++ a/mm/migrate_device.c
@@ -77,6 +77,9 @@ static int migrate_vma_collect_hole(unsi
  * @folio: the folio to split
  * @fault_page: struct page associated with the fault if any
  *
+ * If @folio is not the folio containing @fault_page, the caller must hold a
+ * reference on @folio. The helper consumes that reference.
+ *
  * Returns 0 on success
  */
 static int migrate_vma_split_folio(struct folio *folio,
@@ -86,10 +89,8 @@ static int migrate_vma_split_folio(struc
 	struct folio *fault_folio = fault_page ? page_folio(fault_page) : NULL;
 	struct folio *new_fault_folio = NULL;
 
-	if (folio != fault_folio) {
-		folio_get(folio);
+	if (folio != fault_folio)
 		folio_lock(folio);
-	}
 
 	ret = split_folio(folio);
 	if (ret) {
@@ -310,6 +311,9 @@ again:
 			if (folio_test_large(folio)) {
 				int ret;
 
+				/* migrate_vma_split_folio() consumes this reference */
+				if (folio != fault_folio)
+					folio_get(folio);
 				lazy_mmu_mode_disable();
 				pte_unmap_unlock(ptep, ptl);
 				ret = migrate_vma_split_folio(folio,
@@ -353,6 +357,9 @@ again:
 			if (folio && folio_test_large(folio)) {
 				int ret;
 
+				/* migrate_vma_split_folio() consumes this reference */
+				if (folio != fault_folio)
+					folio_get(folio);
 				lazy_mmu_mode_disable();
 				pte_unmap_unlock(ptep, ptl);
 				ret = migrate_vma_split_folio(folio,
_

Patches currently in -mm which might be from usama.arif@linux.dev are

mm-mempolicy-skip-non-present-pmds-when-queueing-folios.patch
mm-madvise-skip-device-private-pmds-in-cold-and-pageout-walks.patch
mm-huge_memory-skip-device-private-pmds-in-madvise_free_huge_pmd.patch
mm-vmstat-mm-memcontrol-add-_monotonic-vmstat-readers.patch
mm-vmscan-reduce-lru_lock-contention-via-vmstat-derived-scan-balance-cost.patch