[PATCH 6.18.y] mm: Fix a hmm_range_fault() livelock / starvation problem

Thomas Hellström
2026-03-17T11:40:04Z

If hmm_range_fault() fails a folio_trylock() in do_swap_page,
trying to acquire the lock of a device-private folio for migration,
to ram, the function will spin until it succeeds grabbing the lock.

However, if the process holding the lock is depending on a work
item to be completed, which is scheduled on the same CPU as the
spinning hmm_range_fault(), that work item might be starved and
we end up in a livelock / starvation situation which is never
resolved.

This can happen, for example if the process holding the
device-private folio lock is stuck in
migrate_device_unmap()->lru_add_drain_all()
sinc lru_add_drain_all() requires a short work-item
to be run on all online cpus to complete.

A prerequisite for this to happen is:
a) Both zone device and system memory folios are considered in
migrate_device_unmap(), so that there is a reason to call
lru_add_drain_all() for a system memory folio while a
folio lock is held on a zone device folio.
b) The zone device folio has an initial mapcount > 1 which causes
at least one migration PTE entry insertion to be deferred to
try_to_migrate(), which can happen after the call to
lru_add_drain_all().
c) No or voluntary only preemption.

This all seems pretty unlikely to happen, but indeed is hit by
the "xe_exec_system_allocator" igt test.

Resolve this by waiting for the folio to be unlocked if the
folio_trylock() fails in do_swap_page().

Rename migration_entry_wait_on_locked() to
softleaf_entry_wait_unlock() and update its documentation to
indicate the new use-case.

Future code improvements might consider moving
the lru_add_drain_all() call in migrate_device_unmap() to be
called *after* all pages have migration entries inserted.
That would eliminate also b) above.

v2:
- Instead of a cond_resched() in hmm_range_fault(),
eliminate the problem by waiting for the folio to be unlocked
in do_swap_page() (Alistair Popple, Andrew Morton)
v3:
- Add a stub migration_entry_wait_on_locked() for the
!CONFIG_MIGRATION case. (Kernel Test Robot)
v4:
- Rename migrate_entry_wait_on_locked() to
softleaf_entry_wait_on_locked() and update docs (Alistair Popple)
v5:
- Add a WARN_ON_ONCE() for the !CONFIG_MIGRATION
version of softleaf_entry_wait_on_locked().
- Modify wording around function names in the commit message
(Andrew Morton)

Suggested-by: Alistair Popple <apopple@nvidia.com>
Fixes: 1afaeb8293c9 ("mm/migrate: Trylock device page in do_swap_page")
Cc: Ralph Campbell <rcampbell@nvidia.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Jason Gunthorpe <jgg@mellanox.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Leon Romanovsky <leon@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: linux-mm@kvack.org
Cc: <dri-devel@lists.freedesktop.org>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: <stable@vger.kernel.org> # v6.15+
Reviewed-by: John Hubbard <jhubbard@nvidia.com> #v3
Reviewed-by: Alistair Popple <apopple@nvidia.com>
Link: https://patch.msgid.link/20260210115653.92413-1-thomas.hellstrom@linux.intel.com
(cherry picked from commit a69d1ab971a624c6f112cea61536569d579c3215)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
(cherry picked from commit b570f37a2ce480be26c665345c5514686a8a0274)
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
---
include/linux/migrate.h | 8 ++++++++
mm/filemap.c | 13 +++++++++----
mm/memory.c | 3 ++-
3 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/include/linux/migrate.h b/include/linux/migrate.h
index 1f0ac122c3bf..8061efd89041 100644
--- a/include/linux/migrate.h
+++ b/include/linux/migrate.h
@@ -97,6 +97,14 @@ static inline int set_movable_ops(const struct movable_operations *ops, enum pag
return -ENOSYS;
}

+static inline void migration_entry_wait_on_locked(swp_entry_t entry, spinlock_t *ptl)
+ __releases(ptl)
+{
+ WARN_ON_ONCE(1);
+
+ spin_unlock(ptl);
+}
+
#endif /* CONFIG_MIGRATION */

#ifdef CONFIG_NUMA_BALANCING
diff --git a/mm/filemap.c b/mm/filemap.c
index 024b71da5224..8a7f4ce69aff 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1386,14 +1386,16 @@ static inline int folio_wait_bit_common(struct folio *folio, int bit_nr,

#ifdef CONFIG_MIGRATION
/**
- * migration_entry_wait_on_locked - Wait for a migration entry to be removed
- * @entry: migration swap entry.
+ * migration_entry_wait_on_locked - Wait for a migration entry or
+ * device_private entry to be removed.
+ * @entry: migration or device_private swap entry.
* @ptl: already locked ptl. This function will drop the lock.
*
- * Wait for a migration entry referencing the given page to be removed. This is
+ * Wait for a migration entry referencing the given page, or device_private
+ * entry referencing a dvice_private page to be unlocked. This is
* equivalent to folio_put_wait_locked(folio, TASK_UNINTERRUPTIBLE) except
* this can be called without taking a reference on the page. Instead this
- * should be called while holding the ptl for the migration entry referencing
+ * should be called while holding the ptl for @entry referencing
* the page.
*
* Returns after unlocking the ptl.
@@ -1435,6 +1437,9 @@ void migration_entry_wait_on_locked(swp_entry_t entry, spinlock_t *ptl)
* If a migration entry exists for the page the migration path must hold
* a valid reference to the page, and it must take the ptl to remove the
* migration entry. So the page is valid until the ptl is dropped.
+ * Similarly any path attempting to drop the last reference to a
+ * device-private page needs to grab the ptl to remove the device-private
+ * entry.
*/
spin_unlock(ptl);

diff --git a/mm/memory.c b/mm/memory.c
index 61748b762876..e43f0a4702c4 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4642,7 +4642,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
unlock_page(vmf->page);
put_page(vmf->page);
} else {
- pte_unmap_unlock(vmf->pte, vmf->ptl);
+ pte_unmap(vmf->pte);
+ migration_entry_wait_on_locked(entry, vmf->ptl);
}
} else if (is_hwpoison_entry(entry)) {
ret = VM_FAULT_HWPOISON;
--
2.53.0