In the Linux kernel, the following vulnerability has been resolved:
mm: Fix a hmm_range_fault() livelock / starvation problem
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(),
- Add a stub migration_entry_wait_on_locked() for the
- Rename migrate_entry_wait_on_locked() to
- Add a WARN_ON_ONCE() for the !CONFIG_MIGRATION
- Modify wording around function names in the commit message
(cherry picked from commit a69d1ab971a624c6f112cea61536569d579c3215)