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() The lru_add_drain_all() function 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 using a cond_resched() after each iteration in hmm_range_fault(). Future code improvements might consider moving the lru_add_drain_all() call in migrate_device_unmap() out of the folio locked region. Also, hmm_range_fault() can be a very long-running function so a cond_resched() at the end of each iteration can be motivated even in the absence of an -EBUSY. Fixes: d28c2c9a4877 ("mm/hmm: make full use of walk_page_range()") Cc: Ralph Campbell Cc: Christoph Hellwig Cc: Jason Gunthorpe Cc: Jason Gunthorpe Cc: Leon Romanovsky Cc: Andrew Morton Cc: Matthew Brost Cc: linux-mm@kvack.org Cc: # v5.5+ Cc: Signed-off-by: Thomas Hellström --- mm/hmm.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/mm/hmm.c b/mm/hmm.c index 4ec74c18bef6..160c9e4e5a92 100644 --- a/mm/hmm.c +++ b/mm/hmm.c @@ -674,6 +674,13 @@ int hmm_range_fault(struct hmm_range *range) return -EBUSY; ret = walk_page_range(mm, hmm_vma_walk.last, range->end, &hmm_walk_ops, &hmm_vma_walk); + /* + * Conditionally reschedule to let other work items get + * a chance to unlock device-private pages whose locks + * we're spinning on. + */ + cond_resched(); + /* * When -EBUSY is returned the loop restarts with * hmm_vma_walk.last set to an address that has not been stored -- 2.52.0