Since e303297 (mm: extended batches for generic mmu_gather) we are batching
pages to be freed until either tlb_next_batch cannot allocate a new batch or we
are done.

This works just fine most of the time but we can get in troubles with
non-preemptible kernel (CONFIG_PREEMPT_NONE or CONFIG_PREEMPT_VOLUNTARY) on
large machines where too aggressive batching might lead to soft lockups during
process exit path (exit_mmap) because there are no scheduling points down the
free_pages_and_swap_cache path and so the freeing can take long enough to
trigger the soft lockup.

The lockup is harmless except when the system is setup to panic on
softlockup which is not that unusual.

The simplest way to work around this issue is to explicitly cond_resched per
batch in tlb_flush_mmu (1020 pages on x86_64).

...

--- a/mm/memory.c
+++ b/mm/memory.c

@@ -239,6 +239,7 @@ void tlb_flush_mmu(struct mmu_gather *tlb)
 	for (batch = &tlb->local; batch; batch = batch->next) {
 		free_pages_and_swap_cache(batch->pages, batch->nr);
 		batch->nr = 0;
+		cond_resched();
 	}
 	tlb->active = &tlb->local;
 }