mm/tlb: Remove tlb_remove_table() non-concurrent condition

Will noted that only checking mm_users is incorrect; we should also
check mm_count in order to cover CPUs that have a lazy reference to
this mm (and could do speculative TLB operations).

If removing this turns out to be a performance issue, we can
re-instate a more complete check, but in tlb_table_flush() eliding the
call_rcu_sched().

Fixes: 2672391 ("mm, powerpc: move the RCU page-table freeing into generic code")
Reported-by: Will Deacon <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Acked-by: Rik van Riel <[email protected]>
Acked-by: Will Deacon <[email protected]>
Cc: Nicholas Piggin <[email protected]>
Cc: David Miller <[email protected]>
Cc: Martin Schwidefsky <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: [email protected]
Signed-off-by: Linus Torvalds <[email protected]>

mm/memory.c

@@ -375,15 +375,6 @@ void tlb_remove_table(struct mmu_gather *tlb, void *table)
 {
 	struct mmu_table_batch **batch = &tlb->batch;
 
-	/*
-	 * When there's less then two users of this mm there cannot be a
-	 * concurrent page-table walk.
-	 */
-	if (atomic_read(&tlb->mm->mm_users) < 2) {
-		__tlb_remove_table(table);
-		return;
-	}
-
 	if (*batch == NULL) {
 		*batch = (struct mmu_table_batch *)__get_free_page(GFP_NOWAIT | __GFP_NOWARN);
 		if (*batch == NULL) {