stackdepot: stop preallocating after the final pool

From: Caleb Kan <[email protected]>

depot_init_pool() decides whether another pool is needed before it increments pools_num. When it registers the last allowed pool, pools_num is still one below stack_max_pools, so the existing comparison clears new_pool.

With new_pool cleared, stack_depot_save_flags() can allocate another order-2 pool after a lookup miss. depot_keep_new_pool() retains its pointer in the global new_pool, so the allocation is not lost. However, pools_num has already reached stack_max_pools, and depot_init_pool() rejects every attempt to register the pool. It remains allocated and unusable until reboot. The non-NULL pointer prevents later saves from allocating more spare pools.

Account for the pool being registered in the limit check so registering the final pool installs STACK_DEPOT_POISON and prevents the extra allocation.

---
Previously posted as patch 1/11 of the stackdepot RFC v2 series:
https://lore.kernel.org/r/[email protected]

 lib/stackdepot.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index dd2717ff94bf..90c52f2e0d3f 100644
@@ -323,7 +323,7 @@ static bool depot_init_pool(void **prealloc)
 	 * NULL; do not reset to NULL if we have reached the maximum number of
 	 * pools.
 	 */
-	if (pools_num < stack_max_pools)
+	if (pools_num + 1 < stack_max_pools)
 		WRITE_ONCE(new_pool, NULL);
 	else
 		WRITE_ONCE(new_pool, STACK_DEPOT_POISON);