On Wed, May 06, 2020 at 12:22:37PM -0400, Qian Cai wrote: > == task struck leaks == > There are leaks from task struct from time to time where someone forgot to call put_task_struct() somewhere leading to leaks. For example, > > https://lore.kernel.org/lkml/C1CCBDAC-A453-4FF2-908F-0B6E356223D1@xxxxxx/ > > It was such a pain to debug this kind of leaks at the moment, as all we could do was to audit the code by checking all new put_task_struct() and get_task_struct() call sites which is error-prone because there could be other new call sites like get_pid_task() which would also need to be balanced with put_task_struct() as well. > > What do you think about adding some aux call traces for kmemleak in general? For example, if the tracking object is a task struct, it would save call traces for the first and last call of both get_task_struct() and put_task_struct(). Then, it could be expanded to track other refcount-based leaks in the future. > > == call_rcu() leaks == > Another issue that might be relevant is that it seems sometimes, kmemleak will give a lot of false positives (hundreds) because the memory was supposed to be freed by call_rcu() (for example, in dst_release()) but for some reasons, it takes a long time probably waiting for grace periods or some kind of RCU self-stall, but the memory had already became an orphan. I am not sure how we are going to resolve this properly until we have to figure out why call_rcu() is taking so long to finish? I know nothing about kmemleak, but I won't let that stop me from making random suggestions... One approach is to do an rcu_barrier() inside kmemleak just before printing leaked blocks, and check to see if any are still leaked after the rcu_barrier(). If kmemleak works on crash dumps, another approach is to scan RCU's callback lists. This will miss those callbacks that rcu_do_batch() was in the middle of invoking, though. It also misses cases where someone passes a linked structure to call_rcu(), and then frees the structure piece by piece within the callback function. > Another solution is to add aux call traces for both skb_dst_drop() and skb_dst_set() for this case, but that there are many places to free memory via call_rcu() like inode free etc. And call_rcu() has no idea where the memory starts. And again, sometimes there is memory linked from that passed to call_rcu() that will be freed by the callback function. In theory, these linked-structure cases could be handled by checking the callback function and then traversing the links. I wouldn't be that ambitious, but don't let me discourage you. ;-) Thanx, Paul
