From: "Paul E. McKenney" <paulmck@kernel.org>
To: Qian Cai <cai@lca.pw>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
Linux-MM <linux-mm@kvack.org>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: Kmemleak infrastructure improvement for task_struct leaks and call_rcu()
Date: Wed, 6 May 2020 10:40:19 -0700 [thread overview]
Message-ID: <20200506174019.GA2869@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <45D2D811-C3B0-442B-9744-415B4AC5CCDB@lca.pw>
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@lca.pw/
>
> 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
next prev parent reply other threads:[~2020-05-06 17:40 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-06 16:22 Kmemleak infrastructure improvement for task_struct leaks and call_rcu() Qian Cai
2020-05-06 17:40 ` Paul E. McKenney [this message]
2020-05-07 17:14 ` Catalin Marinas
2020-05-07 17:54 ` Paul E. McKenney
2020-05-07 17:16 ` Catalin Marinas
2020-05-07 17:29 ` Qian Cai
2020-05-09 9:44 ` Catalin Marinas
2020-05-10 21:27 ` Qian Cai
2020-05-12 14:15 ` Catalin Marinas
2020-05-12 18:09 ` Qian Cai
2020-05-13 9:59 ` Catalin Marinas
2020-05-17 22:32 ` Qian Cai
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