linux-kernel - Re: Kmemleak infrastructure improvement for task_struct leaks and call

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Message-ID: <20200509094455.GA4351@gaia>
Date:   Sat, 9 May 2020 10:44:56 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Qian Cai <cai@....pw>
Cc:     Linux-MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: Kmemleak infrastructure improvement for task_struct leaks and
 call_rcu()

On Thu, May 07, 2020 at 01:29:04PM -0400, Qian Cai wrote:
> On May 7, 2020, at 1:16 PM, Catalin Marinas <catalin.marinas@....com> wrote:
> > I don't mind adding additional tracking info if it helps with debugging.
> > But if it's for improving false positives, I'd prefer to look deeper
> > into figure out why the pointer reference graph tracking failed.
> 
> No, the task struct leaks are real leaks. It is just painful to figure
> out the missing or misplaced put_task_struct() from the kmemleak
> reports at the moment.

We could log the callers to get_task_struct() and put_task_struct(),
something like __builtin_return_address(0) (how does this work if the
function is inlined?). If it's not the full backtrace, it shouldn't slow
down kmemleak considerably. I don't think it's worth logging only the
first/last calls to get/put. You'd hope that put is called in reverse
order to get.

I think it may be better if this is added as a new allocation pointed to
from kmemleak_object rather than increasing this structure since it will
be added on a case by case basis. When dumping the leak information, it
would also dump the get/put calls, in the order they were called. We
could add some simple refcount tracking (++ for get, -- for put) to
easily notice any imbalance.

I'm pretty busy next week but happy to review if you have a patch ;).

-- 
Catalin