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Date:   Thu, 9 May 2019 11:33:53 -0700
From:   Sultan Alsawaf <sultan@kerneltoast.com>
To:     Oleg Nesterov <oleg@redhat.com>
Cc:     Christian Brauner <christian@brauner.io>,
        Daniel Colascione <dancol@google.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Tim Murray <timmurray@google.com>,
        Michal Hocko <mhocko@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Arve =?iso-8859-1?B?SGr4bm5lduVn?= <arve@android.com>,
        Todd Kjos <tkjos@android.com>,
        Martijn Coenen <maco@android.com>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        "open list:ANDROID DRIVERS" <devel@driverdev.osuosl.org>,
        linux-mm <linux-mm@kvack.org>,
        kernel-team <kernel-team@android.com>,
        Andy Lutomirski <luto@amacapital.net>,
        "Serge E. Hallyn" <serge@hallyn.com>,
        Kees Cook <keescook@chromium.org>,
        Joel Fernandes <joel@joelfernandes.org>
Subject: Re: [RFC] simple_lmk: Introduce Simple Low Memory Killer for Android
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On Thu, May 09, 2019 at 05:56:46PM +0200, Oleg Nesterov wrote:
> Impossible ;) I bet lockdep should report the deadlock as soon as find_victims()
> calls find_lock_task_mm() when you already have a locked victim.

I hope you're not a betting man ;)

With the following configured:
CONFIG_DEBUG_RT_MUTEXES=y
CONFIG_DEBUG_SPINLOCK=y
# CONFIG_DEBUG_SPINLOCK_BITE_ON_BUG is not set
CONFIG_DEBUG_SPINLOCK_PANIC_ON_BUG=y
CONFIG_DEBUG_MUTEXES=y
CONFIG_DEBUG_WW_MUTEX_SLOWPATH=y
CONFIG_DEBUG_LOCK_ALLOC=y
CONFIG_PROVE_LOCKING=y
CONFIG_LOCKDEP=y
CONFIG_LOCK_STAT=y
CONFIG_DEBUG_LOCKDEP=y
CONFIG_DEBUG_ATOMIC_SLEEP=y
# CONFIG_DEBUG_LOCKING_API_SELFTESTS is not set
# CONFIG_LOCK_TORTURE_TEST is not set

And a printk added in vtsk_is_duplicate() to print when a duplicate is detected,
and my phone's memory cut in half to make simple_lmk do something, this is what
I observed:
taimen:/ # dmesg | grep lockdep
[    0.000000] \x09RCU lockdep checking is enabled.
taimen:/ # dmesg | grep simple_lmk
[   23.211091] simple_lmk: Killing android.carrier with adj 906 to free 37420 kiB
[   23.211160] simple_lmk: Killing oadcastreceiver with adj 906 to free 36784 kiB
[   23.248457] simple_lmk: Killing .apps.translate with adj 904 to free 45884 kiB
[   23.248720] simple_lmk: Killing ndroid.settings with adj 904 to free 42868 kiB
[   23.313417] simple_lmk: DUPLICATE VTSK!
[   23.313440] simple_lmk: Killing ndroid.keychain with adj 906 to free 33680 kiB
[   23.313513] simple_lmk: Killing com.whatsapp with adj 904 to free 51436 kiB
[   34.646695] simple_lmk: DUPLICATE VTSK!
[   34.646717] simple_lmk: Killing ndroid.apps.gcs with adj 906 to free 37956 kiB
[   34.646792] simple_lmk: Killing droid.apps.maps with adj 904 to free 63600 kiB
taimen:/ # dmesg | grep lockdep
[    0.000000] \x09RCU lockdep checking is enabled.
taimen:/ # 

> As for https://github.com/kerneltoast/android_kernel_google_wahoo/commit/afc8c9bf2dbde95941253c168d1adb64cfa2e3ad
> Well,
> 
> 	mmdrop(mm);
> 	simple_lmk_mm_freed(mm);
> 
> looks racy because mmdrop(mm) can free this mm_struct. Yes, simple_lmk_mm_freed()
> does not dereference this pointer, but the same memory can be re-allocated as
> another ->mm for the new task which can be found by find_victims(), and _in theory_
> this all can happen in between, so the "victims[i].mm == mm" can be false positive.
> 
> And this also means that simple_lmk_mm_freed() should clear victims[i].mm when
> it detects "victims[i].mm == mm", otherwise we have the same theoretical race,
> victims_to_kill is only cleared when the last victim goes away.

Fair point. Putting simple_lmk_mm_freed(mm) right before mmdrop(mm), and
sprinkling in a cmpxchg in simple_lmk_mm_freed() should fix that up.

> Another nit... you can drop tasklist_lock right after the 1st "find_victims" loop.

True!

> And it seems that you do not really need to walk the "victims" array twice after that,
> you can do everything in a single loop, but this is cosmetic.

Won't this result in potentially holding the task lock way longer than necessary
for multiple tasks that aren't going to be killed?

Thanks,
Sultan