Re: [RFC] simple_lmk: Introduce Simple Low Memory Killer for Android

[Oleg Nesterov <oleg@redhat.com>]

On 05/09, Sultan Alsawaf wrote:
>
> 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 ;)

I am starting to think I am ;)

If you have task1 != task2 this code

	task_lock(task1);
	task_lock(task2);

should trigger print_deadlock_bug(), task1->alloc_lock and task2->alloc_lock are
the "same" lock from lockdep pov, held_lock's will have the same hlock_class().

> CONFIG_PROVE_LOCKING=y

OK,

> And a printk added in vtsk_is_duplicate() to print when a duplicate is detected,

in this case find_lock_task_mm() won't be called, and this is what saves us from
the actual deadlock.


> 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.

this reports that CONFIG_PROVE_RCU 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

yes, looks like simple_lmk has at least 2 locked victims. And I have no idea why
you do not see anything else in dmesg. May be debug_locks_off() was already called.

But see above, "grep lockdep" won't work.  Perhaps you can do
"grep -e WARNING -e BUG -e locking".

Oleg.