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

[Oleg Nesterov <oleg@redhat.com>]

On 05/07, Sultan Alsawaf wrote:
>
> On Tue, May 07, 2019 at 05:31:54PM +0200, Oleg Nesterov wrote:
>
> > Did you test this patch with lockdep enabled?
> >
> > If I read the patch correctly, lockdep should complain. vtsk_is_duplicate()
> > ensures that we do not take the same ->alloc_lock twice or more, but lockdep
> > can't know this.
>
> Yeah, lockdep is fine with this, at least on 4.4.

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.

Nevermind, I guess this code won't run with lockdep enabled...


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.


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

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.

Oleg.