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

Oleg Nesterov oleg at redhat.com
Thu May 9 15:56:46 UTC 2019

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


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.


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