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

[Sultan Alsawaf <sultan@kerneltoast.com>]

On Tue, May 14, 2019 at 12:44:53PM -0400, Steven Rostedt wrote:
> OK, this has gotten my attention.
> 
> This thread is quite long, do you have a git repo I can look at, and
> also where is the first task_lock() taken before the
> find_lock_task_mm()?
> 
> -- Steve

Hi Steve,

This is the git repo I work on: https://github.com/kerneltoast/android_kernel_google_wahoo

With the newest simple_lmk iteration being this commit: https://github.com/kerneltoast/android_kernel_google_wahoo/commit/6b145b8c28b39f7047393169117f72ea7387d91c

This repo is based off the 4.4 kernel that Google ships on the Pixel 2/2XL.

simple_lmk iterates through the entire task list more than once and locks
potential victims using find_lock_task_mm(). It keeps these potential victims
locked across the multiple times that the task list is iterated.

The locking pattern that Oleg said should cause lockdep to complain is that
iterating through the entire task list more than once can lead to locking the
same task that was locked earlier with find_lock_task_mm(), and thus deadlock.
But there is a check in simple_lmk that avoids locking potential victims that
were already found, which avoids the deadlock, but lockdep doesn't know about
the check (which is done with vtsk_is_duplicate()) and should therefore
complain.

Lockdep does not complain though.

Sultan