Re: [PATCH v4 3/3] mm/oom_kill: Have the OOM reaper and exit_mmap() traverse the maple tree in opposite orders

[zhongjinji <zhongjinji@honor.com>]

On Thu, 14 Aug 2025 21:55:55 +0800 <zhongjinji@honor.com> wrote:

> > When a process is OOM killed, if the OOM reaper and the thread running
> > exit_mmap() execute at the same time, both will traverse the vma's maple
> > tree along the same path. They may easily unmap the same vma, causing them
> > to compete for the pte spinlock. This increases unnecessary load, causing
> > the execution time of the OOM reaper and the thread running exit_mmap() to
> > increase.
> 
> Please tell me what I'm missing here.
> 
> OOM kills are a rare event.  And this race sounds like it will rarely
> occur even if an oom-killing is happening.  And the delay will be
> relatively short.
> 
> If I'm correct then we're addressing rare*rare*small, so why bother?

When there are apps that consume a large amount of memory, encountering OOM on
low-memory Android devices is not uncommon. On Android devices, programs like lmkd
(a user-space daemon in the Android system) also call process_mrelease() to reap 
memory when an app is killed.

> > When a process exits, exit_mmap() traverses the vma's maple tree from low to high
> > address. To reduce the chance of unmapping the same vma simultaneously,
> > the OOM reaper should traverse vma's tree from high to low address. This reduces
> > lock contention when unmapping the same vma.
> 
> Sharing some before-and-after runtime measurements would be useful.  Or
> at least, detailed anecdotes.

Here is my test data on Android. The test process is as follows: start the same app,
then kill it, and finally capture the perfetto trace.
In the test, the way to trigger the OOM reaper is: intercept the kill signal and
actively add the process to the OOM reaper queue as what OOM does.

Note: #RxComputationT, vdp:vidtask:m, and tp-background are threads of the same process,
and they are the last threads to exit.

Thread             TID         State        Wall duration (ms)
# with oom reaper and traverse reverse
#RxComputationT    13708       Running      60.690572
oom_reaper         81          Running      46.492032

# with oom reaper but traverses
vdp:vidtask:m      14040       Running      81.848297
oom_reaper         81          Running      69.32

# without oom reaper
tp-background      12424       Running      106.021874